Adding upstream version 6.1.76.upstream/6.1.76upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 3570 insertions, 0 deletions

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
new file mode 100644
index 000000000..21f01d32c
--- /dev/null
+++ b/Documentation/trace/ftrace.rst
@@ -0,0 +1,3570 @@
+========================
+ftrace - Function Tracer
+========================
+
+Copyright 2008 Red Hat Inc.
+
+:Author:   Steven Rostedt <srostedt@redhat.com>
+:License:  The GNU Free Documentation License, Version 1.2
+          (dual licensed under the GPL v2)
+:Original Reviewers:  Elias Oltmanns, Randy Dunlap, Andrew Morton,
+		      John Kacur, and David Teigland.
+
+- Written for: 2.6.28-rc2
+- Updated for: 3.10
+- Updated for: 4.13 - Copyright 2017 VMware Inc. Steven Rostedt
+- Converted to rst format - Changbin Du <changbin.du@intel.com>
+
+Introduction
+------------
+
+Ftrace is an internal tracer designed to help out developers and
+designers of systems to find what is going on inside the kernel.
+It can be used for debugging or analyzing latencies and
+performance issues that take place outside of user-space.
+
+Although ftrace is typically considered the function tracer, it
+is really a framework of several assorted tracing utilities.
+There's latency tracing to examine what occurs between interrupts
+disabled and enabled, as well as for preemption and from a time
+a task is woken to the task is actually scheduled in.
+
+One of the most common uses of ftrace is the event tracing.
+Throughout the kernel is hundreds of static event points that
+can be enabled via the tracefs file system to see what is
+going on in certain parts of the kernel.
+
+See events.rst for more information.
+
+
+Implementation Details
+----------------------
+
+See Documentation/trace/ftrace-design.rst for details for arch porters and such.
+
+
+The File System
+---------------
+
+Ftrace uses the tracefs file system to hold the control files as
+well as the files to display output.
+
+When tracefs is configured into the kernel (which selecting any ftrace
+option will do) the directory /sys/kernel/tracing will be created. To mount
+this directory, you can add to your /etc/fstab file::
+
+ tracefs       /sys/kernel/tracing       tracefs defaults        0       0
+
+Or you can mount it at run time with::
+
+ mount -t tracefs nodev /sys/kernel/tracing
+
+For quicker access to that directory you may want to make a soft link to
+it::
+
+ ln -s /sys/kernel/tracing /tracing
+
+.. attention::
+
+  Before 4.1, all ftrace tracing control files were within the debugfs
+  file system, which is typically located at /sys/kernel/debug/tracing.
+  For backward compatibility, when mounting the debugfs file system,
+  the tracefs file system will be automatically mounted at:
+
+  /sys/kernel/debug/tracing
+
+  All files located in the tracefs file system will be located in that
+  debugfs file system directory as well.
+
+.. attention::
+
+  Any selected ftrace option will also create the tracefs file system.
+  The rest of the document will assume that you are in the ftrace directory
+  (cd /sys/kernel/tracing) and will only concentrate on the files within that
+  directory and not distract from the content with the extended
+  "/sys/kernel/tracing" path name.
+
+That's it! (assuming that you have ftrace configured into your kernel)
+
+After mounting tracefs you will have access to the control and output files
+of ftrace. Here is a list of some of the key files:
+
+
+ Note: all time values are in microseconds.
+
+  current_tracer:
+
+	This is used to set or display the current tracer
+	that is configured. Changing the current tracer clears
+	the ring buffer content as well as the "snapshot" buffer.
+
+  available_tracers:
+
+	This holds the different types of tracers that
+	have been compiled into the kernel. The
+	tracers listed here can be configured by
+	echoing their name into current_tracer.
+
+  tracing_on:
+
+	This sets or displays whether writing to the trace
+	ring buffer is enabled. Echo 0 into this file to disable
+	the tracer or 1 to enable it. Note, this only disables
+	writing to the ring buffer, the tracing overhead may
+	still be occurring.
+
+	The kernel function tracing_off() can be used within the
+	kernel to disable writing to the ring buffer, which will
+	set this file to "0". User space can re-enable tracing by
+	echoing "1" into the file.
+
+	Note, the function and event trigger "traceoff" will also
+	set this file to zero and stop tracing. Which can also
+	be re-enabled by user space using this file.
+
+  trace:
+
+	This file holds the output of the trace in a human
+	readable format (described below). Opening this file for
+	writing with the O_TRUNC flag clears the ring buffer content.
+        Note, this file is not a consumer. If tracing is off
+        (no tracer running, or tracing_on is zero), it will produce
+        the same output each time it is read. When tracing is on,
+        it may produce inconsistent results as it tries to read
+        the entire buffer without consuming it.
+
+  trace_pipe:
+
+	The output is the same as the "trace" file but this
+	file is meant to be streamed with live tracing.
+	Reads from this file will block until new data is
+	retrieved.  Unlike the "trace" file, this file is a
+	consumer. This means reading from this file causes
+	sequential reads to display more current data. Once
+	data is read from this file, it is consumed, and
+	will not be read again with a sequential read. The
+	"trace" file is static, and if the tracer is not
+	adding more data, it will display the same
+	information every time it is read.
+
+  trace_options:
+
+	This file lets the user control the amount of data
+	that is displayed in one of the above output
+	files. Options also exist to modify how a tracer
+	or events work (stack traces, timestamps, etc).
+
+  options:
+
+	This is a directory that has a file for every available
+	trace option (also in trace_options). Options may also be set
+	or cleared by writing a "1" or "0" respectively into the
+	corresponding file with the option name.
+
+  tracing_max_latency:
+
+	Some of the tracers record the max latency.
+	For example, the maximum time that interrupts are disabled.
+	The maximum time is saved in this file. The max trace will also be
+	stored,	and displayed by "trace". A new max trace will only be
+	recorded if the latency is greater than the value in this file
+	(in microseconds).
+
+	By echoing in a time into this file, no latency will be recorded
+	unless it is greater than the time in this file.
+
+  tracing_thresh:
+
+	Some latency tracers will record a trace whenever the
+	latency is greater than the number in this file.
+	Only active when the file contains a number greater than 0.
+	(in microseconds)
+
+  buffer_size_kb:
+
+	This sets or displays the number of kilobytes each CPU
+	buffer holds. By default, the trace buffers are the same size
+	for each CPU. The displayed number is the size of the
+	CPU buffer and not total size of all buffers. The
+	trace buffers are allocated in pages (blocks of memory
+	that the kernel uses for allocation, usually 4 KB in size).
+	A few extra pages may be allocated to accommodate buffer management
+	meta-data. If the last page allocated has room for more bytes
+	than requested, the rest of the page will be used,
+	making the actual allocation bigger than requested or shown.
+	( Note, the size may not be a multiple of the page size
+	due to buffer management meta-data. )
+
+	Buffer sizes for individual CPUs may vary
+	(see "per_cpu/cpu0/buffer_size_kb" below), and if they do
+	this file will show "X".
+
+  buffer_total_size_kb:
+
+	This displays the total combined size of all the trace buffers.
+
+  free_buffer:
+
+	If a process is performing tracing, and the ring buffer	should be
+	shrunk "freed" when the process is finished, even if it were to be
+	killed by a signal, this file can be used for that purpose. On close
+	of this file, the ring buffer will be resized to its minimum size.
+	Having a process that is tracing also open this file, when the process
+	exits its file descriptor for this file will be closed, and in doing so,
+	the ring buffer will be "freed".
+
+	It may also stop tracing if disable_on_free option is set.
+
+  tracing_cpumask:
+
+	This is a mask that lets the user only trace on specified CPUs.
+	The format is a hex string representing the CPUs.
+
+  set_ftrace_filter:
+
+	When dynamic ftrace is configured in (see the
+	section below "dynamic ftrace"), the code is dynamically
+	modified (code text rewrite) to disable calling of the
+	function profiler (mcount). This lets tracing be configured
+	in with practically no overhead in performance.  This also
+	has a side effect of enabling or disabling specific functions
+	to be traced. Echoing names of functions into this file
+	will limit the trace to only those functions.
+	This influences the tracers "function" and "function_graph"
+	and thus also function profiling (see "function_profile_enabled").
+
+	The functions listed in "available_filter_functions" are what
+	can be written into this file.
+
+	This interface also allows for commands to be used. See the
+	"Filter commands" section for more details.
+
+	As a speed up, since processing strings can be quite expensive
+	and requires a check of all functions registered to tracing, instead
+	an index can be written into this file. A number (starting with "1")
+	written will instead select the same corresponding at the line position
+	of the "available_filter_functions" file.
+
+  set_ftrace_notrace:
+
+	This has an effect opposite to that of
+	set_ftrace_filter. Any function that is added here will not
+	be traced. If a function exists in both set_ftrace_filter
+	and set_ftrace_notrace,	the function will _not_ be traced.
+
+  set_ftrace_pid:
+
+	Have the function tracer only trace the threads whose PID are
+	listed in this file.
+
+	If the "function-fork" option is set, then when a task whose
+	PID is listed in this file forks, the child's PID will
+	automatically be added to this file, and the child will be
+	traced by the function tracer as well. This option will also
+	cause PIDs of tasks that exit to be removed from the file.
+
+  set_ftrace_notrace_pid:
+
+        Have the function tracer ignore threads whose PID are listed in
+        this file.
+
+        If the "function-fork" option is set, then when a task whose
+	PID is listed in this file forks, the child's PID will
+	automatically be added to this file, and the child will not be
+	traced by the function tracer as well. This option will also
+	cause PIDs of tasks that exit to be removed from the file.
+
+        If a PID is in both this file and "set_ftrace_pid", then this
+        file takes precedence, and the thread will not be traced.
+
+  set_event_pid:
+
+	Have the events only trace a task with a PID listed in this file.
+	Note, sched_switch and sched_wake_up will also trace events
+	listed in this file.
+
+	To have the PIDs of children of tasks with their PID in this file
+	added on fork, enable the "event-fork" option. That option will also
+	cause the PIDs of tasks to be removed from this file when the task
+	exits.
+
+  set_event_notrace_pid:
+
+	Have the events not trace a task with a PID listed in this file.
+	Note, sched_switch and sched_wakeup will trace threads not listed
+	in this file, even if a thread's PID is in the file if the
+        sched_switch or sched_wakeup events also trace a thread that should
+        be traced.
+
+	To have the PIDs of children of tasks with their PID in this file
+	added on fork, enable the "event-fork" option. That option will also
+	cause the PIDs of tasks to be removed from this file when the task
+	exits.
+
+  set_graph_function:
+
+	Functions listed in this file will cause the function graph
+	tracer to only trace these functions and the functions that
+	they call. (See the section "dynamic ftrace" for more details).
+	Note, set_ftrace_filter and set_ftrace_notrace still affects
+	what functions are being traced.
+
+  set_graph_notrace:
+
+	Similar to set_graph_function, but will disable function graph
+	tracing when the function is hit until it exits the function.
+	This makes it possible to ignore tracing functions that are called
+	by a specific function.
+
+  available_filter_functions:
+
+	This lists the functions that ftrace has processed and can trace.
+	These are the function names that you can pass to
+	"set_ftrace_filter", "set_ftrace_notrace",
+	"set_graph_function", or "set_graph_notrace".
+	(See the section "dynamic ftrace" below for more details.)
+
+  dyn_ftrace_total_info:
+
+	This file is for debugging purposes. The number of functions that
+	have been converted to nops and are available to be traced.
+
+  enabled_functions:
+
+	This file is more for debugging ftrace, but can also be useful
+	in seeing if any function has a callback attached to it.
+	Not only does the trace infrastructure use ftrace function
+	trace utility, but other subsystems might too. This file
+	displays all functions that have a callback attached to them
+	as well as the number of callbacks that have been attached.
+	Note, a callback may also call multiple functions which will
+	not be listed in this count.
+
+	If the callback registered to be traced by a function with
+	the "save regs" attribute (thus even more overhead), a 'R'
+	will be displayed on the same line as the function that
+	is returning registers.
+
+	If the callback registered to be traced by a function with
+	the "ip modify" attribute (thus the regs->ip can be changed),
+	an 'I' will be displayed on the same line as the function that
+	can be overridden.
+
+	If the architecture supports it, it will also show what callback
+	is being directly called by the function. If the count is greater
+	than 1 it most likely will be ftrace_ops_list_func().
+
+	If the callback of a function jumps to a trampoline that is
+	specific to the callback and which is not the standard trampoline,
+	its address will be printed as well as the function that the
+	trampoline calls.
+
+  function_profile_enabled:
+
+	When set it will enable all functions with either the function
+	tracer, or if configured, the function graph tracer. It will
+	keep a histogram of the number of functions that were called
+	and if the function graph tracer was configured, it will also keep
+	track of the time spent in those functions. The histogram
+	content can be displayed in the files:
+
+	trace_stat/function<cpu> ( function0, function1, etc).
+
+  trace_stat:
+
+	A directory that holds different tracing stats.
+
+  kprobe_events:
+
+	Enable dynamic trace points. See kprobetrace.rst.
+
+  kprobe_profile:
+
+	Dynamic trace points stats. See kprobetrace.rst.
+
+  max_graph_depth:
+
+	Used with the function graph tracer. This is the max depth
+	it will trace into a function. Setting this to a value of
+	one will show only the first kernel function that is called
+	from user space.
+
+  printk_formats:
+
+	This is for tools that read the raw format files. If an event in
+	the ring buffer references a string, only a pointer to the string
+	is recorded into the buffer and not the string itself. This prevents
+	tools from knowing what that string was. This file displays the string
+	and address for	the string allowing tools to map the pointers to what
+	the strings were.
+
+  saved_cmdlines:
+
+	Only the pid of the task is recorded in a trace event unless
+	the event specifically saves the task comm as well. Ftrace
+	makes a cache of pid mappings to comms to try to display
+	comms for events. If a pid for a comm is not listed, then
+	"<...>" is displayed in the output.
+
+	If the option "record-cmd" is set to "0", then comms of tasks
+	will not be saved during recording. By default, it is enabled.
+
+  saved_cmdlines_size:
+
+	By default, 128 comms are saved (see "saved_cmdlines" above). To
+	increase or decrease the amount of comms that are cached, echo
+	the number of comms to cache into this file.
+
+  saved_tgids:
+
+	If the option "record-tgid" is set, on each scheduling context switch
+	the Task Group ID of a task is saved in a table mapping the PID of
+	the thread to its TGID. By default, the "record-tgid" option is
+	disabled.
+
+  snapshot:
+
+	This displays the "snapshot" buffer and also lets the user
+	take a snapshot of the current running trace.
+	See the "Snapshot" section below for more details.
+
+  stack_max_size:
+
+	When the stack tracer is activated, this will display the
+	maximum stack size it has encountered.
+	See the "Stack Trace" section below.
+
+  stack_trace:
+
+	This displays the stack back trace of the largest stack
+	that was encountered when the stack tracer is activated.
+	See the "Stack Trace" section below.
+
+  stack_trace_filter:
+
+	This is similar to "set_ftrace_filter" but it limits what
+	functions the stack tracer will check.
+
+  trace_clock:
+
+	Whenever an event is recorded into the ring buffer, a
+	"timestamp" is added. This stamp comes from a specified
+	clock. By default, ftrace uses the "local" clock. This
+	clock is very fast and strictly per cpu, but on some
+	systems it may not be monotonic with respect to other
+	CPUs. In other words, the local clocks may not be in sync
+	with local clocks on other CPUs.
+
+	Usual clocks for tracing::
+
+	  # cat trace_clock
+	  [local] global counter x86-tsc
+
+	The clock with the square brackets around it is the one in effect.
+
+	local:
+		Default clock, but may not be in sync across CPUs
+
+	global:
+		This clock is in sync with all CPUs but may
+		be a bit slower than the local clock.
+
+	counter:
+		This is not a clock at all, but literally an atomic
+		counter. It counts up one by one, but is in sync
+		with all CPUs. This is useful when you need to
+		know exactly the order events occurred with respect to
+		each other on different CPUs.
+
+	uptime:
+		This uses the jiffies counter and the time stamp
+		is relative to the time since boot up.
+
+	perf:
+		This makes ftrace use the same clock that perf uses.
+		Eventually perf will be able to read ftrace buffers
+		and this will help out in interleaving the data.
+
+	x86-tsc:
+		Architectures may define their own clocks. For
+		example, x86 uses its own TSC cycle clock here.
+
+	ppc-tb:
+		This uses the powerpc timebase register value.
+		This is in sync across CPUs and can also be used
+		to correlate events across hypervisor/guest if
+		tb_offset is known.
+
+	mono:
+		This uses the fast monotonic clock (CLOCK_MONOTONIC)
+		which is monotonic and is subject to NTP rate adjustments.
+
+	mono_raw:
+		This is the raw monotonic clock (CLOCK_MONOTONIC_RAW)
+		which is monotonic but is not subject to any rate adjustments
+		and ticks at the same rate as the hardware clocksource.
+
+	boot:
+		This is the boot clock (CLOCK_BOOTTIME) and is based on the
+		fast monotonic clock, but also accounts for time spent in
+		suspend. Since the clock access is designed for use in
+		tracing in the suspend path, some side effects are possible
+		if clock is accessed after the suspend time is accounted before
+		the fast mono clock is updated. In this case, the clock update
+		appears to happen slightly sooner than it normally would have.
+		Also on 32-bit systems, it's possible that the 64-bit boot offset
+		sees a partial update. These effects are rare and post
+		processing should be able to handle them. See comments in the
+		ktime_get_boot_fast_ns() function for more information.
+
+	tai:
+		This is the tai clock (CLOCK_TAI) and is derived from the wall-
+		clock time. However, this clock does not experience
+		discontinuities and backwards jumps caused by NTP inserting leap
+		seconds. Since the clock access is designed for use in tracing,
+		side effects are possible. The clock access may yield wrong
+		readouts in case the internal TAI offset is updated e.g., caused
+		by setting the system time or using adjtimex() with an offset.
+		These effects are rare and post processing should be able to
+		handle them. See comments in the ktime_get_tai_fast_ns()
+		function for more information.
+
+	To set a clock, simply echo the clock name into this file::
+
+	  # echo global > trace_clock
+
+	Setting a clock clears the ring buffer content as well as the
+	"snapshot" buffer.
+
+  trace_marker:
+
+	This is a very useful file for synchronizing user space
+	with events happening in the kernel. Writing strings into
+	this file will be written into the ftrace buffer.
+
+	It is useful in applications to open this file at the start
+	of the application and just reference the file descriptor
+	for the file::
+
+		void trace_write(const char *fmt, ...)
+		{
+			va_list ap;
+			char buf[256];
+			int n;
+
+			if (trace_fd < 0)
+				return;
+
+			va_start(ap, fmt);
+			n = vsnprintf(buf, 256, fmt, ap);
+			va_end(ap);
+
+			write(trace_fd, buf, n);
+		}
+
+	start::
+
+		trace_fd = open("trace_marker", O_WRONLY);
+
+	Note: Writing into the trace_marker file can also initiate triggers
+	      that are written into /sys/kernel/tracing/events/ftrace/print/trigger
+	      See "Event triggers" in Documentation/trace/events.rst and an
+              example in Documentation/trace/histogram.rst (Section 3.)
+
+  trace_marker_raw:
+
+	This is similar to trace_marker above, but is meant for binary data
+	to be written to it, where a tool can be used to parse the data
+	from trace_pipe_raw.
+
+  uprobe_events:
+
+	Add dynamic tracepoints in programs.
+	See uprobetracer.rst
+
+  uprobe_profile:
+
+	Uprobe statistics. See uprobetrace.txt
+
+  instances:
+
+	This is a way to make multiple trace buffers where different
+	events can be recorded in different buffers.
+	See "Instances" section below.
+
+  events:
+
+	This is the trace event directory. It holds event tracepoints
+	(also known as static tracepoints) that have been compiled
+	into the kernel. It shows what event tracepoints exist
+	and how they are grouped by system. There are "enable"
+	files at various levels that can enable the tracepoints
+	when a "1" is written to them.
+
+	See events.rst for more information.
+
+  set_event:
+
+	By echoing in the event into this file, will enable that event.
+
+	See events.rst for more information.
+
+  available_events:
+
+	A list of events that can be enabled in tracing.
+
+	See events.rst for more information.
+
+  timestamp_mode:
+
+	Certain tracers may change the timestamp mode used when
+	logging trace events into the event buffer.  Events with
+	different modes can coexist within a buffer but the mode in
+	effect when an event is logged determines which timestamp mode
+	is used for that event.  The default timestamp mode is
+	'delta'.
+
+	Usual timestamp modes for tracing:
+
+	  # cat timestamp_mode
+	  [delta] absolute
+
+	  The timestamp mode with the square brackets around it is the
+	  one in effect.
+
+	  delta: Default timestamp mode - timestamp is a delta against
+	         a per-buffer timestamp.
+
+	  absolute: The timestamp is a full timestamp, not a delta
+                 against some other value.  As such it takes up more
+                 space and is less efficient.
+
+  hwlat_detector:
+
+	Directory for the Hardware Latency Detector.
+	See "Hardware Latency Detector" section below.
+
+  per_cpu:
+
+	This is a directory that contains the trace per_cpu information.
+
+  per_cpu/cpu0/buffer_size_kb:
+
+	The ftrace buffer is defined per_cpu. That is, there's a separate
+	buffer for each CPU to allow writes to be done atomically,
+	and free from cache bouncing. These buffers may have different
+	size buffers. This file is similar to the buffer_size_kb
+	file, but it only displays or sets the buffer size for the
+	specific CPU. (here cpu0).
+
+  per_cpu/cpu0/trace:
+
+	This is similar to the "trace" file, but it will only display
+	the data specific for the CPU. If written to, it only clears
+	the specific CPU buffer.
+
+  per_cpu/cpu0/trace_pipe
+
+	This is similar to the "trace_pipe" file, and is a consuming
+	read, but it will only display (and consume) the data specific
+	for the CPU.
+
+  per_cpu/cpu0/trace_pipe_raw
+
+	For tools that can parse the ftrace ring buffer binary format,
+	the trace_pipe_raw file can be used to extract the data
+	from the ring buffer directly. With the use of the splice()
+	system call, the buffer data can be quickly transferred to
+	a file or to the network where a server is collecting the
+	data.
+
+	Like trace_pipe, this is a consuming reader, where multiple
+	reads will always produce different data.
+
+  per_cpu/cpu0/snapshot:
+
+	This is similar to the main "snapshot" file, but will only
+	snapshot the current CPU (if supported). It only displays
+	the content of the snapshot for a given CPU, and if
+	written to, only clears this CPU buffer.
+
+  per_cpu/cpu0/snapshot_raw:
+
+	Similar to the trace_pipe_raw, but will read the binary format
+	from the snapshot buffer for the given CPU.
+
+  per_cpu/cpu0/stats:
+
+	This displays certain stats about the ring buffer:
+
+	entries:
+		The number of events that are still in the buffer.
+
+	overrun:
+		The number of lost events due to overwriting when
+		the buffer was full.
+
+	commit overrun:
+		Should always be zero.
+		This gets set if so many events happened within a nested
+		event (ring buffer is re-entrant), that it fills the
+		buffer and starts dropping events.
+
+	bytes:
+		Bytes actually read (not overwritten).
+
+	oldest event ts:
+		The oldest timestamp in the buffer
+
+	now ts:
+		The current timestamp
+
+	dropped events:
+		Events lost due to overwrite option being off.
+
+	read events:
+		The number of events read.
+
+The Tracers
+-----------
+
+Here is the list of current tracers that may be configured.
+
+  "function"
+
+	Function call tracer to trace all kernel functions.
+
+  "function_graph"
+
+	Similar to the function tracer except that the
+	function tracer probes the functions on their entry
+	whereas the function graph tracer traces on both entry
+	and exit of the functions. It then provides the ability
+	to draw a graph of function calls similar to C code
+	source.
+
+  "blk"
+
+	The block tracer. The tracer used by the blktrace user
+	application.
+
+  "hwlat"
+
+	The Hardware Latency tracer is used to detect if the hardware
+	produces any latency. See "Hardware Latency Detector" section
+	below.
+
+  "irqsoff"
+
+	Traces the areas that disable interrupts and saves
+	the trace with the longest max latency.
+	See tracing_max_latency. When a new max is recorded,
+	it replaces the old trace. It is best to view this
+	trace with the latency-format option enabled, which
+	happens automatically when the tracer is selected.
+
+  "preemptoff"
+
+	Similar to irqsoff but traces and records the amount of
+	time for which preemption is disabled.
+
+  "preemptirqsoff"
+
+	Similar to irqsoff and preemptoff, but traces and
+	records the largest time for which irqs and/or preemption
+	is disabled.
+
+  "wakeup"
+
+	Traces and records the max latency that it takes for
+	the highest priority task to get scheduled after
+	it has been woken up.
+        Traces all tasks as an average developer would expect.
+
+  "wakeup_rt"
+
+        Traces and records the max latency that it takes for just
+        RT tasks (as the current "wakeup" does). This is useful
+        for those interested in wake up timings of RT tasks.
+
+  "wakeup_dl"
+
+	Traces and records the max latency that it takes for
+	a SCHED_DEADLINE task to be woken (as the "wakeup" and
+	"wakeup_rt" does).
+
+  "mmiotrace"
+
+	A special tracer that is used to trace binary module.
+	It will trace all the calls that a module makes to the
+	hardware. Everything it writes and reads from the I/O
+	as well.
+
+  "branch"
+
+	This tracer can be configured when tracing likely/unlikely
+	calls within the kernel. It will trace when a likely and
+	unlikely branch is hit and if it was correct in its prediction
+	of being correct.
+
+  "nop"
+
+	This is the "trace nothing" tracer. To remove all
+	tracers from tracing simply echo "nop" into
+	current_tracer.
+
+Error conditions
+----------------
+
+  For most ftrace commands, failure modes are obvious and communicated
+  using standard return codes.
+
+  For other more involved commands, extended error information may be
+  available via the tracing/error_log file.  For the commands that
+  support it, reading the tracing/error_log file after an error will
+  display more detailed information about what went wrong, if
+  information is available.  The tracing/error_log file is a circular
+  error log displaying a small number (currently, 8) of ftrace errors
+  for the last (8) failed commands.
+
+  The extended error information and usage takes the form shown in
+  this example::
+
+    # echo xxx > /sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger
+    echo: write error: Invalid argument
+
+    # cat /sys/kernel/debug/tracing/error_log
+    [ 5348.887237] location: error: Couldn't yyy: zzz
+      Command: xxx
+               ^
+    [ 7517.023364] location: error: Bad rrr: sss
+      Command: ppp qqq
+                   ^
+
+  To clear the error log, echo the empty string into it::
+
+    # echo > /sys/kernel/debug/tracing/error_log
+
+Examples of using the tracer
+----------------------------
+
+Here are typical examples of using the tracers when controlling
+them only with the tracefs interface (without using any
+user-land utilities).
+
+Output format:
+--------------
+
+Here is an example of the output format of the file "trace"::
+
+  # tracer: function
+  #
+  # entries-in-buffer/entries-written: 140080/250280   #P:4
+  #
+  #                              _-----=> irqs-off
+  #                             / _----=> need-resched
+  #                            | / _---=> hardirq/softirq
+  #                            || / _--=> preempt-depth
+  #                            ||| /     delay
+  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
+  #              | |       |   ||||       |         |
+              bash-1977  [000] .... 17284.993652: sys_close <-system_call_fastpath
+              bash-1977  [000] .... 17284.993653: __close_fd <-sys_close
+              bash-1977  [000] .... 17284.993653: _raw_spin_lock <-__close_fd
+              sshd-1974  [003] .... 17284.993653: __srcu_read_unlock <-fsnotify
+              bash-1977  [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock
+              bash-1977  [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd
+              bash-1977  [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock
+              bash-1977  [000] .... 17284.993657: filp_close <-__close_fd
+              bash-1977  [000] .... 17284.993657: dnotify_flush <-filp_close
+              sshd-1974  [003] .... 17284.993658: sys_select <-system_call_fastpath
+              ....
+
+A header is printed with the tracer name that is represented by
+the trace. In this case the tracer is "function". Then it shows the
+number of events in the buffer as well as the total number of entries
+that were written. The difference is the number of entries that were
+lost due to the buffer filling up (250280 - 140080 = 110200 events
+lost).
+
+The header explains the content of the events. Task name "bash", the task
+PID "1977", the CPU that it was running on "000", the latency format
+(explained below), the timestamp in <secs>.<usecs> format, the
+function name that was traced "sys_close" and the parent function that
+called this function "system_call_fastpath". The timestamp is the time
+at which the function was entered.
+
+Latency trace format
+--------------------
+
+When the latency-format option is enabled or when one of the latency
+tracers is set, the trace file gives somewhat more information to see
+why a latency happened. Here is a typical trace::
+
+  # tracer: irqsoff
+  #
+  # irqsoff latency trace v1.1.5 on 3.8.0-test+
+  # --------------------------------------------------------------------
+  # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+  #    -----------------
+  #    | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0)
+  #    -----------------
+  #  => started at: __lock_task_sighand
+  #  => ended at:   _raw_spin_unlock_irqrestore
+  #
+  #
+  #                  _------=> CPU#            
+  #                 / _-----=> irqs-off        
+  #                | / _----=> need-resched    
+  #                || / _---=> hardirq/softirq 
+  #                ||| / _--=> preempt-depth   
+  #                |||| /     delay             
+  #  cmd     pid   ||||| time  |   caller      
+  #     \   /      |||||  \    |   /           
+        ps-6143    2d...    0us!: trace_hardirqs_off <-__lock_task_sighand
+        ps-6143    2d..1  259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore
+        ps-6143    2d..1  263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore
+        ps-6143    2d..1  306us : <stack trace>
+   => trace_hardirqs_on_caller
+   => trace_hardirqs_on
+   => _raw_spin_unlock_irqrestore
+   => do_task_stat
+   => proc_tgid_stat
+   => proc_single_show
+   => seq_read
+   => vfs_read
+   => sys_read
+   => system_call_fastpath
+
+
+This shows that the current tracer is "irqsoff" tracing the time
+for which interrupts were disabled. It gives the trace version (which
+never changes) and the version of the kernel upon which this was executed on
+(3.8). Then it displays the max latency in microseconds (259 us). The number
+of trace entries displayed and the total number (both are four: #4/4).
+VP, KP, SP, and HP are always zero and are reserved for later use.
+#P is the number of online CPUs (#P:4).
+
+The task is the process that was running when the latency
+occurred. (ps pid: 6143).
+
+The start and stop (the functions in which the interrupts were
+disabled and enabled respectively) that caused the latencies:
+
+  - __lock_task_sighand is where the interrupts were disabled.
+  - _raw_spin_unlock_irqrestore is where they were enabled again.
+
+The next lines after the header are the trace itself. The header
+explains which is which.
+
+  cmd: The name of the process in the trace.
+
+  pid: The PID of that process.
+
+  CPU#: The CPU which the process was running on.
+
+  irqs-off: 'd' interrupts are disabled. '.' otherwise.
+	.. caution:: If the architecture does not support a way to
+		read the irq flags variable, an 'X' will always
+		be printed here.
+
+  need-resched:
+	- 'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set,
+	- 'n' only TIF_NEED_RESCHED is set,
+	- 'p' only PREEMPT_NEED_RESCHED is set,
+	- '.' otherwise.
+
+  hardirq/softirq:
+	- 'Z' - NMI occurred inside a hardirq
+	- 'z' - NMI is running
+	- 'H' - hard irq occurred inside a softirq.
+	- 'h' - hard irq is running
+	- 's' - soft irq is running
+	- '.' - normal context.
+
+  preempt-depth: The level of preempt_disabled
+
+The above is mostly meaningful for kernel developers.
+
+  time:
+	When the latency-format option is enabled, the trace file
+	output includes a timestamp relative to the start of the
+	trace. This differs from the output when latency-format
+	is disabled, which includes an absolute timestamp.
+
+  delay:
+	This is just to help catch your eye a bit better. And
+	needs to be fixed to be only relative to the same CPU.
+	The marks are determined by the difference between this
+	current trace and the next trace.
+
+	  - '$' - greater than 1 second
+	  - '@' - greater than 100 millisecond
+	  - '*' - greater than 10 millisecond
+	  - '#' - greater than 1000 microsecond
+	  - '!' - greater than 100 microsecond
+	  - '+' - greater than 10 microsecond
+	  - ' ' - less than or equal to 10 microsecond.
+
+  The rest is the same as the 'trace' file.
+
+  Note, the latency tracers will usually end with a back trace
+  to easily find where the latency occurred.
+
+trace_options
+-------------
+
+The trace_options file (or the options directory) is used to control
+what gets printed in the trace output, or manipulate the tracers.
+To see what is available, simply cat the file::
+
+  cat trace_options
+	print-parent
+	nosym-offset
+	nosym-addr
+	noverbose
+	noraw
+	nohex
+	nobin
+	noblock
+	trace_printk
+	annotate
+	nouserstacktrace
+	nosym-userobj
+	noprintk-msg-only
+	context-info
+	nolatency-format
+	record-cmd
+	norecord-tgid
+	overwrite
+	nodisable_on_free
+	irq-info
+	markers
+	noevent-fork
+	function-trace
+	nofunction-fork
+	nodisplay-graph
+	nostacktrace
+	nobranch
+
+To disable one of the options, echo in the option prepended with
+"no"::
+
+  echo noprint-parent > trace_options
+
+To enable an option, leave off the "no"::
+
+  echo sym-offset > trace_options
+
+Here are the available options:
+
+  print-parent
+	On function traces, display the calling (parent)
+	function as well as the function being traced.
+	::
+
+	  print-parent:
+	   bash-4000  [01]  1477.606694: simple_strtoul <-kstrtoul
+
+	  noprint-parent:
+	   bash-4000  [01]  1477.606694: simple_strtoul
+
+
+  sym-offset
+	Display not only the function name, but also the
+	offset in the function. For example, instead of
+	seeing just "ktime_get", you will see
+	"ktime_get+0xb/0x20".
+	::
+
+	  sym-offset:
+	   bash-4000  [01]  1477.606694: simple_strtoul+0x6/0xa0
+
+  sym-addr
+	This will also display the function address as well
+	as the function name.
+	::
+
+	  sym-addr:
+	   bash-4000  [01]  1477.606694: simple_strtoul <c0339346>
+
+  verbose
+	This deals with the trace file when the
+        latency-format option is enabled.
+	::
+
+	    bash  4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
+	    (+0.000ms): simple_strtoul (kstrtoul)
+
+  raw
+	This will display raw numbers. This option is best for
+	use with user applications that can translate the raw
+	numbers better than having it done in the kernel.
+
+  hex
+	Similar to raw, but the numbers will be in a hexadecimal format.
+
+  bin
+	This will print out the formats in raw binary.
+
+  block
+	When set, reading trace_pipe will not block when polled.
+
+  trace_printk
+	Can disable trace_printk() from writing into the buffer.
+
+  annotate
+	It is sometimes confusing when the CPU buffers are full
+	and one CPU buffer had a lot of events recently, thus
+	a shorter time frame, were another CPU may have only had
+	a few events, which lets it have older events. When
+	the trace is reported, it shows the oldest events first,
+	and it may look like only one CPU ran (the one with the
+	oldest events). When the annotate option is set, it will
+	display when a new CPU buffer started::
+
+			  <idle>-0     [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on
+			  <idle>-0     [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on
+			  <idle>-0     [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore
+		##### CPU 2 buffer started ####
+			  <idle>-0     [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle
+			  <idle>-0     [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog
+			  <idle>-0     [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock
+
+  userstacktrace
+	This option changes the trace. It records a
+	stacktrace of the current user space thread after
+	each trace event.
+
+  sym-userobj
+	when user stacktrace are enabled, look up which
+	object the address belongs to, and print a
+	relative address. This is especially useful when
+	ASLR is on, otherwise you don't get a chance to
+	resolve the address to object/file/line after
+	the app is no longer running
+
+	The lookup is performed when you read
+	trace,trace_pipe. Example::
+
+		  a.out-1623  [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
+		  x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
+
+
+  printk-msg-only
+	When set, trace_printk()s will only show the format
+	and not their parameters (if trace_bprintk() or
+	trace_bputs() was used to save the trace_printk()).
+
+  context-info
+	Show only the event data. Hides the comm, PID,
+	timestamp, CPU, and other useful data.
+
+  latency-format
+	This option changes the trace output. When it is enabled,
+	the trace displays additional information about the
+	latency, as described in "Latency trace format".
+
+  pause-on-trace
+	When set, opening the trace file for read, will pause
+	writing to the ring buffer (as if tracing_on was set to zero).
+	This simulates the original behavior of the trace file.
+	When the file is closed, tracing will be enabled again.
+
+  hash-ptr
+        When set, "%p" in the event printk format displays the
+        hashed pointer value instead of real address.
+        This will be useful if you want to find out which hashed
+        value is corresponding to the real value in trace log.
+
+  record-cmd
+	When any event or tracer is enabled, a hook is enabled
+	in the sched_switch trace point to fill comm cache
+	with mapped pids and comms. But this may cause some
+	overhead, and if you only care about pids, and not the
+	name of the task, disabling this option can lower the
+	impact of tracing. See "saved_cmdlines".
+
+  record-tgid
+	When any event or tracer is enabled, a hook is enabled
+	in the sched_switch trace point to fill the cache of
+	mapped Thread Group IDs (TGID) mapping to pids. See
+	"saved_tgids".
+
+  overwrite
+	This controls what happens when the trace buffer is
+	full. If "1" (default), the oldest events are
+	discarded and overwritten. If "0", then the newest
+	events are discarded.
+	(see per_cpu/cpu0/stats for overrun and dropped)
+
+  disable_on_free
+	When the free_buffer is closed, tracing will
+	stop (tracing_on set to 0).
+
+  irq-info
+	Shows the interrupt, preempt count, need resched data.
+	When disabled, the trace looks like::
+
+		# tracer: function
+		#
+		# entries-in-buffer/entries-written: 144405/9452052   #P:4
+		#
+		#           TASK-PID   CPU#      TIMESTAMP  FUNCTION
+		#              | |       |          |         |
+			  <idle>-0     [002]  23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up
+			  <idle>-0     [002]  23636.756054: activate_task <-ttwu_do_activate.constprop.89
+			  <idle>-0     [002]  23636.756055: enqueue_task <-activate_task
+
+
+  markers
+	When set, the trace_marker is writable (only by root).
+	When disabled, the trace_marker will error with EINVAL
+	on write.
+
+  event-fork
+	When set, tasks with PIDs listed in set_event_pid will have
+	the PIDs of their children added to set_event_pid when those
+	tasks fork. Also, when tasks with PIDs in set_event_pid exit,
+	their PIDs will be removed from the file.
+
+        This affects PIDs listed in set_event_notrace_pid as well.
+
+  function-trace
+	The latency tracers will enable function tracing
+	if this option is enabled (default it is). When
+	it is disabled, the latency tracers do not trace
+	functions. This keeps the overhead of the tracer down
+	when performing latency tests.
+
+  function-fork
+	When set, tasks with PIDs listed in set_ftrace_pid will
+	have the PIDs of their children added to set_ftrace_pid
+	when those tasks fork. Also, when tasks with PIDs in
+	set_ftrace_pid exit, their PIDs will be removed from the
+	file.
+
+        This affects PIDs in set_ftrace_notrace_pid as well.
+
+  display-graph
+	When set, the latency tracers (irqsoff, wakeup, etc) will
+	use function graph tracing instead of function tracing.
+
+  stacktrace
+	When set, a stack trace is recorded after any trace event
+	is recorded.
+
+  branch
+	Enable branch tracing with the tracer. This enables branch
+	tracer along with the currently set tracer. Enabling this
+	with the "nop" tracer is the same as just enabling the
+	"branch" tracer.
+
+.. tip:: Some tracers have their own options. They only appear in this
+       file when the tracer is active. They always appear in the
+       options directory.
+
+
+Here are the per tracer options:
+
+Options for function tracer:
+
+  func_stack_trace
+	When set, a stack trace is recorded after every
+	function that is recorded. NOTE! Limit the functions
+	that are recorded before enabling this, with
+	"set_ftrace_filter" otherwise the system performance
+	will be critically degraded. Remember to disable
+	this option before clearing the function filter.
+
+Options for function_graph tracer:
+
+ Since the function_graph tracer has a slightly different output
+ it has its own options to control what is displayed.
+
+  funcgraph-overrun
+	When set, the "overrun" of the graph stack is
+	displayed after each function traced. The
+	overrun, is when the stack depth of the calls
+	is greater than what is reserved for each task.
+	Each task has a fixed array of functions to
+	trace in the call graph. If the depth of the
+	calls exceeds that, the function is not traced.
+	The overrun is the number of functions missed
+	due to exceeding this array.
+
+  funcgraph-cpu
+	When set, the CPU number of the CPU where the trace
+	occurred is displayed.
+
+  funcgraph-overhead
+	When set, if the function takes longer than
+	A certain amount, then a delay marker is
+	displayed. See "delay" above, under the
+	header description.
+
+  funcgraph-proc
+	Unlike other tracers, the process' command line
+	is not displayed by default, but instead only
+	when a task is traced in and out during a context
+	switch. Enabling this options has the command
+	of each process displayed at every line.
+
+  funcgraph-duration
+	At the end of each function (the return)
+	the duration of the amount of time in the
+	function is displayed in microseconds.
+
+  funcgraph-abstime
+	When set, the timestamp is displayed at each line.
+
+  funcgraph-irqs
+	When disabled, functions that happen inside an
+	interrupt will not be traced.
+
+  funcgraph-tail
+	When set, the return event will include the function
+	that it represents. By default this is off, and
+	only a closing curly bracket "}" is displayed for
+	the return of a function.
+
+  sleep-time
+	When running function graph tracer, to include
+	the time a task schedules out in its function.
+	When enabled, it will account time the task has been
+	scheduled out as part of the function call.
+
+  graph-time
+	When running function profiler with function graph tracer,
+	to include the time to call nested functions. When this is
+	not set, the time reported for the function will only
+	include the time the function itself executed for, not the
+	time for functions that it called.
+
+Options for blk tracer:
+
+  blk_classic
+	Shows a more minimalistic output.
+
+
+irqsoff
+-------
+
+When interrupts are disabled, the CPU can not react to any other
+external event (besides NMIs and SMIs). This prevents the timer
+interrupt from triggering or the mouse interrupt from letting
+the kernel know of a new mouse event. The result is a latency
+with the reaction time.
+
+The irqsoff tracer tracks the time for which interrupts are
+disabled. When a new maximum latency is hit, the tracer saves
+the trace leading up to that latency point so that every time a
+new maximum is reached, the old saved trace is discarded and the
+new trace is saved.
+
+To reset the maximum, echo 0 into tracing_max_latency. Here is
+an example::
+
+  # echo 0 > options/function-trace
+  # echo irqsoff > current_tracer
+  # echo 1 > tracing_on
+  # echo 0 > tracing_max_latency
+  # ls -ltr
+  [...]
+  # echo 0 > tracing_on
+  # cat trace
+  # tracer: irqsoff
+  #
+  # irqsoff latency trace v1.1.5 on 3.8.0-test+
+  # --------------------------------------------------------------------
+  # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+  #    -----------------
+  #    | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0)
+  #    -----------------
+  #  => started at: run_timer_softirq
+  #  => ended at:   run_timer_softirq
+  #
+  #
+  #                  _------=> CPU#            
+  #                 / _-----=> irqs-off        
+  #                | / _----=> need-resched    
+  #                || / _---=> hardirq/softirq 
+  #                ||| / _--=> preempt-depth   
+  #                |||| /     delay             
+  #  cmd     pid   ||||| time  |   caller      
+  #     \   /      |||||  \    |   /           
+    <idle>-0       0d.s2    0us+: _raw_spin_lock_irq <-run_timer_softirq
+    <idle>-0       0dNs3   17us : _raw_spin_unlock_irq <-run_timer_softirq
+    <idle>-0       0dNs3   17us+: trace_hardirqs_on <-run_timer_softirq
+    <idle>-0       0dNs3   25us : <stack trace>
+   => _raw_spin_unlock_irq
+   => run_timer_softirq
+   => __do_softirq
+   => call_softirq
+   => do_softirq
+   => irq_exit
+   => smp_apic_timer_interrupt
+   => apic_timer_interrupt
+   => rcu_idle_exit
+   => cpu_idle
+   => rest_init
+   => start_kernel
+   => x86_64_start_reservations
+   => x86_64_start_kernel
+
+Here we see that we had a latency of 16 microseconds (which is
+very good). The _raw_spin_lock_irq in run_timer_softirq disabled
+interrupts. The difference between the 16 and the displayed
+timestamp 25us occurred because the clock was incremented
+between the time of recording the max latency and the time of
+recording the function that had that latency.
+
+Note the above example had function-trace not set. If we set
+function-trace, we get a much larger output::
+
+ with echo 1 > options/function-trace
+
+  # tracer: irqsoff
+  #
+  # irqsoff latency trace v1.1.5 on 3.8.0-test+
+  # --------------------------------------------------------------------
+  # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+  #    -----------------
+  #    | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0)
+  #    -----------------
+  #  => started at: ata_scsi_queuecmd
+  #  => ended at:   ata_scsi_queuecmd
+  #
+  #
+  #                  _------=> CPU#            
+  #                 / _-----=> irqs-off        
+  #                | / _----=> need-resched    
+  #                || / _---=> hardirq/softirq 
+  #                ||| / _--=> preempt-depth   
+  #                |||| /     delay             
+  #  cmd     pid   ||||| time  |   caller      
+  #     \   /      |||||  \    |   /           
+      bash-2042    3d...    0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd
+      bash-2042    3d...    0us : add_preempt_count <-_raw_spin_lock_irqsave
+      bash-2042    3d..1    1us : ata_scsi_find_dev <-ata_scsi_queuecmd
+      bash-2042    3d..1    1us : __ata_scsi_find_dev <-ata_scsi_find_dev
+      bash-2042    3d..1    2us : ata_find_dev.part.14 <-__ata_scsi_find_dev
+      bash-2042    3d..1    2us : ata_qc_new_init <-__ata_scsi_queuecmd
+      bash-2042    3d..1    3us : ata_sg_init <-__ata_scsi_queuecmd
+      bash-2042    3d..1    4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd
+      bash-2042    3d..1    4us : ata_build_rw_tf <-ata_scsi_rw_xlat
+  [...]
+      bash-2042    3d..1   67us : delay_tsc <-__delay
+      bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
+      bash-2042    3d..2   67us : sub_preempt_count <-delay_tsc
+      bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
+      bash-2042    3d..2   68us : sub_preempt_count <-delay_tsc
+      bash-2042    3d..1   68us+: ata_bmdma_start <-ata_bmdma_qc_issue
+      bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
+      bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
+      bash-2042    3d..1   72us+: trace_hardirqs_on <-ata_scsi_queuecmd
+      bash-2042    3d..1  120us : <stack trace>
+   => _raw_spin_unlock_irqrestore
+   => ata_scsi_queuecmd
+   => scsi_dispatch_cmd
+   => scsi_request_fn
+   => __blk_run_queue_uncond
+   => __blk_run_queue
+   => blk_queue_bio
+   => submit_bio_noacct
+   => submit_bio
+   => submit_bh
+   => __ext3_get_inode_loc
+   => ext3_iget
+   => ext3_lookup
+   => lookup_real
+   => __lookup_hash
+   => walk_component
+   => lookup_last
+   => path_lookupat
+   => filename_lookup
+   => user_path_at_empty
+   => user_path_at
+   => vfs_fstatat
+   => vfs_stat
+   => sys_newstat
+   => system_call_fastpath
+
+
+Here we traced a 71 microsecond latency. But we also see all the
+functions that were called during that time. Note that by
+enabling function tracing, we incur an added overhead. This
+overhead may extend the latency times. But nevertheless, this
+trace has provided some very helpful debugging information.
+
+If we prefer function graph output instead of function, we can set
+display-graph option::
+
+ with echo 1 > options/display-graph
+
+  # tracer: irqsoff
+  #
+  # irqsoff latency trace v1.1.5 on 4.20.0-rc6+
+  # --------------------------------------------------------------------
+  # latency: 3751 us, #274/274, CPU#0 | (M:desktop VP:0, KP:0, SP:0 HP:0 #P:4)
+  #    -----------------
+  #    | task: bash-1507 (uid:0 nice:0 policy:0 rt_prio:0)
+  #    -----------------
+  #  => started at: free_debug_processing
+  #  => ended at:   return_to_handler
+  #
+  #
+  #                                       _-----=> irqs-off
+  #                                      / _----=> need-resched
+  #                                     | / _---=> hardirq/softirq
+  #                                     || / _--=> preempt-depth
+  #                                     ||| /
+  #   REL TIME      CPU  TASK/PID       ||||     DURATION                  FUNCTION CALLS
+  #      |          |     |    |        ||||      |   |                     |   |   |   |
+          0 us |   0)   bash-1507    |  d... |   0.000 us    |  _raw_spin_lock_irqsave();
+          0 us |   0)   bash-1507    |  d..1 |   0.378 us    |    do_raw_spin_trylock();
+          1 us |   0)   bash-1507    |  d..2 |               |    set_track() {
+          2 us |   0)   bash-1507    |  d..2 |               |      save_stack_trace() {
+          2 us |   0)   bash-1507    |  d..2 |               |        __save_stack_trace() {
+          3 us |   0)   bash-1507    |  d..2 |               |          __unwind_start() {
+          3 us |   0)   bash-1507    |  d..2 |               |            get_stack_info() {
+          3 us |   0)   bash-1507    |  d..2 |   0.351 us    |              in_task_stack();
+          4 us |   0)   bash-1507    |  d..2 |   1.107 us    |            }
+  [...]
+       3750 us |   0)   bash-1507    |  d..1 |   0.516 us    |      do_raw_spin_unlock();
+       3750 us |   0)   bash-1507    |  d..1 |   0.000 us    |  _raw_spin_unlock_irqrestore();
+       3764 us |   0)   bash-1507    |  d..1 |   0.000 us    |  tracer_hardirqs_on();
+      bash-1507    0d..1 3792us : <stack trace>
+   => free_debug_processing
+   => __slab_free
+   => kmem_cache_free
+   => vm_area_free
+   => remove_vma
+   => exit_mmap
+   => mmput
+   => begin_new_exec
+   => load_elf_binary
+   => search_binary_handler
+   => __do_execve_file.isra.32
+   => __x64_sys_execve
+   => do_syscall_64
+   => entry_SYSCALL_64_after_hwframe
+
+preemptoff
+----------
+
+When preemption is disabled, we may be able to receive
+interrupts but the task cannot be preempted and a higher
+priority task must wait for preemption to be enabled again
+before it can preempt a lower priority task.
+
+The preemptoff tracer traces the places that disable preemption.
+Like the irqsoff tracer, it records the maximum latency for
+which preemption was disabled. The control of preemptoff tracer
+is much like the irqsoff tracer.
+::
+
+  # echo 0 > options/function-trace
+  # echo preemptoff > current_tracer
+  # echo 1 > tracing_on
+  # echo 0 > tracing_max_latency
+  # ls -ltr
+  [...]
+  # echo 0 > tracing_on
+  # cat trace
+  # tracer: preemptoff
+  #
+  # preemptoff latency trace v1.1.5 on 3.8.0-test+
+  # --------------------------------------------------------------------
+  # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+  #    -----------------
+  #    | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0)
+  #    -----------------
+  #  => started at: do_IRQ
+  #  => ended at:   do_IRQ
+  #
+  #
+  #                  _------=> CPU#            
+  #                 / _-----=> irqs-off        
+  #                | / _----=> need-resched    
+  #                || / _---=> hardirq/softirq 
+  #                ||| / _--=> preempt-depth   
+  #                |||| /     delay             
+  #  cmd     pid   ||||| time  |   caller      
+  #     \   /      |||||  \    |   /           
+      sshd-1991    1d.h.    0us+: irq_enter <-do_IRQ
+      sshd-1991    1d..1   46us : irq_exit <-do_IRQ
+      sshd-1991    1d..1   47us+: trace_preempt_on <-do_IRQ
+      sshd-1991    1d..1   52us : <stack trace>
+   => sub_preempt_count
+   => irq_exit
+   => do_IRQ
+   => ret_from_intr
+
+
+This has some more changes. Preemption was disabled when an
+interrupt came in (notice the 'h'), and was enabled on exit.
+But we also see that interrupts have been disabled when entering
+the preempt off section and leaving it (the 'd'). We do not know if
+interrupts were enabled in the mean time or shortly after this
+was over.
+::
+
+  # tracer: preemptoff
+  #
+  # preemptoff latency trace v1.1.5 on 3.8.0-test+
+  # --------------------------------------------------------------------
+  # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+  #    -----------------
+  #    | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0)
+  #    -----------------
+  #  => started at: wake_up_new_task
+  #  => ended at:   task_rq_unlock
+  #
+  #
+  #                  _------=> CPU#            
+  #                 / _-----=> irqs-off        
+  #                | / _----=> need-resched    
+  #                || / _---=> hardirq/softirq 
+  #                ||| / _--=> preempt-depth   
+  #                |||| /     delay             
+  #  cmd     pid   ||||| time  |   caller      
+  #     \   /      |||||  \    |   /           
+      bash-1994    1d..1    0us : _raw_spin_lock_irqsave <-wake_up_new_task
+      bash-1994    1d..1    0us : select_task_rq_fair <-select_task_rq
+      bash-1994    1d..1    1us : __rcu_read_lock <-select_task_rq_fair
+      bash-1994    1d..1    1us : source_load <-select_task_rq_fair
+      bash-1994    1d..1    1us : source_load <-select_task_rq_fair
+  [...]
+      bash-1994    1d..1   12us : irq_enter <-smp_apic_timer_interrupt
+      bash-1994    1d..1   12us : rcu_irq_enter <-irq_enter
+      bash-1994    1d..1   13us : add_preempt_count <-irq_enter
+      bash-1994    1d.h1   13us : exit_idle <-smp_apic_timer_interrupt
+      bash-1994    1d.h1   13us : hrtimer_interrupt <-smp_apic_timer_interrupt
+      bash-1994    1d.h1   13us : _raw_spin_lock <-hrtimer_interrupt
+      bash-1994    1d.h1   14us : add_preempt_count <-_raw_spin_lock
+      bash-1994    1d.h2   14us : ktime_get_update_offsets <-hrtimer_interrupt
+  [...]
+      bash-1994    1d.h1   35us : lapic_next_event <-clockevents_program_event
+      bash-1994    1d.h1   35us : irq_exit <-smp_apic_timer_interrupt
+      bash-1994    1d.h1   36us : sub_preempt_count <-irq_exit
+      bash-1994    1d..2   36us : do_softirq <-irq_exit
+      bash-1994    1d..2   36us : __do_softirq <-call_softirq
+      bash-1994    1d..2   36us : __local_bh_disable <-__do_softirq
+      bash-1994    1d.s2   37us : add_preempt_count <-_raw_spin_lock_irq
+      bash-1994    1d.s3   38us : _raw_spin_unlock <-run_timer_softirq
+      bash-1994    1d.s3   39us : sub_preempt_count <-_raw_spin_unlock
+      bash-1994    1d.s2   39us : call_timer_fn <-run_timer_softirq
+  [...]
+      bash-1994    1dNs2   81us : cpu_needs_another_gp <-rcu_process_callbacks
+      bash-1994    1dNs2   82us : __local_bh_enable <-__do_softirq
+      bash-1994    1dNs2   82us : sub_preempt_count <-__local_bh_enable
+      bash-1994    1dN.2   82us : idle_cpu <-irq_exit
+      bash-1994    1dN.2   83us : rcu_irq_exit <-irq_exit
+      bash-1994    1dN.2   83us : sub_preempt_count <-irq_exit
+      bash-1994    1.N.1   84us : _raw_spin_unlock_irqrestore <-task_rq_unlock
+      bash-1994    1.N.1   84us+: trace_preempt_on <-task_rq_unlock
+      bash-1994    1.N.1  104us : <stack trace>
+   => sub_preempt_count
+   => _raw_spin_unlock_irqrestore
+   => task_rq_unlock
+   => wake_up_new_task
+   => do_fork
+   => sys_clone
+   => stub_clone
+
+
+The above is an example of the preemptoff trace with
+function-trace set. Here we see that interrupts were not disabled
+the entire time. The irq_enter code lets us know that we entered
+an interrupt 'h'. Before that, the functions being traced still
+show that it is not in an interrupt, but we can see from the
+functions themselves that this is not the case.
+
+preemptirqsoff
+--------------
+
+Knowing the locations that have interrupts disabled or
+preemption disabled for the longest times is helpful. But
+sometimes we would like to know when either preemption and/or
+interrupts are disabled.
+
+Consider the following code::
+
+    local_irq_disable();
+    call_function_with_irqs_off();
+    preempt_disable();
+    call_function_with_irqs_and_preemption_off();
+    local_irq_enable();
+    call_function_with_preemption_off();
+    preempt_enable();
+
+The irqsoff tracer will record the total length of
+call_function_with_irqs_off() and
+call_function_with_irqs_and_preemption_off().
+
+The preemptoff tracer will record the total length of
+call_function_with_irqs_and_preemption_off() and
+call_function_with_preemption_off().
+
+But neither will trace the time that interrupts and/or
+preemption is disabled. This total time is the time that we can
+not schedule. To record this time, use the preemptirqsoff
+tracer.
+
+Again, using this trace is much like the irqsoff and preemptoff
+tracers.
+::
+
+  # echo 0 > options/function-trace
+  # echo preemptirqsoff > current_tracer
+  # echo 1 > tracing_on
+  # echo 0 > tracing_max_latency
+  # ls -ltr
+  [...]
+  # echo 0 > tracing_on
+  # cat trace
+  # tracer: preemptirqsoff
+  #
+  # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
+  # --------------------------------------------------------------------
+  # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+  #    -----------------
+  #    | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0)
+  #    -----------------
+  #  => started at: ata_scsi_queuecmd
+  #  => ended at:   ata_scsi_queuecmd
+  #
+  #
+  #                  _------=> CPU#            
+  #                 / _-----=> irqs-off        
+  #                | / _----=> need-resched    
+  #                || / _---=> hardirq/softirq 
+  #                ||| / _--=> preempt-depth   
+  #                |||| /     delay             
+  #  cmd     pid   ||||| time  |   caller      
+  #     \   /      |||||  \    |   /           
+        ls-2230    3d...    0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd
+        ls-2230    3...1  100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
+        ls-2230    3...1  101us+: trace_preempt_on <-ata_scsi_queuecmd
+        ls-2230    3...1  111us : <stack trace>
+   => sub_preempt_count
+   => _raw_spin_unlock_irqrestore
+   => ata_scsi_queuecmd
+   => scsi_dispatch_cmd
+   => scsi_request_fn
+   => __blk_run_queue_uncond
+   => __blk_run_queue
+   => blk_queue_bio
+   => submit_bio_noacct
+   => submit_bio
+   => submit_bh
+   => ext3_bread
+   => ext3_dir_bread
+   => htree_dirblock_to_tree
+   => ext3_htree_fill_tree
+   => ext3_readdir
+   => vfs_readdir
+   => sys_getdents
+   => system_call_fastpath
+
+
+The trace_hardirqs_off_thunk is called from assembly on x86 when
+interrupts are disabled in the assembly code. Without the
+function tracing, we do not know if interrupts were enabled
+within the preemption points. We do see that it started with
+preemption enabled.
+
+Here is a trace with function-trace set::
+
+  # tracer: preemptirqsoff
+  #
+  # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
+  # --------------------------------------------------------------------
+  # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+  #    -----------------
+  #    | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0)
+  #    -----------------
+  #  => started at: schedule
+  #  => ended at:   mutex_unlock
+  #
+  #
+  #                  _------=> CPU#            
+  #                 / _-----=> irqs-off        
+  #                | / _----=> need-resched    
+  #                || / _---=> hardirq/softirq 
+  #                ||| / _--=> preempt-depth   
+  #                |||| /     delay             
+  #  cmd     pid   ||||| time  |   caller      
+  #     \   /      |||||  \    |   /           
+  kworker/-59      3...1    0us : __schedule <-schedule
+  kworker/-59      3d..1    0us : rcu_preempt_qs <-rcu_note_context_switch
+  kworker/-59      3d..1    1us : add_preempt_count <-_raw_spin_lock_irq
+  kworker/-59      3d..2    1us : deactivate_task <-__schedule
+  kworker/-59      3d..2    1us : dequeue_task <-deactivate_task
+  kworker/-59      3d..2    2us : update_rq_clock <-dequeue_task
+  kworker/-59      3d..2    2us : dequeue_task_fair <-dequeue_task
+  kworker/-59      3d..2    2us : update_curr <-dequeue_task_fair
+  kworker/-59      3d..2    2us : update_min_vruntime <-update_curr
+  kworker/-59      3d..2    3us : cpuacct_charge <-update_curr
+  kworker/-59      3d..2    3us : __rcu_read_lock <-cpuacct_charge
+  kworker/-59      3d..2    3us : __rcu_read_unlock <-cpuacct_charge
+  kworker/-59      3d..2    3us : update_cfs_rq_blocked_load <-dequeue_task_fair
+  kworker/-59      3d..2    4us : clear_buddies <-dequeue_task_fair
+  kworker/-59      3d..2    4us : account_entity_dequeue <-dequeue_task_fair
+  kworker/-59      3d..2    4us : update_min_vruntime <-dequeue_task_fair
+  kworker/-59      3d..2    4us : update_cfs_shares <-dequeue_task_fair
+  kworker/-59      3d..2    5us : hrtick_update <-dequeue_task_fair
+  kworker/-59      3d..2    5us : wq_worker_sleeping <-__schedule
+  kworker/-59      3d..2    5us : kthread_data <-wq_worker_sleeping
+  kworker/-59      3d..2    5us : put_prev_task_fair <-__schedule
+  kworker/-59      3d..2    6us : pick_next_task_fair <-pick_next_task
+  kworker/-59      3d..2    6us : clear_buddies <-pick_next_task_fair
+  kworker/-59      3d..2    6us : set_next_entity <-pick_next_task_fair
+  kworker/-59      3d..2    6us : update_stats_wait_end <-set_next_entity
+        ls-2269    3d..2    7us : finish_task_switch <-__schedule
+        ls-2269    3d..2    7us : _raw_spin_unlock_irq <-finish_task_switch
+        ls-2269    3d..2    8us : do_IRQ <-ret_from_intr
+        ls-2269    3d..2    8us : irq_enter <-do_IRQ
+        ls-2269    3d..2    8us : rcu_irq_enter <-irq_enter
+        ls-2269    3d..2    9us : add_preempt_count <-irq_enter
+        ls-2269    3d.h2    9us : exit_idle <-do_IRQ
+  [...]
+        ls-2269    3d.h3   20us : sub_preempt_count <-_raw_spin_unlock
+        ls-2269    3d.h2   20us : irq_exit <-do_IRQ
+        ls-2269    3d.h2   21us : sub_preempt_count <-irq_exit
+        ls-2269    3d..3   21us : do_softirq <-irq_exit
+        ls-2269    3d..3   21us : __do_softirq <-call_softirq
+        ls-2269    3d..3   21us+: __local_bh_disable <-__do_softirq
+        ls-2269    3d.s4   29us : sub_preempt_count <-_local_bh_enable_ip
+        ls-2269    3d.s5   29us : sub_preempt_count <-_local_bh_enable_ip
+        ls-2269    3d.s5   31us : do_IRQ <-ret_from_intr
+        ls-2269    3d.s5   31us : irq_enter <-do_IRQ
+        ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
+  [...]
+        ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
+        ls-2269    3d.s5   32us : add_preempt_count <-irq_enter
+        ls-2269    3d.H5   32us : exit_idle <-do_IRQ
+        ls-2269    3d.H5   32us : handle_irq <-do_IRQ
+        ls-2269    3d.H5   32us : irq_to_desc <-handle_irq
+        ls-2269    3d.H5   33us : handle_fasteoi_irq <-handle_irq
+  [...]
+        ls-2269    3d.s5  158us : _raw_spin_unlock_irqrestore <-rtl8139_poll
+        ls-2269    3d.s3  158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action
+        ls-2269    3d.s3  159us : __local_bh_enable <-__do_softirq
+        ls-2269    3d.s3  159us : sub_preempt_count <-__local_bh_enable
+        ls-2269    3d..3  159us : idle_cpu <-irq_exit
+        ls-2269    3d..3  159us : rcu_irq_exit <-irq_exit
+        ls-2269    3d..3  160us : sub_preempt_count <-irq_exit
+        ls-2269    3d...  161us : __mutex_unlock_slowpath <-mutex_unlock
+        ls-2269    3d...  162us+: trace_hardirqs_on <-mutex_unlock
+        ls-2269    3d...  186us : <stack trace>
+   => __mutex_unlock_slowpath
+   => mutex_unlock
+   => process_output
+   => n_tty_write
+   => tty_write
+   => vfs_write
+   => sys_write
+   => system_call_fastpath
+
+This is an interesting trace. It started with kworker running and
+scheduling out and ls taking over. But as soon as ls released the
+rq lock and enabled interrupts (but not preemption) an interrupt
+triggered. When the interrupt finished, it started running softirqs.
+But while the softirq was running, another interrupt triggered.
+When an interrupt is running inside a softirq, the annotation is 'H'.
+
+
+wakeup
+------
+
+One common case that people are interested in tracing is the
+time it takes for a task that is woken to actually wake up.
+Now for non Real-Time tasks, this can be arbitrary. But tracing
+it none the less can be interesting. 
+
+Without function tracing::
+
+  # echo 0 > options/function-trace
+  # echo wakeup > current_tracer
+  # echo 1 > tracing_on
+  # echo 0 > tracing_max_latency
+  # chrt -f 5 sleep 1
+  # echo 0 > tracing_on
+  # cat trace
+  # tracer: wakeup
+  #
+  # wakeup latency trace v1.1.5 on 3.8.0-test+
+  # --------------------------------------------------------------------
+  # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+  #    -----------------
+  #    | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0)
+  #    -----------------
+  #
+  #                  _------=> CPU#            
+  #                 / _-----=> irqs-off        
+  #                | / _----=> need-resched    
+  #                || / _---=> hardirq/softirq 
+  #                ||| / _--=> preempt-depth   
+  #                |||| /     delay             
+  #  cmd     pid   ||||| time  |   caller      
+  #     \   /      |||||  \    |   /           
+    <idle>-0       3dNs7    0us :      0:120:R   + [003]   312:100:R kworker/3:1H
+    <idle>-0       3dNs7    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
+    <idle>-0       3d..3   15us : __schedule <-schedule
+    <idle>-0       3d..3   15us :      0:120:R ==> [003]   312:100:R kworker/3:1H
+
+The tracer only traces the highest priority task in the system
+to avoid tracing the normal circumstances. Here we see that
+the kworker with a nice priority of -20 (not very nice), took
+just 15 microseconds from the time it woke up, to the time it
+ran.
+
+Non Real-Time tasks are not that interesting. A more interesting
+trace is to concentrate only on Real-Time tasks.
+
+wakeup_rt
+---------
+
+In a Real-Time environment it is very important to know the
+wakeup time it takes for the highest priority task that is woken
+up to the time that it executes. This is also known as "schedule
+latency". I stress the point that this is about RT tasks. It is
+also important to know the scheduling latency of non-RT tasks,
+but the average schedule latency is better for non-RT tasks.
+Tools like LatencyTop are more appropriate for such
+measurements.
+
+Real-Time environments are interested in the worst case latency.
+That is the longest latency it takes for something to happen,
+and not the average. We can have a very fast scheduler that may
+only have a large latency once in a while, but that would not
+work well with Real-Time tasks.  The wakeup_rt tracer was designed
+to record the worst case wakeups of RT tasks. Non-RT tasks are
+not recorded because the tracer only records one worst case and
+tracing non-RT tasks that are unpredictable will overwrite the
+worst case latency of RT tasks (just run the normal wakeup
+tracer for a while to see that effect).
+
+Since this tracer only deals with RT tasks, we will run this
+slightly differently than we did with the previous tracers.
+Instead of performing an 'ls', we will run 'sleep 1' under
+'chrt' which changes the priority of the task.
+::
+
+  # echo 0 > options/function-trace
+  # echo wakeup_rt > current_tracer
+  # echo 1 > tracing_on
+  # echo 0 > tracing_max_latency
+  # chrt -f 5 sleep 1
+  # echo 0 > tracing_on
+  # cat trace
+  # tracer: wakeup
+  #
+  # tracer: wakeup_rt
+  #
+  # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
+  # --------------------------------------------------------------------
+  # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+  #    -----------------
+  #    | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5)
+  #    -----------------
+  #
+  #                  _------=> CPU#            
+  #                 / _-----=> irqs-off        
+  #                | / _----=> need-resched    
+  #                || / _---=> hardirq/softirq 
+  #                ||| / _--=> preempt-depth   
+  #                |||| /     delay             
+  #  cmd     pid   ||||| time  |   caller      
+  #     \   /      |||||  \    |   /           
+    <idle>-0       3d.h4    0us :      0:120:R   + [003]  2389: 94:R sleep
+    <idle>-0       3d.h4    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
+    <idle>-0       3d..3    5us : __schedule <-schedule
+    <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep
+
+
+Running this on an idle system, we see that it only took 5 microseconds
+to perform the task switch.  Note, since the trace point in the schedule
+is before the actual "switch", we stop the tracing when the recorded task
+is about to schedule in. This may change if we add a new marker at the
+end of the scheduler.
+
+Notice that the recorded task is 'sleep' with the PID of 2389
+and it has an rt_prio of 5. This priority is user-space priority
+and not the internal kernel priority. The policy is 1 for
+SCHED_FIFO and 2 for SCHED_RR.
+
+Note, that the trace data shows the internal priority (99 - rtprio).
+::
+
+  <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep
+
+The 0:120:R means idle was running with a nice priority of 0 (120 - 120)
+and in the running state 'R'. The sleep task was scheduled in with
+2389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94)
+and it too is in the running state.
+
+Doing the same with chrt -r 5 and function-trace set.
+::
+
+  echo 1 > options/function-trace
+
+  # tracer: wakeup_rt
+  #
+  # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
+  # --------------------------------------------------------------------
+  # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+  #    -----------------
+  #    | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5)
+  #    -----------------
+  #
+  #                  _------=> CPU#            
+  #                 / _-----=> irqs-off        
+  #                | / _----=> need-resched    
+  #                || / _---=> hardirq/softirq 
+  #                ||| / _--=> preempt-depth   
+  #                |||| /     delay             
+  #  cmd     pid   ||||| time  |   caller      
+  #     \   /      |||||  \    |   /           
+    <idle>-0       3d.h4    1us+:      0:120:R   + [003]  2448: 94:R sleep
+    <idle>-0       3d.h4    2us : ttwu_do_activate.constprop.87 <-try_to_wake_up
+    <idle>-0       3d.h3    3us : check_preempt_curr <-ttwu_do_wakeup
+    <idle>-0       3d.h3    3us : resched_curr <-check_preempt_curr
+    <idle>-0       3dNh3    4us : task_woken_rt <-ttwu_do_wakeup
+    <idle>-0       3dNh3    4us : _raw_spin_unlock <-try_to_wake_up
+    <idle>-0       3dNh3    4us : sub_preempt_count <-_raw_spin_unlock
+    <idle>-0       3dNh2    5us : ttwu_stat <-try_to_wake_up
+    <idle>-0       3dNh2    5us : _raw_spin_unlock_irqrestore <-try_to_wake_up
+    <idle>-0       3dNh2    6us : sub_preempt_count <-_raw_spin_unlock_irqrestore
+    <idle>-0       3dNh1    6us : _raw_spin_lock <-__run_hrtimer
+    <idle>-0       3dNh1    6us : add_preempt_count <-_raw_spin_lock
+    <idle>-0       3dNh2    7us : _raw_spin_unlock <-hrtimer_interrupt
+    <idle>-0       3dNh2    7us : sub_preempt_count <-_raw_spin_unlock
+    <idle>-0       3dNh1    7us : tick_program_event <-hrtimer_interrupt
+    <idle>-0       3dNh1    7us : clockevents_program_event <-tick_program_event
+    <idle>-0       3dNh1    8us : ktime_get <-clockevents_program_event
+    <idle>-0       3dNh1    8us : lapic_next_event <-clockevents_program_event
+    <idle>-0       3dNh1    8us : irq_exit <-smp_apic_timer_interrupt
+    <idle>-0       3dNh1    9us : sub_preempt_count <-irq_exit
+    <idle>-0       3dN.2    9us : idle_cpu <-irq_exit
+    <idle>-0       3dN.2    9us : rcu_irq_exit <-irq_exit
+    <idle>-0       3dN.2   10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit
+    <idle>-0       3dN.2   10us : sub_preempt_count <-irq_exit
+    <idle>-0       3.N.1   11us : rcu_idle_exit <-cpu_idle
+    <idle>-0       3dN.1   11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit
+    <idle>-0       3.N.1   11us : tick_nohz_idle_exit <-cpu_idle
+    <idle>-0       3dN.1   12us : menu_hrtimer_cancel <-tick_nohz_idle_exit
+    <idle>-0       3dN.1   12us : ktime_get <-tick_nohz_idle_exit
+    <idle>-0       3dN.1   12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit
+    <idle>-0       3dN.1   13us : cpu_load_update_nohz <-tick_nohz_idle_exit
+    <idle>-0       3dN.1   13us : _raw_spin_lock <-cpu_load_update_nohz
+    <idle>-0       3dN.1   13us : add_preempt_count <-_raw_spin_lock
+    <idle>-0       3dN.2   13us : __cpu_load_update <-cpu_load_update_nohz
+    <idle>-0       3dN.2   14us : sched_avg_update <-__cpu_load_update
+    <idle>-0       3dN.2   14us : _raw_spin_unlock <-cpu_load_update_nohz
+    <idle>-0       3dN.2   14us : sub_preempt_count <-_raw_spin_unlock
+    <idle>-0       3dN.1   15us : calc_load_nohz_stop <-tick_nohz_idle_exit
+    <idle>-0       3dN.1   15us : touch_softlockup_watchdog <-tick_nohz_idle_exit
+    <idle>-0       3dN.1   15us : hrtimer_cancel <-tick_nohz_idle_exit
+    <idle>-0       3dN.1   15us : hrtimer_try_to_cancel <-hrtimer_cancel
+    <idle>-0       3dN.1   16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel
+    <idle>-0       3dN.1   16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
+    <idle>-0       3dN.1   16us : add_preempt_count <-_raw_spin_lock_irqsave
+    <idle>-0       3dN.2   17us : __remove_hrtimer <-remove_hrtimer.part.16
+    <idle>-0       3dN.2   17us : hrtimer_force_reprogram <-__remove_hrtimer
+    <idle>-0       3dN.2   17us : tick_program_event <-hrtimer_force_reprogram
+    <idle>-0       3dN.2   18us : clockevents_program_event <-tick_program_event
+    <idle>-0       3dN.2   18us : ktime_get <-clockevents_program_event
+    <idle>-0       3dN.2   18us : lapic_next_event <-clockevents_program_event
+    <idle>-0       3dN.2   19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
+    <idle>-0       3dN.2   19us : sub_preempt_count <-_raw_spin_unlock_irqrestore
+    <idle>-0       3dN.1   19us : hrtimer_forward <-tick_nohz_idle_exit
+    <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
+    <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
+    <idle>-0       3dN.1   20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
+    <idle>-0       3dN.1   20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns
+    <idle>-0       3dN.1   21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns
+    <idle>-0       3dN.1   21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
+    <idle>-0       3dN.1   21us : add_preempt_count <-_raw_spin_lock_irqsave
+    <idle>-0       3dN.2   22us : ktime_add_safe <-__hrtimer_start_range_ns
+    <idle>-0       3dN.2   22us : enqueue_hrtimer <-__hrtimer_start_range_ns
+    <idle>-0       3dN.2   22us : tick_program_event <-__hrtimer_start_range_ns
+    <idle>-0       3dN.2   23us : clockevents_program_event <-tick_program_event
+    <idle>-0       3dN.2   23us : ktime_get <-clockevents_program_event
+    <idle>-0       3dN.2   23us : lapic_next_event <-clockevents_program_event
+    <idle>-0       3dN.2   24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns
+    <idle>-0       3dN.2   24us : sub_preempt_count <-_raw_spin_unlock_irqrestore
+    <idle>-0       3dN.1   24us : account_idle_ticks <-tick_nohz_idle_exit
+    <idle>-0       3dN.1   24us : account_idle_time <-account_idle_ticks
+    <idle>-0       3.N.1   25us : sub_preempt_count <-cpu_idle
+    <idle>-0       3.N..   25us : schedule <-cpu_idle
+    <idle>-0       3.N..   25us : __schedule <-preempt_schedule
+    <idle>-0       3.N..   26us : add_preempt_count <-__schedule
+    <idle>-0       3.N.1   26us : rcu_note_context_switch <-__schedule
+    <idle>-0       3.N.1   26us : rcu_sched_qs <-rcu_note_context_switch
+    <idle>-0       3dN.1   27us : rcu_preempt_qs <-rcu_note_context_switch
+    <idle>-0       3.N.1   27us : _raw_spin_lock_irq <-__schedule
+    <idle>-0       3dN.1   27us : add_preempt_count <-_raw_spin_lock_irq
+    <idle>-0       3dN.2   28us : put_prev_task_idle <-__schedule
+    <idle>-0       3dN.2   28us : pick_next_task_stop <-pick_next_task
+    <idle>-0       3dN.2   28us : pick_next_task_rt <-pick_next_task
+    <idle>-0       3dN.2   29us : dequeue_pushable_task <-pick_next_task_rt
+    <idle>-0       3d..3   29us : __schedule <-preempt_schedule
+    <idle>-0       3d..3   30us :      0:120:R ==> [003]  2448: 94:R sleep
+
+This isn't that big of a trace, even with function tracing enabled,
+so I included the entire trace.
+
+The interrupt went off while when the system was idle. Somewhere
+before task_woken_rt() was called, the NEED_RESCHED flag was set,
+this is indicated by the first occurrence of the 'N' flag.
+
+Latency tracing and events
+--------------------------
+As function tracing can induce a much larger latency, but without
+seeing what happens within the latency it is hard to know what
+caused it. There is a middle ground, and that is with enabling
+events.
+::
+
+  # echo 0 > options/function-trace
+  # echo wakeup_rt > current_tracer
+  # echo 1 > events/enable
+  # echo 1 > tracing_on
+  # echo 0 > tracing_max_latency
+  # chrt -f 5 sleep 1
+  # echo 0 > tracing_on
+  # cat trace
+  # tracer: wakeup_rt
+  #
+  # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
+  # --------------------------------------------------------------------
+  # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
+  #    -----------------
+  #    | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5)
+  #    -----------------
+  #
+  #                  _------=> CPU#            
+  #                 / _-----=> irqs-off        
+  #                | / _----=> need-resched    
+  #                || / _---=> hardirq/softirq 
+  #                ||| / _--=> preempt-depth   
+  #                |||| /     delay             
+  #  cmd     pid   ||||| time  |   caller      
+  #     \   /      |||||  \    |   /           
+    <idle>-0       2d.h4    0us :      0:120:R   + [002]  5882: 94:R sleep
+    <idle>-0       2d.h4    0us : ttwu_do_activate.constprop.87 <-try_to_wake_up
+    <idle>-0       2d.h4    1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002
+    <idle>-0       2dNh2    1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8
+    <idle>-0       2.N.2    2us : power_end: cpu_id=2
+    <idle>-0       2.N.2    3us : cpu_idle: state=4294967295 cpu_id=2
+    <idle>-0       2dN.3    4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0
+    <idle>-0       2dN.3    4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000
+    <idle>-0       2.N.2    5us : rcu_utilization: Start context switch
+    <idle>-0       2.N.2    5us : rcu_utilization: End context switch
+    <idle>-0       2d..3    6us : __schedule <-schedule
+    <idle>-0       2d..3    6us :      0:120:R ==> [002]  5882: 94:R sleep
+
+
+Hardware Latency Detector
+-------------------------
+
+The hardware latency detector is executed by enabling the "hwlat" tracer.
+
+NOTE, this tracer will affect the performance of the system as it will
+periodically make a CPU constantly busy with interrupts disabled.
+::
+
+  # echo hwlat > current_tracer
+  # sleep 100
+  # cat trace
+  # tracer: hwlat
+  #
+  # entries-in-buffer/entries-written: 13/13   #P:8
+  #
+  #                              _-----=> irqs-off
+  #                             / _----=> need-resched
+  #                            | / _---=> hardirq/softirq
+  #                            || / _--=> preempt-depth
+  #                            ||| /     delay
+  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
+  #              | |       |   ||||       |         |
+             <...>-1729  [001] d...   678.473449: #1     inner/outer(us):   11/12    ts:1581527483.343962693 count:6
+             <...>-1729  [004] d...   689.556542: #2     inner/outer(us):   16/9     ts:1581527494.889008092 count:1
+             <...>-1729  [005] d...   714.756290: #3     inner/outer(us):   16/16    ts:1581527519.678961629 count:5
+             <...>-1729  [001] d...   718.788247: #4     inner/outer(us):    9/17    ts:1581527523.889012713 count:1
+             <...>-1729  [002] d...   719.796341: #5     inner/outer(us):   13/9     ts:1581527524.912872606 count:1
+             <...>-1729  [006] d...   844.787091: #6     inner/outer(us):    9/12    ts:1581527649.889048502 count:2
+             <...>-1729  [003] d...   849.827033: #7     inner/outer(us):   18/9     ts:1581527654.889013793 count:1
+             <...>-1729  [007] d...   853.859002: #8     inner/outer(us):    9/12    ts:1581527658.889065736 count:1
+             <...>-1729  [001] d...   855.874978: #9     inner/outer(us):    9/11    ts:1581527660.861991877 count:1
+             <...>-1729  [001] d...   863.938932: #10    inner/outer(us):    9/11    ts:1581527668.970010500 count:1 nmi-total:7 nmi-count:1
+             <...>-1729  [007] d...   878.050780: #11    inner/outer(us):    9/12    ts:1581527683.385002600 count:1 nmi-total:5 nmi-count:1
+             <...>-1729  [007] d...   886.114702: #12    inner/outer(us):    9/12    ts:1581527691.385001600 count:1
+
+
+The above output is somewhat the same in the header. All events will have
+interrupts disabled 'd'. Under the FUNCTION title there is:
+
+ #1
+	This is the count of events recorded that were greater than the
+	tracing_threshold (See below).
+
+ inner/outer(us):   11/11
+
+      This shows two numbers as "inner latency" and "outer latency". The test
+      runs in a loop checking a timestamp twice. The latency detected within
+      the two timestamps is the "inner latency" and the latency detected
+      after the previous timestamp and the next timestamp in the loop is
+      the "outer latency".
+
+ ts:1581527483.343962693
+
+      The absolute timestamp that the first latency was recorded in the window.
+
+ count:6
+
+      The number of times a latency was detected during the window.
+
+ nmi-total:7 nmi-count:1
+
+      On architectures that support it, if an NMI comes in during the
+      test, the time spent in NMI is reported in "nmi-total" (in
+      microseconds).
+
+      All architectures that have NMIs will show the "nmi-count" if an
+      NMI comes in during the test.
+
+hwlat files:
+
+  tracing_threshold
+	This gets automatically set to "10" to represent 10
+	microseconds. This is the threshold of latency that
+	needs to be detected before the trace will be recorded.
+
+	Note, when hwlat tracer is finished (another tracer is
+	written into "current_tracer"), the original value for
+	tracing_threshold is placed back into this file.
+
+  hwlat_detector/width
+	The length of time the test runs with interrupts disabled.
+
+  hwlat_detector/window
+	The length of time of the window which the test
+	runs. That is, the test will run for "width"
+	microseconds per "window" microseconds
+
+  tracing_cpumask
+	When the test is started. A kernel thread is created that
+	runs the test. This thread will alternate between CPUs
+	listed in the tracing_cpumask between each period
+	(one "window"). To limit the test to specific CPUs
+	set the mask in this file to only the CPUs that the test
+	should run on.
+
+function
+--------
+
+This tracer is the function tracer. Enabling the function tracer
+can be done from the debug file system. Make sure the
+ftrace_enabled is set; otherwise this tracer is a nop.
+See the "ftrace_enabled" section below.
+::
+
+  # sysctl kernel.ftrace_enabled=1
+  # echo function > current_tracer
+  # echo 1 > tracing_on
+  # usleep 1
+  # echo 0 > tracing_on
+  # cat trace
+  # tracer: function
+  #
+  # entries-in-buffer/entries-written: 24799/24799   #P:4
+  #
+  #                              _-----=> irqs-off
+  #                             / _----=> need-resched
+  #                            | / _---=> hardirq/softirq
+  #                            || / _--=> preempt-depth
+  #                            ||| /     delay
+  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
+  #              | |       |   ||||       |         |
+              bash-1994  [002] ....  3082.063030: mutex_unlock <-rb_simple_write
+              bash-1994  [002] ....  3082.063031: __mutex_unlock_slowpath <-mutex_unlock
+              bash-1994  [002] ....  3082.063031: __fsnotify_parent <-fsnotify_modify
+              bash-1994  [002] ....  3082.063032: fsnotify <-fsnotify_modify
+              bash-1994  [002] ....  3082.063032: __srcu_read_lock <-fsnotify
+              bash-1994  [002] ....  3082.063032: add_preempt_count <-__srcu_read_lock
+              bash-1994  [002] ...1  3082.063032: sub_preempt_count <-__srcu_read_lock
+              bash-1994  [002] ....  3082.063033: __srcu_read_unlock <-fsnotify
+  [...]
+
+
+Note: function tracer uses ring buffers to store the above
+entries. The newest data may overwrite the oldest data.
+Sometimes using echo to stop the trace is not sufficient because
+the tracing could have overwritten the data that you wanted to
+record. For this reason, it is sometimes better to disable
+tracing directly from a program. This allows you to stop the
+tracing at the point that you hit the part that you are
+interested in. To disable the tracing directly from a C program,
+something like following code snippet can be used::
+
+	int trace_fd;
+	[...]
+	int main(int argc, char *argv[]) {
+		[...]
+		trace_fd = open(tracing_file("tracing_on"), O_WRONLY);
+		[...]
+		if (condition_hit()) {
+			write(trace_fd, "0", 1);
+		}
+		[...]
+	}
+
+
+Single thread tracing
+---------------------
+
+By writing into set_ftrace_pid you can trace a
+single thread. For example::
+
+  # cat set_ftrace_pid
+  no pid
+  # echo 3111 > set_ftrace_pid
+  # cat set_ftrace_pid
+  3111
+  # echo function > current_tracer
+  # cat trace | head
+  # tracer: function
+  #
+  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
+  #              | |       |          |         |
+      yum-updatesd-3111  [003]  1637.254676: finish_task_switch <-thread_return
+      yum-updatesd-3111  [003]  1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
+      yum-updatesd-3111  [003]  1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
+      yum-updatesd-3111  [003]  1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
+      yum-updatesd-3111  [003]  1637.254685: fget_light <-do_sys_poll
+      yum-updatesd-3111  [003]  1637.254686: pipe_poll <-do_sys_poll
+  # echo > set_ftrace_pid
+  # cat trace |head
+  # tracer: function
+  #
+  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
+  #              | |       |          |         |
+  ##### CPU 3 buffer started ####
+      yum-updatesd-3111  [003]  1701.957688: free_poll_entry <-poll_freewait
+      yum-updatesd-3111  [003]  1701.957689: remove_wait_queue <-free_poll_entry
+      yum-updatesd-3111  [003]  1701.957691: fput <-free_poll_entry
+      yum-updatesd-3111  [003]  1701.957692: audit_syscall_exit <-sysret_audit
+      yum-updatesd-3111  [003]  1701.957693: path_put <-audit_syscall_exit
+
+If you want to trace a function when executing, you could use
+something like this simple program.
+::
+
+	#include <stdio.h>
+	#include <stdlib.h>
+	#include <sys/types.h>
+	#include <sys/stat.h>
+	#include <fcntl.h>
+	#include <unistd.h>
+	#include <string.h>
+
+	#define _STR(x) #x
+	#define STR(x) _STR(x)
+	#define MAX_PATH 256
+
+	const char *find_tracefs(void)
+	{
+	       static char tracefs[MAX_PATH+1];
+	       static int tracefs_found;
+	       char type[100];
+	       FILE *fp;
+
+	       if (tracefs_found)
+		       return tracefs;
+
+	       if ((fp = fopen("/proc/mounts","r")) == NULL) {
+		       perror("/proc/mounts");
+		       return NULL;
+	       }
+
+	       while (fscanf(fp, "%*s %"
+		             STR(MAX_PATH)
+		             "s %99s %*s %*d %*d\n",
+		             tracefs, type) == 2) {
+		       if (strcmp(type, "tracefs") == 0)
+		               break;
+	       }
+	       fclose(fp);
+
+	       if (strcmp(type, "tracefs") != 0) {
+		       fprintf(stderr, "tracefs not mounted");
+		       return NULL;
+	       }
+
+	       strcat(tracefs, "/tracing/");
+	       tracefs_found = 1;
+
+	       return tracefs;
+	}
+
+	const char *tracing_file(const char *file_name)
+	{
+	       static char trace_file[MAX_PATH+1];
+	       snprintf(trace_file, MAX_PATH, "%s/%s", find_tracefs(), file_name);
+	       return trace_file;
+	}
+
+	int main (int argc, char **argv)
+	{
+		if (argc < 1)
+		        exit(-1);
+
+		if (fork() > 0) {
+		        int fd, ffd;
+		        char line[64];
+		        int s;
+
+		        ffd = open(tracing_file("current_tracer"), O_WRONLY);
+		        if (ffd < 0)
+		                exit(-1);
+		        write(ffd, "nop", 3);
+
+		        fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
+		        s = sprintf(line, "%d\n", getpid());
+		        write(fd, line, s);
+
+		        write(ffd, "function", 8);
+
+		        close(fd);
+		        close(ffd);
+
+		        execvp(argv[1], argv+1);
+		}
+
+		return 0;
+	}
+
+Or this simple script!
+::
+
+  #!/bin/bash
+
+  tracefs=`sed -ne 's/^tracefs \(.*\) tracefs.*/\1/p' /proc/mounts`
+  echo 0 > $tracefs/tracing_on
+  echo $$ > $tracefs/set_ftrace_pid
+  echo function > $tracefs/current_tracer
+  echo 1 > $tracefs/tracing_on
+  exec "$@"
+
+
+function graph tracer
+---------------------------
+
+This tracer is similar to the function tracer except that it
+probes a function on its entry and its exit. This is done by
+using a dynamically allocated stack of return addresses in each
+task_struct. On function entry the tracer overwrites the return
+address of each function traced to set a custom probe. Thus the
+original return address is stored on the stack of return address
+in the task_struct.
+
+Probing on both ends of a function leads to special features
+such as:
+
+- measure of a function's time execution
+- having a reliable call stack to draw function calls graph
+
+This tracer is useful in several situations:
+
+- you want to find the reason of a strange kernel behavior and
+  need to see what happens in detail on any areas (or specific
+  ones).
+
+- you are experiencing weird latencies but it's difficult to
+  find its origin.
+
+- you want to find quickly which path is taken by a specific
+  function
+
+- you just want to peek inside a working kernel and want to see
+  what happens there.
+
+::
+
+  # tracer: function_graph
+  #
+  # CPU  DURATION                  FUNCTION CALLS
+  # |     |   |                     |   |   |   |
+
+   0)               |  sys_open() {
+   0)               |    do_sys_open() {
+   0)               |      getname() {
+   0)               |        kmem_cache_alloc() {
+   0)   1.382 us    |          __might_sleep();
+   0)   2.478 us    |        }
+   0)               |        strncpy_from_user() {
+   0)               |          might_fault() {
+   0)   1.389 us    |            __might_sleep();
+   0)   2.553 us    |          }
+   0)   3.807 us    |        }
+   0)   7.876 us    |      }
+   0)               |      alloc_fd() {
+   0)   0.668 us    |        _spin_lock();
+   0)   0.570 us    |        expand_files();
+   0)   0.586 us    |        _spin_unlock();
+
+
+There are several columns that can be dynamically
+enabled/disabled. You can use every combination of options you
+want, depending on your needs.
+
+- The cpu number on which the function executed is default
+  enabled.  It is sometimes better to only trace one cpu (see
+  tracing_cpu_mask file) or you might sometimes see unordered
+  function calls while cpu tracing switch.
+
+	- hide: echo nofuncgraph-cpu > trace_options
+	- show: echo funcgraph-cpu > trace_options
+
+- The duration (function's time of execution) is displayed on
+  the closing bracket line of a function or on the same line
+  than the current function in case of a leaf one. It is default
+  enabled.
+
+	- hide: echo nofuncgraph-duration > trace_options
+	- show: echo funcgraph-duration > trace_options
+
+- The overhead field precedes the duration field in case of
+  reached duration thresholds.
+
+	- hide: echo nofuncgraph-overhead > trace_options
+	- show: echo funcgraph-overhead > trace_options
+	- depends on: funcgraph-duration
+
+  ie::
+
+    3) # 1837.709 us |          } /* __switch_to */
+    3)               |          finish_task_switch() {
+    3)   0.313 us    |            _raw_spin_unlock_irq();
+    3)   3.177 us    |          }
+    3) # 1889.063 us |        } /* __schedule */
+    3) ! 140.417 us  |      } /* __schedule */
+    3) # 2034.948 us |    } /* schedule */
+    3) * 33998.59 us |  } /* schedule_preempt_disabled */
+
+    [...]
+
+    1)   0.260 us    |              msecs_to_jiffies();
+    1)   0.313 us    |              __rcu_read_unlock();
+    1) + 61.770 us   |            }
+    1) + 64.479 us   |          }
+    1)   0.313 us    |          rcu_bh_qs();
+    1)   0.313 us    |          __local_bh_enable();
+    1) ! 217.240 us  |        }
+    1)   0.365 us    |        idle_cpu();
+    1)               |        rcu_irq_exit() {
+    1)   0.417 us    |          rcu_eqs_enter_common.isra.47();
+    1)   3.125 us    |        }
+    1) ! 227.812 us  |      }
+    1) ! 457.395 us  |    }
+    1) @ 119760.2 us |  }
+
+    [...]
+
+    2)               |    handle_IPI() {
+    1)   6.979 us    |                  }
+    2)   0.417 us    |      scheduler_ipi();
+    1)   9.791 us    |                }
+    1) + 12.917 us   |              }
+    2)   3.490 us    |    }
+    1) + 15.729 us   |            }
+    1) + 18.542 us   |          }
+    2) $ 3594274 us  |  }
+
+Flags::
+
+  + means that the function exceeded 10 usecs.
+  ! means that the function exceeded 100 usecs.
+  # means that the function exceeded 1000 usecs.
+  * means that the function exceeded 10 msecs.
+  @ means that the function exceeded 100 msecs.
+  $ means that the function exceeded 1 sec.
+
+
+- The task/pid field displays the thread cmdline and pid which
+  executed the function. It is default disabled.
+
+	- hide: echo nofuncgraph-proc > trace_options
+	- show: echo funcgraph-proc > trace_options
+
+  ie::
+
+    # tracer: function_graph
+    #
+    # CPU  TASK/PID        DURATION                  FUNCTION CALLS
+    # |    |    |           |   |                     |   |   |   |
+    0)    sh-4802     |               |                  d_free() {
+    0)    sh-4802     |               |                    call_rcu() {
+    0)    sh-4802     |               |                      __call_rcu() {
+    0)    sh-4802     |   0.616 us    |                        rcu_process_gp_end();
+    0)    sh-4802     |   0.586 us    |                        check_for_new_grace_period();
+    0)    sh-4802     |   2.899 us    |                      }
+    0)    sh-4802     |   4.040 us    |                    }
+    0)    sh-4802     |   5.151 us    |                  }
+    0)    sh-4802     | + 49.370 us   |                }
+
+
+- The absolute time field is an absolute timestamp given by the
+  system clock since it started. A snapshot of this time is
+  given on each entry/exit of functions
+
+	- hide: echo nofuncgraph-abstime > trace_options
+	- show: echo funcgraph-abstime > trace_options
+
+  ie::
+
+    #
+    #      TIME       CPU  DURATION                  FUNCTION CALLS
+    #       |         |     |   |                     |   |   |   |
+    360.774522 |   1)   0.541 us    |                                          }
+    360.774522 |   1)   4.663 us    |                                        }
+    360.774523 |   1)   0.541 us    |                                        __wake_up_bit();
+    360.774524 |   1)   6.796 us    |                                      }
+    360.774524 |   1)   7.952 us    |                                    }
+    360.774525 |   1)   9.063 us    |                                  }
+    360.774525 |   1)   0.615 us    |                                  journal_mark_dirty();
+    360.774527 |   1)   0.578 us    |                                  __brelse();
+    360.774528 |   1)               |                                  reiserfs_prepare_for_journal() {
+    360.774528 |   1)               |                                    unlock_buffer() {
+    360.774529 |   1)               |                                      wake_up_bit() {
+    360.774529 |   1)               |                                        bit_waitqueue() {
+    360.774530 |   1)   0.594 us    |                                          __phys_addr();
+
+
+The function name is always displayed after the closing bracket
+for a function if the start of that function is not in the
+trace buffer.
+
+Display of the function name after the closing bracket may be
+enabled for functions whose start is in the trace buffer,
+allowing easier searching with grep for function durations.
+It is default disabled.
+
+	- hide: echo nofuncgraph-tail > trace_options
+	- show: echo funcgraph-tail > trace_options
+
+  Example with nofuncgraph-tail (default)::
+
+    0)               |      putname() {
+    0)               |        kmem_cache_free() {
+    0)   0.518 us    |          __phys_addr();
+    0)   1.757 us    |        }
+    0)   2.861 us    |      }
+
+  Example with funcgraph-tail::
+
+    0)               |      putname() {
+    0)               |        kmem_cache_free() {
+    0)   0.518 us    |          __phys_addr();
+    0)   1.757 us    |        } /* kmem_cache_free() */
+    0)   2.861 us    |      } /* putname() */
+
+You can put some comments on specific functions by using
+trace_printk() For example, if you want to put a comment inside
+the __might_sleep() function, you just have to include
+<linux/ftrace.h> and call trace_printk() inside __might_sleep()::
+
+	trace_printk("I'm a comment!\n")
+
+will produce::
+
+   1)               |             __might_sleep() {
+   1)               |                /* I'm a comment! */
+   1)   1.449 us    |             }
+
+
+You might find other useful features for this tracer in the
+following "dynamic ftrace" section such as tracing only specific
+functions or tasks.
+
+dynamic ftrace
+--------------
+
+If CONFIG_DYNAMIC_FTRACE is set, the system will run with
+virtually no overhead when function tracing is disabled. The way
+this works is the mcount function call (placed at the start of
+every kernel function, produced by the -pg switch in gcc),
+starts of pointing to a simple return. (Enabling FTRACE will
+include the -pg switch in the compiling of the kernel.)
+
+At compile time every C file object is run through the
+recordmcount program (located in the scripts directory). This
+program will parse the ELF headers in the C object to find all
+the locations in the .text section that call mcount. Starting
+with gcc version 4.6, the -mfentry has been added for x86, which
+calls "__fentry__" instead of "mcount". Which is called before
+the creation of the stack frame.
+
+Note, not all sections are traced. They may be prevented by either
+a notrace, or blocked another way and all inline functions are not
+traced. Check the "available_filter_functions" file to see what functions
+can be traced.
+
+A section called "__mcount_loc" is created that holds
+references to all the mcount/fentry call sites in the .text section.
+The recordmcount program re-links this section back into the
+original object. The final linking stage of the kernel will add all these
+references into a single table.
+
+On boot up, before SMP is initialized, the dynamic ftrace code
+scans this table and updates all the locations into nops. It
+also records the locations, which are added to the
+available_filter_functions list.  Modules are processed as they
+are loaded and before they are executed.  When a module is
+unloaded, it also removes its functions from the ftrace function
+list. This is automatic in the module unload code, and the
+module author does not need to worry about it.
+
+When tracing is enabled, the process of modifying the function
+tracepoints is dependent on architecture. The old method is to use
+kstop_machine to prevent races with the CPUs executing code being
+modified (which can cause the CPU to do undesirable things, especially
+if the modified code crosses cache (or page) boundaries), and the nops are
+patched back to calls. But this time, they do not call mcount
+(which is just a function stub). They now call into the ftrace
+infrastructure.
+
+The new method of modifying the function tracepoints is to place
+a breakpoint at the location to be modified, sync all CPUs, modify
+the rest of the instruction not covered by the breakpoint. Sync
+all CPUs again, and then remove the breakpoint with the finished
+version to the ftrace call site.
+
+Some archs do not even need to monkey around with the synchronization,
+and can just slap the new code on top of the old without any
+problems with other CPUs executing it at the same time.
+
+One special side-effect to the recording of the functions being
+traced is that we can now selectively choose which functions we
+wish to trace and which ones we want the mcount calls to remain
+as nops.
+
+Two files are used, one for enabling and one for disabling the
+tracing of specified functions. They are:
+
+  set_ftrace_filter
+
+and
+
+  set_ftrace_notrace
+
+A list of available functions that you can add to these files is
+listed in:
+
+   available_filter_functions
+
+::
+
+  # cat available_filter_functions
+  put_prev_task_idle
+  kmem_cache_create
+  pick_next_task_rt
+  cpus_read_lock
+  pick_next_task_fair
+  mutex_lock
+  [...]
+
+If I am only interested in sys_nanosleep and hrtimer_interrupt::
+
+  # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter
+  # echo function > current_tracer
+  # echo 1 > tracing_on
+  # usleep 1
+  # echo 0 > tracing_on
+  # cat trace
+  # tracer: function
+  #
+  # entries-in-buffer/entries-written: 5/5   #P:4
+  #
+  #                              _-----=> irqs-off
+  #                             / _----=> need-resched
+  #                            | / _---=> hardirq/softirq
+  #                            || / _--=> preempt-depth
+  #                            ||| /     delay
+  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
+  #              | |       |   ||||       |         |
+            usleep-2665  [001] ....  4186.475355: sys_nanosleep <-system_call_fastpath
+            <idle>-0     [001] d.h1  4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt
+            usleep-2665  [001] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
+            <idle>-0     [003] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
+            <idle>-0     [002] d.h1  4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt
+
+To see which functions are being traced, you can cat the file:
+::
+
+  # cat set_ftrace_filter
+  hrtimer_interrupt
+  sys_nanosleep
+
+
+Perhaps this is not enough. The filters also allow glob(7) matching.
+
+  ``<match>*``
+	will match functions that begin with <match>
+  ``*<match>``
+	will match functions that end with <match>
+  ``*<match>*``
+	will match functions that have <match> in it
+  ``<match1>*<match2>``
+	will match functions that begin with <match1> and end with <match2>
+
+.. note::
+      It is better to use quotes to enclose the wild cards,
+      otherwise the shell may expand the parameters into names
+      of files in the local directory.
+
+::
+
+  # echo 'hrtimer_*' > set_ftrace_filter
+
+Produces::
+
+  # tracer: function
+  #
+  # entries-in-buffer/entries-written: 897/897   #P:4
+  #
+  #                              _-----=> irqs-off
+  #                             / _----=> need-resched
+  #                            | / _---=> hardirq/softirq
+  #                            || / _--=> preempt-depth
+  #                            ||| /     delay
+  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
+  #              | |       |   ||||       |         |
+            <idle>-0     [003] dN.1  4228.547803: hrtimer_cancel <-tick_nohz_idle_exit
+            <idle>-0     [003] dN.1  4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel
+            <idle>-0     [003] dN.2  4228.547805: hrtimer_force_reprogram <-__remove_hrtimer
+            <idle>-0     [003] dN.1  4228.547805: hrtimer_forward <-tick_nohz_idle_exit
+            <idle>-0     [003] dN.1  4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
+            <idle>-0     [003] d..1  4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt
+            <idle>-0     [003] d..1  4228.547859: hrtimer_start <-__tick_nohz_idle_enter
+            <idle>-0     [003] d..2  4228.547860: hrtimer_force_reprogram <-__rem
+
+Notice that we lost the sys_nanosleep.
+::
+
+  # cat set_ftrace_filter
+  hrtimer_run_queues
+  hrtimer_run_pending
+  hrtimer_init
+  hrtimer_cancel
+  hrtimer_try_to_cancel
+  hrtimer_forward
+  hrtimer_start
+  hrtimer_reprogram
+  hrtimer_force_reprogram
+  hrtimer_get_next_event
+  hrtimer_interrupt
+  hrtimer_nanosleep
+  hrtimer_wakeup
+  hrtimer_get_remaining
+  hrtimer_get_res
+  hrtimer_init_sleeper
+
+
+This is because the '>' and '>>' act just like they do in bash.
+To rewrite the filters, use '>'
+To append to the filters, use '>>'
+
+To clear out a filter so that all functions will be recorded
+again::
+
+ # echo > set_ftrace_filter
+ # cat set_ftrace_filter
+ #
+
+Again, now we want to append.
+
+::
+
+  # echo sys_nanosleep > set_ftrace_filter
+  # cat set_ftrace_filter
+  sys_nanosleep
+  # echo 'hrtimer_*' >> set_ftrace_filter
+  # cat set_ftrace_filter
+  hrtimer_run_queues
+  hrtimer_run_pending
+  hrtimer_init
+  hrtimer_cancel
+  hrtimer_try_to_cancel
+  hrtimer_forward
+  hrtimer_start
+  hrtimer_reprogram
+  hrtimer_force_reprogram
+  hrtimer_get_next_event
+  hrtimer_interrupt
+  sys_nanosleep
+  hrtimer_nanosleep
+  hrtimer_wakeup
+  hrtimer_get_remaining
+  hrtimer_get_res
+  hrtimer_init_sleeper
+
+
+The set_ftrace_notrace prevents those functions from being
+traced.
+::
+
+  # echo '*preempt*' '*lock*' > set_ftrace_notrace
+
+Produces::
+
+  # tracer: function
+  #
+  # entries-in-buffer/entries-written: 39608/39608   #P:4
+  #
+  #                              _-----=> irqs-off
+  #                             / _----=> need-resched
+  #                            | / _---=> hardirq/softirq
+  #                            || / _--=> preempt-depth
+  #                            ||| /     delay
+  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
+  #              | |       |   ||||       |         |
+              bash-1994  [000] ....  4342.324896: file_ra_state_init <-do_dentry_open
+              bash-1994  [000] ....  4342.324897: open_check_o_direct <-do_last
+              bash-1994  [000] ....  4342.324897: ima_file_check <-do_last
+              bash-1994  [000] ....  4342.324898: process_measurement <-ima_file_check
+              bash-1994  [000] ....  4342.324898: ima_get_action <-process_measurement
+              bash-1994  [000] ....  4342.324898: ima_match_policy <-ima_get_action
+              bash-1994  [000] ....  4342.324899: do_truncate <-do_last
+              bash-1994  [000] ....  4342.324899: setattr_should_drop_suidgid <-do_truncate
+              bash-1994  [000] ....  4342.324899: notify_change <-do_truncate
+              bash-1994  [000] ....  4342.324900: current_fs_time <-notify_change
+              bash-1994  [000] ....  4342.324900: current_kernel_time <-current_fs_time
+              bash-1994  [000] ....  4342.324900: timespec_trunc <-current_fs_time
+
+We can see that there's no more lock or preempt tracing.
+
+Selecting function filters via index
+------------------------------------
+
+Because processing of strings is expensive (the address of the function
+needs to be looked up before comparing to the string being passed in),
+an index can be used as well to enable functions. This is useful in the
+case of setting thousands of specific functions at a time. By passing
+in a list of numbers, no string processing will occur. Instead, the function
+at the specific location in the internal array (which corresponds to the
+functions in the "available_filter_functions" file), is selected.
+
+::
+
+  # echo 1 > set_ftrace_filter
+
+Will select the first function listed in "available_filter_functions"
+
+::
+
+  # head -1 available_filter_functions
+  trace_initcall_finish_cb
+
+  # cat set_ftrace_filter
+  trace_initcall_finish_cb
+
+  # head -50 available_filter_functions | tail -1
+  x86_pmu_commit_txn
+
+  # echo 1 50 > set_ftrace_filter
+  # cat set_ftrace_filter
+  trace_initcall_finish_cb
+  x86_pmu_commit_txn
+
+Dynamic ftrace with the function graph tracer
+---------------------------------------------
+
+Although what has been explained above concerns both the
+function tracer and the function-graph-tracer, there are some
+special features only available in the function-graph tracer.
+
+If you want to trace only one function and all of its children,
+you just have to echo its name into set_graph_function::
+
+ echo __do_fault > set_graph_function
+
+will produce the following "expanded" trace of the __do_fault()
+function::
+
+   0)               |  __do_fault() {
+   0)               |    filemap_fault() {
+   0)               |      find_lock_page() {
+   0)   0.804 us    |        find_get_page();
+   0)               |        __might_sleep() {
+   0)   1.329 us    |        }
+   0)   3.904 us    |      }
+   0)   4.979 us    |    }
+   0)   0.653 us    |    _spin_lock();
+   0)   0.578 us    |    page_add_file_rmap();
+   0)   0.525 us    |    native_set_pte_at();
+   0)   0.585 us    |    _spin_unlock();
+   0)               |    unlock_page() {
+   0)   0.541 us    |      page_waitqueue();
+   0)   0.639 us    |      __wake_up_bit();
+   0)   2.786 us    |    }
+   0) + 14.237 us   |  }
+   0)               |  __do_fault() {
+   0)               |    filemap_fault() {
+   0)               |      find_lock_page() {
+   0)   0.698 us    |        find_get_page();
+   0)               |        __might_sleep() {
+   0)   1.412 us    |        }
+   0)   3.950 us    |      }
+   0)   5.098 us    |    }
+   0)   0.631 us    |    _spin_lock();
+   0)   0.571 us    |    page_add_file_rmap();
+   0)   0.526 us    |    native_set_pte_at();
+   0)   0.586 us    |    _spin_unlock();
+   0)               |    unlock_page() {
+   0)   0.533 us    |      page_waitqueue();
+   0)   0.638 us    |      __wake_up_bit();
+   0)   2.793 us    |    }
+   0) + 14.012 us   |  }
+
+You can also expand several functions at once::
+
+ echo sys_open > set_graph_function
+ echo sys_close >> set_graph_function
+
+Now if you want to go back to trace all functions you can clear
+this special filter via::
+
+ echo > set_graph_function
+
+
+ftrace_enabled
+--------------
+
+Note, the proc sysctl ftrace_enable is a big on/off switch for the
+function tracer. By default it is enabled (when function tracing is
+enabled in the kernel). If it is disabled, all function tracing is
+disabled. This includes not only the function tracers for ftrace, but
+also for any other uses (perf, kprobes, stack tracing, profiling, etc). It
+cannot be disabled if there is a callback with FTRACE_OPS_FL_PERMANENT set
+registered.
+
+Please disable this with care.
+
+This can be disable (and enabled) with::
+
+  sysctl kernel.ftrace_enabled=0
+  sysctl kernel.ftrace_enabled=1
+
+ or
+
+  echo 0 > /proc/sys/kernel/ftrace_enabled
+  echo 1 > /proc/sys/kernel/ftrace_enabled
+
+
+Filter commands
+---------------
+
+A few commands are supported by the set_ftrace_filter interface.
+Trace commands have the following format::
+
+  <function>:<command>:<parameter>
+
+The following commands are supported:
+
+- mod:
+  This command enables function filtering per module. The
+  parameter defines the module. For example, if only the write*
+  functions in the ext3 module are desired, run:
+
+   echo 'write*:mod:ext3' > set_ftrace_filter
+
+  This command interacts with the filter in the same way as
+  filtering based on function names. Thus, adding more functions
+  in a different module is accomplished by appending (>>) to the
+  filter file. Remove specific module functions by prepending
+  '!'::
+
+   echo '!writeback*:mod:ext3' >> set_ftrace_filter
+
+  Mod command supports module globbing. Disable tracing for all
+  functions except a specific module::
+
+   echo '!*:mod:!ext3' >> set_ftrace_filter
+
+  Disable tracing for all modules, but still trace kernel::
+
+   echo '!*:mod:*' >> set_ftrace_filter
+
+  Enable filter only for kernel::
+
+   echo '*write*:mod:!*' >> set_ftrace_filter
+
+  Enable filter for module globbing::
+
+   echo '*write*:mod:*snd*' >> set_ftrace_filter
+
+- traceon/traceoff:
+  These commands turn tracing on and off when the specified
+  functions are hit. The parameter determines how many times the
+  tracing system is turned on and off. If unspecified, there is
+  no limit. For example, to disable tracing when a schedule bug
+  is hit the first 5 times, run::
+
+   echo '__schedule_bug:traceoff:5' > set_ftrace_filter
+
+  To always disable tracing when __schedule_bug is hit::
+
+   echo '__schedule_bug:traceoff' > set_ftrace_filter
+
+  These commands are cumulative whether or not they are appended
+  to set_ftrace_filter. To remove a command, prepend it by '!'
+  and drop the parameter::
+
+   echo '!__schedule_bug:traceoff:0' > set_ftrace_filter
+
+  The above removes the traceoff command for __schedule_bug
+  that have a counter. To remove commands without counters::
+
+   echo '!__schedule_bug:traceoff' > set_ftrace_filter
+
+- snapshot:
+  Will cause a snapshot to be triggered when the function is hit.
+  ::
+
+   echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter
+
+  To only snapshot once:
+  ::
+
+   echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter
+
+  To remove the above commands::
+
+   echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter
+   echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter
+
+- enable_event/disable_event:
+  These commands can enable or disable a trace event. Note, because
+  function tracing callbacks are very sensitive, when these commands
+  are registered, the trace point is activated, but disabled in
+  a "soft" mode. That is, the tracepoint will be called, but
+  just will not be traced. The event tracepoint stays in this mode
+  as long as there's a command that triggers it.
+  ::
+
+   echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \
+   	 set_ftrace_filter
+
+  The format is::
+
+    <function>:enable_event:<system>:<event>[:count]
+    <function>:disable_event:<system>:<event>[:count]
+
+  To remove the events commands::
+
+   echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \
+   	 set_ftrace_filter
+   echo '!schedule:disable_event:sched:sched_switch' > \
+   	 set_ftrace_filter
+
+- dump:
+  When the function is hit, it will dump the contents of the ftrace
+  ring buffer to the console. This is useful if you need to debug
+  something, and want to dump the trace when a certain function
+  is hit. Perhaps it's a function that is called before a triple
+  fault happens and does not allow you to get a regular dump.
+
+- cpudump:
+  When the function is hit, it will dump the contents of the ftrace
+  ring buffer for the current CPU to the console. Unlike the "dump"
+  command, it only prints out the contents of the ring buffer for the
+  CPU that executed the function that triggered the dump.
+
+- stacktrace:
+  When the function is hit, a stack trace is recorded.
+
+trace_pipe
+----------
+
+The trace_pipe outputs the same content as the trace file, but
+the effect on the tracing is different. Every read from
+trace_pipe is consumed. This means that subsequent reads will be
+different. The trace is live.
+::
+
+  # echo function > current_tracer
+  # cat trace_pipe > /tmp/trace.out &
+  [1] 4153
+  # echo 1 > tracing_on
+  # usleep 1
+  # echo 0 > tracing_on
+  # cat trace
+  # tracer: function
+  #
+  # entries-in-buffer/entries-written: 0/0   #P:4
+  #
+  #                              _-----=> irqs-off
+  #                             / _----=> need-resched
+  #                            | / _---=> hardirq/softirq
+  #                            || / _--=> preempt-depth
+  #                            ||| /     delay
+  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
+  #              | |       |   ||||       |         |
+
+  #
+  # cat /tmp/trace.out
+             bash-1994  [000] ....  5281.568961: mutex_unlock <-rb_simple_write
+             bash-1994  [000] ....  5281.568963: __mutex_unlock_slowpath <-mutex_unlock
+             bash-1994  [000] ....  5281.568963: __fsnotify_parent <-fsnotify_modify
+             bash-1994  [000] ....  5281.568964: fsnotify <-fsnotify_modify
+             bash-1994  [000] ....  5281.568964: __srcu_read_lock <-fsnotify
+             bash-1994  [000] ....  5281.568964: add_preempt_count <-__srcu_read_lock
+             bash-1994  [000] ...1  5281.568965: sub_preempt_count <-__srcu_read_lock
+             bash-1994  [000] ....  5281.568965: __srcu_read_unlock <-fsnotify
+             bash-1994  [000] ....  5281.568967: sys_dup2 <-system_call_fastpath
+
+
+Note, reading the trace_pipe file will block until more input is
+added. This is contrary to the trace file. If any process opened
+the trace file for reading, it will actually disable tracing and
+prevent new entries from being added. The trace_pipe file does
+not have this limitation.
+
+trace entries
+-------------
+
+Having too much or not enough data can be troublesome in
+diagnosing an issue in the kernel. The file buffer_size_kb is
+used to modify the size of the internal trace buffers. The
+number listed is the number of entries that can be recorded per
+CPU. To know the full size, multiply the number of possible CPUs
+with the number of entries.
+::
+
+  # cat buffer_size_kb
+  1408 (units kilobytes)
+
+Or simply read buffer_total_size_kb
+::
+
+  # cat buffer_total_size_kb 
+  5632
+
+To modify the buffer, simple echo in a number (in 1024 byte segments).
+::
+
+  # echo 10000 > buffer_size_kb
+  # cat buffer_size_kb
+  10000 (units kilobytes)
+
+It will try to allocate as much as possible. If you allocate too
+much, it can cause Out-Of-Memory to trigger.
+::
+
+  # echo 1000000000000 > buffer_size_kb
+  -bash: echo: write error: Cannot allocate memory
+  # cat buffer_size_kb
+  85
+
+The per_cpu buffers can be changed individually as well:
+::
+
+  # echo 10000 > per_cpu/cpu0/buffer_size_kb
+  # echo 100 > per_cpu/cpu1/buffer_size_kb
+
+When the per_cpu buffers are not the same, the buffer_size_kb
+at the top level will just show an X
+::
+
+  # cat buffer_size_kb
+  X
+
+This is where the buffer_total_size_kb is useful:
+::
+
+  # cat buffer_total_size_kb 
+  12916
+
+Writing to the top level buffer_size_kb will reset all the buffers
+to be the same again.
+
+Snapshot
+--------
+CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature
+available to all non latency tracers. (Latency tracers which
+record max latency, such as "irqsoff" or "wakeup", can't use
+this feature, since those are already using the snapshot
+mechanism internally.)
+
+Snapshot preserves a current trace buffer at a particular point
+in time without stopping tracing. Ftrace swaps the current
+buffer with a spare buffer, and tracing continues in the new
+current (=previous spare) buffer.
+
+The following tracefs files in "tracing" are related to this
+feature:
+
+  snapshot:
+
+	This is used to take a snapshot and to read the output
+	of the snapshot. Echo 1 into this file to allocate a
+	spare buffer and to take a snapshot (swap), then read
+	the snapshot from this file in the same format as
+	"trace" (described above in the section "The File
+	System"). Both reads snapshot and tracing are executable
+	in parallel. When the spare buffer is allocated, echoing
+	0 frees it, and echoing else (positive) values clear the
+	snapshot contents.
+	More details are shown in the table below.
+
+	+--------------+------------+------------+------------+
+	|status\\input |     0      |     1      |    else    |
+	+==============+============+============+============+
+	|not allocated |(do nothing)| alloc+swap |(do nothing)|
+	+--------------+------------+------------+------------+
+	|allocated     |    free    |    swap    |   clear    |
+	+--------------+------------+------------+------------+
+
+Here is an example of using the snapshot feature.
+::
+
+  # echo 1 > events/sched/enable
+  # echo 1 > snapshot
+  # cat snapshot
+  # tracer: nop
+  #
+  # entries-in-buffer/entries-written: 71/71   #P:8
+  #
+  #                              _-----=> irqs-off
+  #                             / _----=> need-resched
+  #                            | / _---=> hardirq/softirq
+  #                            || / _--=> preempt-depth
+  #                            ||| /     delay
+  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
+  #              | |       |   ||||       |         |
+            <idle>-0     [005] d...  2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120   prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120
+             sleep-2242  [005] d...  2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120   prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120
+  [...]
+          <idle>-0     [002] d...  2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120  
+
+  # cat trace  
+  # tracer: nop
+  #
+  # entries-in-buffer/entries-written: 77/77   #P:8
+  #
+  #                              _-----=> irqs-off
+  #                             / _----=> need-resched
+  #                            | / _---=> hardirq/softirq
+  #                            || / _--=> preempt-depth
+  #                            ||| /     delay
+  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
+  #              | |       |   ||||       |         |
+            <idle>-0     [007] d...  2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120
+   snapshot-test-2-2229  [002] d...  2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120
+  [...]
+
+
+If you try to use this snapshot feature when current tracer is
+one of the latency tracers, you will get the following results.
+::
+
+  # echo wakeup > current_tracer
+  # echo 1 > snapshot
+  bash: echo: write error: Device or resource busy
+  # cat snapshot
+  cat: snapshot: Device or resource busy
+
+
+Instances
+---------
+In the tracefs tracing directory, there is a directory called "instances".
+This directory can have new directories created inside of it using
+mkdir, and removing directories with rmdir. The directory created
+with mkdir in this directory will already contain files and other
+directories after it is created.
+::
+
+  # mkdir instances/foo
+  # ls instances/foo
+  buffer_size_kb  buffer_total_size_kb  events  free_buffer  per_cpu
+  set_event  snapshot  trace  trace_clock  trace_marker  trace_options
+  trace_pipe  tracing_on
+
+As you can see, the new directory looks similar to the tracing directory
+itself. In fact, it is very similar, except that the buffer and
+events are agnostic from the main directory, or from any other
+instances that are created.
+
+The files in the new directory work just like the files with the
+same name in the tracing directory except the buffer that is used
+is a separate and new buffer. The files affect that buffer but do not
+affect the main buffer with the exception of trace_options. Currently,
+the trace_options affect all instances and the top level buffer
+the same, but this may change in future releases. That is, options
+may become specific to the instance they reside in.
+
+Notice that none of the function tracer files are there, nor is
+current_tracer and available_tracers. This is because the buffers
+can currently only have events enabled for them.
+::
+
+  # mkdir instances/foo
+  # mkdir instances/bar
+  # mkdir instances/zoot
+  # echo 100000 > buffer_size_kb
+  # echo 1000 > instances/foo/buffer_size_kb
+  # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb
+  # echo function > current_trace
+  # echo 1 > instances/foo/events/sched/sched_wakeup/enable
+  # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable
+  # echo 1 > instances/foo/events/sched/sched_switch/enable
+  # echo 1 > instances/bar/events/irq/enable
+  # echo 1 > instances/zoot/events/syscalls/enable
+  # cat trace_pipe
+  CPU:2 [LOST 11745 EVENTS]
+              bash-2044  [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist
+              bash-2044  [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave
+              bash-2044  [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist
+              bash-2044  [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist
+              bash-2044  [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock
+              bash-2044  [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype
+              bash-2044  [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist
+              bash-2044  [002] d... 10594.481034: zone_statistics <-get_page_from_freelist
+              bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
+              bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
+              bash-2044  [002] .... 10594.481035: arch_dup_task_struct <-copy_process
+  [...]
+
+  # cat instances/foo/trace_pipe
+              bash-1998  [000] d..4   136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
+              bash-1998  [000] dN.4   136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
+            <idle>-0     [003] d.h3   136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003
+            <idle>-0     [003] d..3   136.676909: sched_switch: prev_comm=swapper/3 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_preempt next_pid=9 next_prio=120
+       rcu_preempt-9     [003] d..3   136.676916: sched_switch: prev_comm=rcu_preempt prev_pid=9 prev_prio=120 prev_state=S ==> next_comm=swapper/3 next_pid=0 next_prio=120
+              bash-1998  [000] d..4   136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
+              bash-1998  [000] dN.4   136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
+              bash-1998  [000] d..3   136.677018: sched_switch: prev_comm=bash prev_pid=1998 prev_prio=120 prev_state=R+ ==> next_comm=kworker/0:1 next_pid=59 next_prio=120
+       kworker/0:1-59    [000] d..4   136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001
+       kworker/0:1-59    [000] d..3   136.677025: sched_switch: prev_comm=kworker/0:1 prev_pid=59 prev_prio=120 prev_state=S ==> next_comm=bash next_pid=1998 next_prio=120
+  [...]
+
+  # cat instances/bar/trace_pipe
+       migration/1-14    [001] d.h3   138.732674: softirq_raise: vec=3 [action=NET_RX]
+            <idle>-0     [001] dNh3   138.732725: softirq_raise: vec=3 [action=NET_RX]
+              bash-1998  [000] d.h1   138.733101: softirq_raise: vec=1 [action=TIMER]
+              bash-1998  [000] d.h1   138.733102: softirq_raise: vec=9 [action=RCU]
+              bash-1998  [000] ..s2   138.733105: softirq_entry: vec=1 [action=TIMER]
+              bash-1998  [000] ..s2   138.733106: softirq_exit: vec=1 [action=TIMER]
+              bash-1998  [000] ..s2   138.733106: softirq_entry: vec=9 [action=RCU]
+              bash-1998  [000] ..s2   138.733109: softirq_exit: vec=9 [action=RCU]
+              sshd-1995  [001] d.h1   138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4
+              sshd-1995  [001] d.h1   138.733280: irq_handler_exit: irq=21 ret=unhandled
+              sshd-1995  [001] d.h1   138.733281: irq_handler_entry: irq=21 name=eth0
+              sshd-1995  [001] d.h1   138.733283: irq_handler_exit: irq=21 ret=handled
+  [...]
+
+  # cat instances/zoot/trace
+  # tracer: nop
+  #
+  # entries-in-buffer/entries-written: 18996/18996   #P:4
+  #
+  #                              _-----=> irqs-off
+  #                             / _----=> need-resched
+  #                            | / _---=> hardirq/softirq
+  #                            || / _--=> preempt-depth
+  #                            ||| /     delay
+  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
+  #              | |       |   ||||       |         |
+              bash-1998  [000] d...   140.733501: sys_write -> 0x2
+              bash-1998  [000] d...   140.733504: sys_dup2(oldfd: a, newfd: 1)
+              bash-1998  [000] d...   140.733506: sys_dup2 -> 0x1
+              bash-1998  [000] d...   140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0)
+              bash-1998  [000] d...   140.733509: sys_fcntl -> 0x1
+              bash-1998  [000] d...   140.733510: sys_close(fd: a)
+              bash-1998  [000] d...   140.733510: sys_close -> 0x0
+              bash-1998  [000] d...   140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8)
+              bash-1998  [000] d...   140.733515: sys_rt_sigprocmask -> 0x0
+              bash-1998  [000] d...   140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8)
+              bash-1998  [000] d...   140.733516: sys_rt_sigaction -> 0x0
+
+You can see that the trace of the top most trace buffer shows only
+the function tracing. The foo instance displays wakeups and task
+switches.
+
+To remove the instances, simply delete their directories:
+::
+
+  # rmdir instances/foo
+  # rmdir instances/bar
+  # rmdir instances/zoot
+
+Note, if a process has a trace file open in one of the instance
+directories, the rmdir will fail with EBUSY.
+
+
+Stack trace
+-----------
+Since the kernel has a fixed sized stack, it is important not to
+waste it in functions. A kernel developer must be conscience of
+what they allocate on the stack. If they add too much, the system
+can be in danger of a stack overflow, and corruption will occur,
+usually leading to a system panic.
+
+There are some tools that check this, usually with interrupts
+periodically checking usage. But if you can perform a check
+at every function call that will become very useful. As ftrace provides
+a function tracer, it makes it convenient to check the stack size
+at every function call. This is enabled via the stack tracer.
+
+CONFIG_STACK_TRACER enables the ftrace stack tracing functionality.
+To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled.
+::
+
+ # echo 1 > /proc/sys/kernel/stack_tracer_enabled
+
+You can also enable it from the kernel command line to trace
+the stack size of the kernel during boot up, by adding "stacktrace"
+to the kernel command line parameter.
+
+After running it for a few minutes, the output looks like:
+::
+
+  # cat stack_max_size
+  2928
+
+  # cat stack_trace
+          Depth    Size   Location    (18 entries)
+          -----    ----   --------
+    0)     2928     224   update_sd_lb_stats+0xbc/0x4ac
+    1)     2704     160   find_busiest_group+0x31/0x1f1
+    2)     2544     256   load_balance+0xd9/0x662
+    3)     2288      80   idle_balance+0xbb/0x130
+    4)     2208     128   __schedule+0x26e/0x5b9
+    5)     2080      16   schedule+0x64/0x66
+    6)     2064     128   schedule_timeout+0x34/0xe0
+    7)     1936     112   wait_for_common+0x97/0xf1
+    8)     1824      16   wait_for_completion+0x1d/0x1f
+    9)     1808     128   flush_work+0xfe/0x119
+   10)     1680      16   tty_flush_to_ldisc+0x1e/0x20
+   11)     1664      48   input_available_p+0x1d/0x5c
+   12)     1616      48   n_tty_poll+0x6d/0x134
+   13)     1568      64   tty_poll+0x64/0x7f
+   14)     1504     880   do_select+0x31e/0x511
+   15)      624     400   core_sys_select+0x177/0x216
+   16)      224      96   sys_select+0x91/0xb9
+   17)      128     128   system_call_fastpath+0x16/0x1b
+
+Note, if -mfentry is being used by gcc, functions get traced before
+they set up the stack frame. This means that leaf level functions
+are not tested by the stack tracer when -mfentry is used.
+
+Currently, -mfentry is used by gcc 4.6.0 and above on x86 only.
+
+More
+----
+More details can be found in the source code, in the `kernel/trace/*.c` files.