perf script: Allow specifying event to switch on processing of other events

Sometime we want to only consider events after something happens, so
allow discarding events till such events is found, e.g.:

Record all scheduler tracepoints and the sys_enter_nanosleep syscall
event for the 'sleep 1' workload:

  # perf record -e sched:*,syscalls:sys_enter_nanosleep sleep 1
  [ perf record: Woken up 31 times to write data ]
  [ perf record: Captured and wrote 0.032 MB perf.data (10 samples) ]
  #

So we have these events in the generated perf data file:

  # perf evlist
  sched:sched_kthread_stop
  sched:sched_kthread_stop_ret
  sched:sched_waking
  sched:sched_wakeup
  sched:sched_wakeup_new
  sched:sched_switch
  sched:sched_migrate_task
  sched:sched_process_free
  sched:sched_process_exit
  sched:sched_wait_task
  sched:sched_process_wait
  sched:sched_process_fork
  sched:sched_process_exec
  sched:sched_stat_wait
  sched:sched_stat_sleep
  sched:sched_stat_iowait
  sched:sched_stat_blocked
  sched:sched_stat_runtime
  sched:sched_pi_setprio
  sched:sched_move_numa
  sched:sched_stick_numa
  sched:sched_swap_numa
  sched:sched_wake_idle_without_ipi
  syscalls:sys_enter_nanosleep
  # Tip: use 'perf evlist --trace-fields' to show fields for tracepoint events
  #

Then show all of the events that actually took place in this 'perf record' session:

  # perf script
          :13637 13637 [002] 108237.581529:            sched:sched_waking: comm=perf pid=13638 prio=120 target_cpu=001
          :13637 13637 [002] 108237.581537:            sched:sched_wakeup: perf:13638 [120] success=1 CPU:001
           sleep 13638 [001] 108237.581992:      sched:sched_process_exec: filename=/usr/bin/sleep pid=13638 old_pid=13638
           sleep 13638 [001] 108237.582286:  syscalls:sys_enter_nanosleep: rqtp: 0x7fff1948ac40, rmtp: 0x00000000
           sleep 13638 [001] 108237.582289:      sched:sched_stat_runtime: comm=sleep pid=13638 runtime=578104 [ns] vruntime=202889459556 [ns]
           sleep 13638 [001] 108237.582291:            sched:sched_switch: sleep:13638 [120] S ==> swapper/1:0 [120]
         swapper     0 [001] 108238.582428:            sched:sched_waking: comm=sleep pid=13638 prio=120 target_cpu=001
         swapper     0 [001] 108238.582458:            sched:sched_wakeup: sleep:13638 [120] success=1 CPU:001
           sleep 13638 [001] 108238.582698:      sched:sched_stat_runtime: comm=sleep pid=13638 runtime=173915 [ns] vruntime=202889633471 [ns]
           sleep 13638 [001] 108238.582782:      sched:sched_process_exit: comm=sleep pid=13638 prio=120
  #

Now lets see only the ones that took place after a certain "marker":

  # perf script --switch-on syscalls:sys_enter_nanosleep
           sleep 13638 [001] 108237.582289:      sched:sched_stat_runtime: comm=sleep pid=13638 runtime=578104 [ns] vruntime=202889459556 [ns]
           sleep 13638 [001] 108237.582291:            sched:sched_switch: sleep:13638 [120] S ==> swapper/1:0 [120]
         swapper     0 [001] 108238.582428:            sched:sched_waking: comm=sleep pid=13638 prio=120 target_cpu=001
         swapper     0 [001] 108238.582458:            sched:sched_wakeup: sleep:13638 [120] success=1 CPU:001
           sleep 13638 [001] 108238.582698:      sched:sched_stat_runtime: comm=sleep pid=13638 runtime=173915 [ns] vruntime=202889633471 [ns]
           sleep 13638 [001] 108238.582782:      sched:sched_process_exit: comm=sleep pid=13638 prio=120
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: William Cohen <wcohen@redhat.com>
Link: https://lkml.kernel.org/n/tip-f1oo0ufdhrkx6nhy2lj1ierm@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-script.txt

@@ -417,6 +417,9 @@ include::itrace.txt[]
 	For itrace only show specified functions and their callees for
 	itrace. Multiple functions can be separated by comma.
 
+--switch-on EVENT_NAME::
+	Only consider events after this event is found.
+
 SEE ALSO
 --------
 linkperf:perf-record[1], linkperf:perf-script-perl[1],

tools/perf/builtin-script.c

@@ -1616,6 +1616,11 @@ static int perf_sample__fprintf_synth(struct perf_sample *sample,
 	return 0;
 }
 
+struct evswitch {
+	struct evsel *on;
+	bool	     discarding;
+};
+
 struct perf_script {
 	struct perf_tool	tool;
 	struct perf_session	*session;
@@ -1628,6 +1633,7 @@ struct perf_script {
 	bool			show_bpf_events;
 	bool			allocated;
 	bool			per_event_dump;
+	struct evswitch		evswitch;
 	struct perf_cpu_map	*cpus;
 	struct perf_thread_map *threads;
 	int			name_width;
@@ -1805,6 +1811,13 @@ static void process_event(struct perf_script *script,
 	if (!show_event(sample, evsel, thread, al))
 		return;
 
+	if (script->evswitch.on && script->evswitch.discarding) {
+		if (script->evswitch.on != evsel)
+			return;
+
+		script->evswitch.discarding = false;
+	}
+
 	++es->samples;
 
 	perf_sample__fprintf_start(sample, thread, evsel,
@@ -3395,6 +3408,7 @@ int cmd_script(int argc, const char **argv)
 	struct utsname uts;
 	char *script_path = NULL;
 	const char **__argv;
+	const char *event_switch_on = NULL;
 	int i, j, err = 0;
 	struct perf_script script = {
 		.tool = {
@@ -3538,6 +3552,8 @@ int cmd_script(int argc, const char **argv)
 		   "file", "file saving guest os /proc/kallsyms"),
 	OPT_STRING(0, "guestmodules", &symbol_conf.default_guest_modules,
 		   "file", "file saving guest os /proc/modules"),
+	OPT_STRING(0, "switch-on", &event_switch_on,
+		   "event", "Consider events from the first ocurrence of this event"),
 	OPT_END()
 	};
 	const char * const script_subcommands[] = { "record", "report", NULL };
@@ -3862,6 +3878,16 @@ int cmd_script(int argc, const char **argv)
 						  script.range_num);
 	}
 
+	if (event_switch_on) {
+		script.evswitch.on = perf_evlist__find_evsel_by_str(session->evlist, event_switch_on);
+		if (script.evswitch.on == NULL) {
+			fprintf(stderr, "switch-on event not found (%s)\n", event_switch_on);
+			err = -ENOENT;
+			goto out_delete;
+		}
+		script.evswitch.discarding = true;
+	}
+
 	err = __cmd_script(&script);
 
 	flush_scripting();