path: root/Documentation/libtracefs-synth2.txt

libtracefs(3)
=============

NAME
----
tracefs_synth_create, tracefs_synth_destroy, tracefs_synth_complete,
tracefs_synth_trace, tracefs_synth_snapshot, tracefs_synth_save
- Creation of synthetic events

SYNOPSIS
--------
[verse]
--
*#include <tracefs.h>*

int *tracefs_synth_create*(struct tracefs_synth pass:[*]_synth_);
int *tracefs_synth_destroy*(struct tracefs_synth pass:[*]_synth_);
bool *tracefs_synth_complete*(struct tracefs_synth pass:[*]_synth_);

int *tracefs_synth_trace*(struct tracefs_synth pass:[*]_synth_,
			enum tracefs_synth_handler _type_, const char pass:[*]_var_);
int *tracefs_synth_snapshot*(struct tracefs_synth pass:[*]_synth_,
			   enum tracefs_synth_handler _type_, const char pass:[*]_var_);
int *tracefs_synth_save*(struct tracefs_synth pass:[*]_synth_,
		       enum tracefs_synth_handler _type_, const char pass:[*]_var_,
		       char pass:[**]_save_fields_);
--

DESCRIPTION
-----------
Synthetic events are dynamic events that are created by matching
two other events which triggers a synthetic event. One event is the starting
event which some field is recorded, and when the second event is executed,
if it has a field (or fields) that matches the starting event's field (or fields)
then it will trigger the synthetic event. The field values other than the matching
fields may be passed from the starting event to the end event to perform calculations
on, or to simply pass as a parameter to the synthetic event.

One common use case is to set "sched_waking" as the starting event. This event is
triggered when a process is awoken. Then set "sched_switch" as the ending event.
This event is triggered when a new task is scheduled on the CPU. By setting
the "common_pid" of both events as the matching fields, the time between the
two events is considered the wake up latency of that process. Use *TRACEFS_TIMESTAMP*
as a field for both events to calculate the delta in nanoseconds, or use
*TRACEFS_TIMESTAMP_USECS* as the compare fields for both events to calculate the
delta in microseconds. This is used as the example below.

*tracefs_synth_create*() creates the synthetic event in the system. The synthetic events apply
across all instances. A synthetic event must be created with *tracefs_synth_alloc*(3) before
it can be created.

*tracefs_synth_destroy*() destroys the synthetic event. It will attempt to stop the running of it in
its instance (top by default), but if its running in another instance this may fail as busy.

*tracefs_synth_complete*() returns true if the synthetic event _synth_ has both
a starting and ending event.

*tracefs_synth_trace*() Instead of doing just a trace on matching of the start and
end events, do the _type_ handler where *TRACEFS_SYNTH_HANDLE_MAX* will do a trace
when the given variable _var_ hits a new max for the matching keys. Or
*TRACEFS_SYNTH_HANDLE_CHANGE* for when the _var_ changes. _var_ must be one of
the _name_ elements used in *tracefs_synth_add_end_field*(3).

*tracefs_synth_snapshot*() When the given variable _var_ is either a new max if
_handler_ is *TRACEFS_SYNTH_HANDLE_MAX* or simply changed if *TRACEFS_SYNTH_HANDLE_CHANGE*
then take a "snapshot" of the buffer. The snapshot moves the normal "trace" buffer
into a "snapshot" buffer, that can be accessed via the "snapshot" file in the
top level tracefs directory, or one of the instances.  _var_ changes. _var_ must be one of
the _name_ elements used in *tracefs_synth_add_end_field*(3).

*tracefs_synth_save*() When the given variable _var_ is either a new max if
_handler_ is *TRACEFS_SYNTH_HANDLE_MAX* or simpy changed if *TRACEFS_SYNTH_HANDLE_CHANGE*
then save the given _save_fields_ list. The fields will be stored in the histogram
"hist" file of the event that can be retrieved with *tracefs_event_file_read*(3).
_var_ must be one of the _name_ elements used in *tracefs_synth_add_end_field*(3).

RETURN VALUE
------------
All functions return zero on success or -1 on error.

ERRORS
------
The following errors are for all the above calls:

*EPERM* Not run as root user when required.

*EINVAL* Either a parameter is not valid (NULL when it should not be)
  or a field that is not compatible for calculations.

*ENODEV* An event or one of its fields is not found.

*EBADE* The fields of the start and end events are not compatible for
  either matching or comparing.

*ENOMEM* not enough memory is available.

And more errors may have happened from the system calls to the system.

EXAMPLE
-------
See *tracefs_sql*(3) for a more indepth use of some of this code.

[source,c]
--
#include <stdlib.h>
#include <tracefs.h>

#define start_event "sched_waking"
#define start_field "pid"

#define end_event "sched_switch"
#define end_field "next_pid"

#define match_name "pid"

static struct tracefs_synth *synth;

static void make_event(void)
{
	struct tep_handle *tep;

	/* Load all events from the system */
	tep = tracefs_local_events(NULL);

	/* Initialize the synthetic event */
	synth = tracefs_synth_alloc(tep, "wakeup_lat",
				    NULL, start_event,
				    NULL, end_event,
				    start_field, end_field,
				    match_name);

	/* The tep is no longer needed */
	tep_free(tep);


	/* Save the "prio" field as "prio" from the start event */
	tracefs_synth_add_start_field(synth, "prio", NULL);

	/* Save the "next_comm" as "comm" from the end event */
	tracefs_synth_add_end_field(synth, "next_comm", "comm");

	/* Save the "prev_prio" as "prev_prio" from the end event */
	tracefs_synth_add_end_field(synth, "prev_prio", NULL);

	/*
	 * Take a microsecond time difference between end and start
	 * and record as "delta"
	 */
	tracefs_synth_add_compare_field(synth, TRACEFS_TIMESTAMP_USECS,
					TRACEFS_TIMESTAMP_USECS,
					TRACEFS_SYNTH_DELTA_END, "delta");

	/* Only record if start event "prio" is less than 100 */
	tracefs_synth_append_start_filter(synth, TRACEFS_FILTER_COMPARE,
					  "prio", TRACEFS_COMPARE_LT, "100");

	/*
	 * Only record if end event "next_prio" is less than 50
	 * or the previous task's prio was not greater than or equal to 100.
	 *   next_prio < 50 || !(prev_prio >= 100)
	 */
	tracefs_synth_append_end_filter(synth, TRACEFS_FILTER_COMPARE,
					"next_prio", TRACEFS_COMPARE_LT, "50");
	tracefs_synth_append_end_filter(synth, TRACEFS_FILTER_OR, NULL, 0, NULL);
	tracefs_synth_append_end_filter(synth, TRACEFS_FILTER_NOT, NULL, 0, NULL);
	tracefs_synth_append_end_filter(synth, TRACEFS_FILTER_OPEN_PAREN, NULL, 0, NULL);
	tracefs_synth_append_end_filter(synth, TRACEFS_FILTER_COMPARE,
					"prev_prio", TRACEFS_COMPARE_GE, "100");
	/*
	 * Note, the above only added: "next_prio < 50 || !(prev_prio >= 100"
	 * That's because, when the synth is executed, the remaining close parenthesis
	 * will be added. That is, the string will end up being:
	 * "next_prio < 50 || !(prev_prio >= 100)" when one of tracefs_sync_create()
	 * or tracefs_sync_echo_cmd() is run.
	 */
}

/* Display how to create the synthetic event */
static void show_event(void)
{
	struct trace_seq s;

	trace_seq_init(&s);

	tracefs_synth_echo_cmd(&s, synth);
	trace_seq_terminate(&s);
	trace_seq_do_printf(&s);
	trace_seq_destroy(&s);
}

int main (int argc, char **argv)
{
	make_event();

	if (argc > 1) {
		if (!strcmp(argv[1], "create")) {
			/* Create the synthetic event */
			tracefs_synth_create(synth);
		} else if (!strcmp(argv[1], "delete")) {
			/* Delete the synthetic event */
			tracefs_synth_destroy(synth);
		} else {
			printf("usage: %s [create|delete]\n", argv[0]);
			exit(-1);
		}
	} else
		show_event();

	tracefs_synth_free(synth);

	return 0;
}
--

FILES
-----
[verse]
--
*tracefs.h*
	Header file to include in order to have access to the library APIs.
*-ltracefs*
	Linker switch to add when building a program that uses the library.
--

SEE ALSO
--------
*libtracefs*(3),
*libtraceevent*(3),
*trace-cmd*(1),
*tracefs_hist_alloc*(3),
*tracefs_hist_alloc_2d*(3),
*tracefs_hist_alloc_nd*(3),
*tracefs_hist_free*(3),
*tracefs_hist_add_key*(3),
*tracefs_hist_add_value*(3),
*tracefs_hist_add_name*(3),
*tracefs_hist_start*(3),
*tracefs_hist_destory*(3),
*tracefs_hist_add_sort_key*(3),
*tracefs_hist_sort_key_direction*(3),
*tracefs_synth_alloc*(3),
*tracefs_synth_add_match_field*(3),
*tracefs_synth_add_compare_field*(3),
*tracefs_synth_add_start_field*(3),
*tracefs_synth_add_end_field*(3),
*tracefs_synth_append_start_filter*(3),
*tracefs_synth_append_end_filter*(3),
*tracefs_synth_free*(3),
*tracefs_synth_echo_cmd*(3),
*tracefs_synth_get_start_hist*(3),
*tracefs_synth_get_name*(3),
*tracefs_synth_raw_fmt*(3),
*tracefs_synth_show_event*(3),
*tracefs_synth_show_start_hist*(3),
*tracefs_synth_show_end_hist*(3),
*tracefs_synth_get_event*(3),

AUTHOR
------
[verse]
--
*Steven Rostedt* <rostedt@goodmis.org>
*Tzvetomir Stoyanov* <tz.stoyanov@gmail.com>
*sameeruddin shaik* <sameeruddin.shaik8@gmail.com>
--
REPORTING BUGS
--------------
Report bugs to  <linux-trace-devel@vger.kernel.org>

LICENSE
-------
libtracefs is Free Software licensed under the GNU LGPL 2.1

RESOURCES
---------
https://git.kernel.org/pub/scm/libs/libtrace/libtracefs.git/

COPYING
-------
Copyright \(C) 2020 VMware, Inc. Free use of this software is granted under
the terms of the GNU Public License (GPL).