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diff --git a/Documentation/trace/tracepoint-analysis.rst b/Documentation/trace/tracepoint-analysis.rst new file mode 100644 index 000000000..716326b9f --- /dev/null +++ b/Documentation/trace/tracepoint-analysis.rst @@ -0,0 +1,338 @@ +========================================================= +Notes on Analysing Behaviour Using Events and Tracepoints +========================================================= +:Author: Mel Gorman (PCL information heavily based on email from Ingo Molnar) + +1. Introduction +=============== + +Tracepoints (see Documentation/trace/tracepoints.rst) can be used without +creating custom kernel modules to register probe functions using the event +tracing infrastructure. + +Simplistically, tracepoints represent important events that can be +taken in conjunction with other tracepoints to build a "Big Picture" of +what is going on within the system. There are a large number of methods for +gathering and interpreting these events. Lacking any current Best Practises, +this document describes some of the methods that can be used. + +This document assumes that debugfs is mounted on /sys/kernel/debug and that +the appropriate tracing options have been configured into the kernel. It is +assumed that the PCL tool tools/perf has been installed and is in your path. + +2. Listing Available Events +=========================== + +2.1 Standard Utilities +---------------------- + +All possible events are visible from /sys/kernel/debug/tracing/events. Simply +calling:: + + $ find /sys/kernel/debug/tracing/events -type d + +will give a fair indication of the number of events available. + +2.2 PCL (Performance Counters for Linux) +---------------------------------------- + +Discovery and enumeration of all counters and events, including tracepoints, +are available with the perf tool. Getting a list of available events is a +simple case of:: + + $ perf list 2>&1 | grep Tracepoint + ext4:ext4_free_inode [Tracepoint event] + ext4:ext4_request_inode [Tracepoint event] + ext4:ext4_allocate_inode [Tracepoint event] + ext4:ext4_write_begin [Tracepoint event] + ext4:ext4_ordered_write_end [Tracepoint event] + [ .... remaining output snipped .... ] + + +3. Enabling Events +================== + +3.1 System-Wide Event Enabling +------------------------------ + +See Documentation/trace/events.rst for a proper description on how events +can be enabled system-wide. A short example of enabling all events related +to page allocation would look something like:: + + $ for i in `find /sys/kernel/debug/tracing/events -name "enable" | grep mm_`; do echo 1 > $i; done + +3.2 System-Wide Event Enabling with SystemTap +--------------------------------------------- + +In SystemTap, tracepoints are accessible using the kernel.trace() function +call. The following is an example that reports every 5 seconds what processes +were allocating the pages. +:: + + global page_allocs + + probe kernel.trace("mm_page_alloc") { + page_allocs[execname()]++ + } + + function print_count() { + printf ("%-25s %-s\n", "#Pages Allocated", "Process Name") + foreach (proc in page_allocs-) + printf("%-25d %s\n", page_allocs[proc], proc) + printf ("\n") + delete page_allocs + } + + probe timer.s(5) { + print_count() + } + +3.3 System-Wide Event Enabling with PCL +--------------------------------------- + +By specifying the -a switch and analysing sleep, the system-wide events +for a duration of time can be examined. +:: + + $ perf stat -a \ + -e kmem:mm_page_alloc -e kmem:mm_page_free \ + -e kmem:mm_page_free_batched \ + sleep 10 + Performance counter stats for 'sleep 10': + + 9630 kmem:mm_page_alloc + 2143 kmem:mm_page_free + 7424 kmem:mm_page_free_batched + + 10.002577764 seconds time elapsed + +Similarly, one could execute a shell and exit it as desired to get a report +at that point. + +3.4 Local Event Enabling +------------------------ + +Documentation/trace/ftrace.rst describes how to enable events on a per-thread +basis using set_ftrace_pid. + +3.5 Local Event Enablement with PCL +----------------------------------- + +Events can be activated and tracked for the duration of a process on a local +basis using PCL such as follows. +:: + + $ perf stat -e kmem:mm_page_alloc -e kmem:mm_page_free \ + -e kmem:mm_page_free_batched ./hackbench 10 + Time: 0.909 + + Performance counter stats for './hackbench 10': + + 17803 kmem:mm_page_alloc + 12398 kmem:mm_page_free + 4827 kmem:mm_page_free_batched + + 0.973913387 seconds time elapsed + +4. Event Filtering +================== + +Documentation/trace/ftrace.rst covers in-depth how to filter events in +ftrace. Obviously using grep and awk of trace_pipe is an option as well +as any script reading trace_pipe. + +5. Analysing Event Variances with PCL +===================================== + +Any workload can exhibit variances between runs and it can be important +to know what the standard deviation is. By and large, this is left to the +performance analyst to do it by hand. In the event that the discrete event +occurrences are useful to the performance analyst, then perf can be used. +:: + + $ perf stat --repeat 5 -e kmem:mm_page_alloc -e kmem:mm_page_free + -e kmem:mm_page_free_batched ./hackbench 10 + Time: 0.890 + Time: 0.895 + Time: 0.915 + Time: 1.001 + Time: 0.899 + + Performance counter stats for './hackbench 10' (5 runs): + + 16630 kmem:mm_page_alloc ( +- 3.542% ) + 11486 kmem:mm_page_free ( +- 4.771% ) + 4730 kmem:mm_page_free_batched ( +- 2.325% ) + + 0.982653002 seconds time elapsed ( +- 1.448% ) + +In the event that some higher-level event is required that depends on some +aggregation of discrete events, then a script would need to be developed. + +Using --repeat, it is also possible to view how events are fluctuating over +time on a system-wide basis using -a and sleep. +:: + + $ perf stat -e kmem:mm_page_alloc -e kmem:mm_page_free \ + -e kmem:mm_page_free_batched \ + -a --repeat 10 \ + sleep 1 + Performance counter stats for 'sleep 1' (10 runs): + + 1066 kmem:mm_page_alloc ( +- 26.148% ) + 182 kmem:mm_page_free ( +- 5.464% ) + 890 kmem:mm_page_free_batched ( +- 30.079% ) + + 1.002251757 seconds time elapsed ( +- 0.005% ) + +6. Higher-Level Analysis with Helper Scripts +============================================ + +When events are enabled the events that are triggering can be read from +/sys/kernel/debug/tracing/trace_pipe in human-readable format although binary +options exist as well. By post-processing the output, further information can +be gathered on-line as appropriate. Examples of post-processing might include + + - Reading information from /proc for the PID that triggered the event + - Deriving a higher-level event from a series of lower-level events. + - Calculating latencies between two events + +Documentation/trace/postprocess/trace-pagealloc-postprocess.pl is an example +script that can read trace_pipe from STDIN or a copy of a trace. When used +on-line, it can be interrupted once to generate a report without exiting +and twice to exit. + +Simplistically, the script just reads STDIN and counts up events but it +also can do more such as + + - Derive high-level events from many low-level events. If a number of pages + are freed to the main allocator from the per-CPU lists, it recognises + that as one per-CPU drain even though there is no specific tracepoint + for that event + - It can aggregate based on PID or individual process number + - In the event memory is getting externally fragmented, it reports + on whether the fragmentation event was severe or moderate. + - When receiving an event about a PID, it can record who the parent was so + that if large numbers of events are coming from very short-lived + processes, the parent process responsible for creating all the helpers + can be identified + +7. Lower-Level Analysis with PCL +================================ + +There may also be a requirement to identify what functions within a program +were generating events within the kernel. To begin this sort of analysis, the +data must be recorded. At the time of writing, this required root: +:: + + $ perf record -c 1 \ + -e kmem:mm_page_alloc -e kmem:mm_page_free \ + -e kmem:mm_page_free_batched \ + ./hackbench 10 + Time: 0.894 + [ perf record: Captured and wrote 0.733 MB perf.data (~32010 samples) ] + +Note the use of '-c 1' to set the event period to sample. The default sample +period is quite high to minimise overhead but the information collected can be +very coarse as a result. + +This record outputted a file called perf.data which can be analysed using +perf report. +:: + + $ perf report + # Samples: 30922 + # + # Overhead Command Shared Object + # ........ ......... ................................ + # + 87.27% hackbench [vdso] + 6.85% hackbench /lib/i686/cmov/libc-2.9.so + 2.62% hackbench /lib/ld-2.9.so + 1.52% perf [vdso] + 1.22% hackbench ./hackbench + 0.48% hackbench [kernel] + 0.02% perf /lib/i686/cmov/libc-2.9.so + 0.01% perf /usr/bin/perf + 0.01% perf /lib/ld-2.9.so + 0.00% hackbench /lib/i686/cmov/libpthread-2.9.so + # + # (For more details, try: perf report --sort comm,dso,symbol) + # + +According to this, the vast majority of events triggered on events +within the VDSO. With simple binaries, this will often be the case so let's +take a slightly different example. In the course of writing this, it was +noticed that X was generating an insane amount of page allocations so let's look +at it: +:: + + $ perf record -c 1 -f \ + -e kmem:mm_page_alloc -e kmem:mm_page_free \ + -e kmem:mm_page_free_batched \ + -p `pidof X` + +This was interrupted after a few seconds and +:: + + $ perf report + # Samples: 27666 + # + # Overhead Command Shared Object + # ........ ....... ....................................... + # + 51.95% Xorg [vdso] + 47.95% Xorg /opt/gfx-test/lib/libpixman-1.so.0.13.1 + 0.09% Xorg /lib/i686/cmov/libc-2.9.so + 0.01% Xorg [kernel] + # + # (For more details, try: perf report --sort comm,dso,symbol) + # + +So, almost half of the events are occurring in a library. To get an idea which +symbol: +:: + + $ perf report --sort comm,dso,symbol + # Samples: 27666 + # + # Overhead Command Shared Object Symbol + # ........ ....... ....................................... ...... + # + 51.95% Xorg [vdso] [.] 0x000000ffffe424 + 47.93% Xorg /opt/gfx-test/lib/libpixman-1.so.0.13.1 [.] pixmanFillsse2 + 0.09% Xorg /lib/i686/cmov/libc-2.9.so [.] _int_malloc + 0.01% Xorg /opt/gfx-test/lib/libpixman-1.so.0.13.1 [.] pixman_region32_copy_f + 0.01% Xorg [kernel] [k] read_hpet + 0.01% Xorg /opt/gfx-test/lib/libpixman-1.so.0.13.1 [.] get_fast_path + 0.00% Xorg [kernel] [k] ftrace_trace_userstack + +To see where within the function pixmanFillsse2 things are going wrong: +:: + + $ perf annotate pixmanFillsse2 + [ ... ] + 0.00 : 34eeb: 0f 18 08 prefetcht0 (%eax) + : } + : + : extern __inline void __attribute__((__gnu_inline__, __always_inline__, _ + : _mm_store_si128 (__m128i *__P, __m128i __B) : { + : *__P = __B; + 12.40 : 34eee: 66 0f 7f 80 40 ff ff movdqa %xmm0,-0xc0(%eax) + 0.00 : 34ef5: ff + 12.40 : 34ef6: 66 0f 7f 80 50 ff ff movdqa %xmm0,-0xb0(%eax) + 0.00 : 34efd: ff + 12.39 : 34efe: 66 0f 7f 80 60 ff ff movdqa %xmm0,-0xa0(%eax) + 0.00 : 34f05: ff + 12.67 : 34f06: 66 0f 7f 80 70 ff ff movdqa %xmm0,-0x90(%eax) + 0.00 : 34f0d: ff + 12.58 : 34f0e: 66 0f 7f 40 80 movdqa %xmm0,-0x80(%eax) + 12.31 : 34f13: 66 0f 7f 40 90 movdqa %xmm0,-0x70(%eax) + 12.40 : 34f18: 66 0f 7f 40 a0 movdqa %xmm0,-0x60(%eax) + 12.31 : 34f1d: 66 0f 7f 40 b0 movdqa %xmm0,-0x50(%eax) + +At a glance, it looks like the time is being spent copying pixmaps to +the card. Further investigation would be needed to determine why pixmaps +are being copied around so much but a starting point would be to take an +ancient build of libpixmap out of the library path where it was totally +forgotten about from months ago! |