xref: /aosp_15_r20/external/libtracefs/Documentation/libtracefs-iterator.txt (revision 287e80b3a36113050663245e7f2c00d274188f18)

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

NAME
----
tracefs_iterate_raw_events, tracefs_iterate_stop, tracefs_follow_event, tracefs_follow_missed_events - Iterate over events in the ring buffer

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

int *tracefs_iterate_raw_events*(struct tep_handle pass:[*]_tep_, struct tracefs_instance pass:[*]_instance_,
				 cpu_set_t pass:[*]_cpus_, int _cpu_size_,
				 int (pass:[*]_callback_)(struct tep_event pass:[*], struct tep_record pass:[*], int, void pass:[*]),
				 void pass:[*]_callback_context_);
void *tracefs_iterate_stop*(struct tracefs_instance pass:[*]_instance_);

int *tracefs_follow_event*(struct tep_handle pass:[*]_tep_, struct tracefs_instance pass:[*]_instance_,
			  const char pass:[*]_system_, const char pass:[*]_event_name_,
			  int (pass:[*]_callback_)(struct tep_event pass:[*],
					  struct tep_record pass:[*],
					  int, void pass:[*]),
			  void pass:[*]_callback_data_);
int *tracefs_follow_missed_events*(struct tracefs_instance pass:[*]_instance_,
			  int (pass:[*]_callback_)(struct tep_event pass:[*],
					  struct tep_record pass:[*],
					  int, void pass:[*]),
			  void pass:[*]_callback_data_);
--

DESCRIPTION
-----------
Trace iterator over raw events.

The *tracefs_iterate_raw_events()* function will read the tracefs raw
data buffers and call the specified _callback_ function for every event it
encounters. Events are iterated in sorted order: oldest first. An initialized
_tep_ handler is required (See *tracefs_local_events*(3)). If _instance_ is
NULL, then the toplevel tracefs buffer is used, otherwise the buffer for
the corresponding _instance_ is read. To filter only on a subset of CPUs,
_cpus_ and _cpu_size_ may be set to only call _callback_ with events that
occurred on the CPUs specified, otherwise if _cpus_ is NULL then the _callback_
function will be called for all events, and _cpu_size_ is ignored. The
_callback_ function will be called with the following parameters: A
pointer to a struct tep_event that corresponds to the type of event the
record is; The record representing the event; The CPU that the event
occurred on; and a pointer to user specified _callback_context_. If the _callback_
returns non-zero, the iteration stops.

Use *tracefs_iterate_stop()* to force a executing *tracefs_iterate_raw_events()*
to halt. This can be called from either a callback that is called by
the iterator (even though a return of non-zero will stop it), or from another
thread.

The *tracefs_follow_event()* is used with *tracefs_iterate_raw_events()* but
intead of the callback being called for every event, it is only called for the
specified _system_ / _event_name_ given to the function. The _callback_ is the
same as for *tracefs_iterate_raw_events()*, and the passed in _callback_context_
will be passed to the _callback_ as well. Note, if it returns something other
than 0, it will stop the loop before the _callback_ of *tracefs_iterate_raw_events()*
is called.

The *tracefs_follow_missed_events()* will call the _callback_ when missed
events are detected. It will set the _record_ parameter of the callback to the
record that came after the missed events and _event_ will be of the type of
event _record_ is. _cpu_ will be set to the CPU that missed the events, and
_callback_data_ will be the content that was passed in to the function.

RETURN VALUE
------------
The *tracefs_iterate_raw_events()* function returns -1 in case of an error or
0 otherwise.

EXAMPLE
-------
[source,c]
--
#include <unistd.h>
#include <tracefs.h>
#include <stdbool.h>
#include <signal.h>

struct my_struct {
	bool		stopped;
};

#define MAX_COUNT 500000
static int counter;

static int callback(struct tep_event *event, struct tep_record *record,
		    int cpu, void *data)
{
	struct my_struct *my_data = data;
	static struct trace_seq seq;

	if (counter++ > MAX_COUNT) {
		my_data->stopped = true;
		return 1;
	}

	if (!seq.buffer)
		trace_seq_init(&seq);

	tep_print_event(event->tep, &seq, record, "%16s-%-5d [%03d] %6.1000d %s: %s\n",
			TEP_PRINT_COMM, TEP_PRINT_PID, TEP_PRINT_CPU,
			TEP_PRINT_TIME, TEP_PRINT_NAME, TEP_PRINT_INFO);
	trace_seq_terminate(&seq);
	trace_seq_do_printf(&seq);
	trace_seq_reset(&seq);
	return 0;
}

static int sched_callback(struct tep_event *event, struct tep_record *record,
			  int cpu, void *data)
{
	static struct tep_format_field *prev_pid;
	static struct tep_format_field *next_pid;
	unsigned long long pid;
	int this_pid = *(int *)data;

	if (!prev_pid) {
		prev_pid = tep_find_field(event, "prev_pid");
		next_pid = tep_find_field(event, "next_pid");
		if (!prev_pid || !next_pid) {
			fprintf(stderr, "No pid fields??\n");
			return -1;
		}
	}

	tep_read_number_field(prev_pid, record->data, &pid);
	if (pid == this_pid)
		printf("WE ARE LEAVING!\n");
	tep_read_number_field(next_pid, record->data, &pid);
	if (pid == this_pid)
		printf("WE ARE ARRIVING!\n");
	return 0;
}

static int missed_callback(struct tep_event *event, struct tep_record *record,
			   int cpu, void *data)
{
	printf("OOPS! cpu %d dropped ", cpu);
	if (record->missed_events > 0)
		printf("%lld ", record->missed_events);
	printf("events\n");
	return 0;
}

static struct tracefs_instance *instance;
static struct my_struct my_data;

static void sig(int s)
{
	tracefs_iterate_stop(instance);
	my_data.stopped = true;
}

int main (int argc, char **argv, char **env)
{
	struct tep_handle *tep;
	int this_pid = getpid();

	instance = tracefs_instance_create("my-buffer");
	if (!instance)
		return -1;

	signal(SIGINT, sig);

	tracefs_event_enable(instance, NULL, NULL);
	sleep(1);
	tracefs_event_disable(instance, NULL, NULL);
	tep = tracefs_local_events(NULL);
	tep_load_plugins(tep);
	tracefs_follow_missed_events(instance, missed_callback, NULL);
	tracefs_follow_event(tep, instance, "sched", "sched_switch", sched_callback, &this_pid);
	tracefs_iterate_raw_events(tep, instance, NULL, 0, callback, &my_data);
	tracefs_instance_destroy(instance);

	if (my_data.stopped) {
		if (counter > MAX_COUNT)
			printf("Finished max count\n");
		else
			printf("Finished via signal\n");
	}

	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)

AUTHOR
------
[verse]
--
*Steven Rostedt* <[email protected]>
*Tzvetomir Stoyanov* <[email protected]>
--
REPORTING BUGS
--------------
Report bugs to  <[email protected]>

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).