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: Arnaldo Carvalho de Melo <acme@redhat.com>