LIBTRACEFS(3)

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version 1.8.4 · 12 versions
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🌸 LIBTRACEFS(3) — libtracefs Manual 🌸

okay bestie, welcome to the libtracefs manual ✨ this page is all about pulling data out of synthetic events, and it's giving "I made a dynamic event and now I want receipts" 💅 no cap, let's get into it.

✨ NAME ✨

  • tracefs_synth_echo_cmd, tracefs_synth_get_start_hist, tracefs_synth_get_name, tracefs_synth_raw_fmt, tracefs_synth_show_event, tracefs_synth_show_start_hist, tracefs_synth_show_end_hist, tracefs_synth_get_event — Retrieve data of synthetic events.

🌈 SYNOPSIS 🌈

#include <tracefs.h>

int tracefs_synth_echo_cmd(struct trace_seq *seq, struct tracefs_synth *synth);
struct tracefs_hist *tracefs_synth_get_start_hist(struct tracefs_synth *synth);

const char *tracefs_synth_get_name(struct tracefs_synth *synth);
int tracefs_synth_raw_fmt(struct trace_seq *seq, struct tracefs_synth *synth);
const char *tracefs_synth_show_event(struct tracefs_synth *synth);
const char *tracefs_synth_show_start_hist(struct tracefs_synth *synth);
const char *tracefs_synth_show_end_hist(struct tracefs_synth *synth);
struct tep_event *tracefs_synth_get_event(struct tep_handle *tep, struct tracefs_synth *synth);

🧚 DESCRIPTION 🧚

Synthetic events are dynamic events that are created by matching two other events, and that match triggers a synthetic event. One event is the starting event, where some field is recorded. Then when the second event is executed, if it has a field (or fields) that matches the starting event's field (or fields), 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. 🔥

See tracefs_synth_alloc(3) for allocation of synthetic events, and tracefs_synth_create() for creating the synthetic event on the system.

tracefs_synth_echo_cmd() acts like tracefs_synth_create(), but instead of creating the synthetic event in the system, it will write the echo commands to manually create it in the seq given. 🎀

tracefs_synth_get_start_hist() returns a struct tracefs_hist descriptor describing the histogram used to create the synthetic event.

oh and there's this enum in the mix too:

enum tracefs_synth_handler {
        TRACEFS_SYNTH_HANDLE_MATCH,
        TRACEFS_SYNTH_HANDLE_MAX,
        TRACEFS_SYNTH_HANDLE_CHANGE,
};

tracefs_synth_get_name() returns the name of the synthetic event or NULL on error. The returned string belongs to the synth event object and is freed with the event by tracefs_synth_free().

tracefs_synth_raw_fmt() writes the raw format strings (dynamic event and histograms) of the synthetic event in the seq given.

tracefs_synth_show_event() returns the format of the dynamic event used by the synthetic event or NULL on error. The returned string belongs to the synth event object and is freed with the event by tracefs_synth_free().

tracefs_synth_show_start_hist() returns the format of the start histogram used by the synthetic event or NULL on error. The returned string belongs to the synth event object and is freed with the event by tracefs_synth_free().

tracefs_synth_show_end_hist() returns the format of the end histogram used by the synthetic event or NULL on error. The returned string belongs to the synth event object and is freed with the event by tracefs_synth_free().

The tracefs_synth_get_event() function returns a tep event, describing the given synthetic event. The API detects any newly created or removed dynamic events. The returned pointer to tep event is controlled by @tep and must not be freed.

💖 RETURN VALUE 💖

tracefs_synth_get_name(), tracefs_synth_show_event(), tracefs_synth_show_start_hist() and tracefs_synth_show_end_hist() return a string owned by the synth event object.

The tracefs_synth_get_event() function returns a pointer to a tep event or NULL in case of an error or if the requested synthetic event is missing. The returned pointer to tep event is controlled by @tep and must not be freed.

All other 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.

#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 🎀

  • 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 🔥

👑 AUTHOR 👑

  • Steven Rostedt <rostedt@goodmis.org[1]>
  • Tzvetomir Stoyanov <tz.stoyanov@gmail.com[2]>
  • sameeruddin shaik <sameeruddin.shaik8@gmail.com[3]>

🙈 REPORTING BUGS 🙈

Report bugs to <linux-trace-devel@vger.kernel.org[4]>

🌸 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).

💌 NOTES 💌

okay so sometimes you're reading a manpage and you hit the part where you're like "wait, who do I even talk to about this?" — and boom, that's this section, bestie. these are the actual humans behind the libtracefs lore. no cap, they're the ones you ping when something's weird or when you just wanna say thanks 🫶

  1. rostedt@goodmis.org mailto:rostedt@goodmis.org
  2. tz.stoyanov@gmail.com mailto:tz.stoyanov@gmail.com
  3. sameeruddin.shaik8@gmail.com mailto:sameeruddin.shaik8@gmail.com
  4. linux-trace-devel@vger.kernel.org mailto:linux-trace-devel@vger.kernel.org
pro tip from your girl: those mailto: lines are literally the exact same address in clickable form, so you don't gotta copy-paste like a peasant 💅 keep them character for character and you're golden ✨

and that's the whole vibe, bestie — you made it to the end of the page 🌈 this little masterpiece is libtracefs version 1.8.4, dated 10/21/2026, and honestly? iconic. go forth and trace some stuff, you absolute main character 🦋👑 mwah 💋

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