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LIBTRACEFS(3) libtracefs Manual LIBTRACEFS(3)


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


#include <tracefs.h>
int tracefs_synth_create(struct tracefs_synth *synth);
int tracefs_synth_destroy(struct tracefs_synth *synth);
bool tracefs_synth_complete(struct tracefs_synth *synth);
int tracefs_synth_set_instance(struct tracefs_synth *synth, struct tracefs_instance *instance);
int tracefs_synth_trace(struct tracefs_synth *synth,

enum tracefs_synth_handler type, const char *var); int tracefs_synth_snapshot(struct tracefs_synth *synth,
enum tracefs_synth_handler type, const char *var); int tracefs_synth_save(struct tracefs_synth *synth,
enum tracefs_synth_handler type, const char *var,
char **save_fields);


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. By default, the histogram triggers are created in the top trace instance, as any synthetic event can be used globally across all instances. In case an application wants to keep the histogram triggers out of the top level instance, it can use tracefs_synth_set_instance() to have the histograms used for creating the synthetic event in an instance other than the top level. A synthetic event descriptor must be created with tracefs_synth_alloc(3) before this can be used to create it on the system.

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).

tracefs_synth_set_instance() Set the trace instance, where the histogram triggers that create the synthetic event will be created. By default, the top instance is used. This API must be called before the call to tracefs_synth_create(), in order to use the new instance when creating the event. Note, that even if the synthetic event is created in an instance, it is still visible by all other instances including the top level. That is, other instances can enable the created synthetic event and have it traced in the buffers that belong to the instance that enabled it.


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


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.


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

#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,
/* The tep is no longer needed */
/* 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,
/* 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;
tracefs_synth_echo_cmd(&s, synth);
trace_seq_destroy(&s); } int main (int argc, char **argv) {
if (argc > 1) {
if (!strcmp(argv[1], "create")) {
/* Create the synthetic event */
} else if (!strcmp(argv[1], "delete")) {
/* Delete the synthetic event */
} else {
printf("usage: %s [create|delete]\n", argv[0]);
} else
return 0; }



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.


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),


Steven Rostedt <[1]>
Tzvetomir Stoyanov <[2]>
sameeruddin shaik <[3]>


Report bugs to <[4]>


libtracefs is Free Software licensed under the GNU LGPL 2.1



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


06/15/2023 libtracefs