[PATCH] [RFC] sched: Add trace_sched_waking() tracepoint to sched_ttwu_pending()

[John Stultz]

Zimuzo reported seeing occasional cases in perfetto traces where
tasks went from sleeping directly to trace_sched_wakeup()
without always seeing a trace_sched_waking().

Looking at the code, trace_sched_wakeup() is only called in
ttwu_do_wakeup()

The call paths that get you to ttwu_do_wakeup() are:
try_to_wake_up() -> ttwu_do_wakeup()
try_to_wake_up() -> ttwu_runnable() -> ttwu_do_wakeup()
try_to_wake_up() -> ttwu_queue() -> ttwu_do_activate() -> ttwu_do_wakeup()
sched_ttwu_pending() -> ttwu_do_activate() -> ttwu_do_wakeup()

where trace_sched_waking() is currently called only in
try_to_wake_up().

So this patch adds a trace_sched_waking() call to
sched_ttwu_pending(), so we see the same state machine
transitions.

With this change, the number of unexpected state transitions
in perfetto was greatly reduced.

This has been in my drafts for awhile, so I wanted to send
this out for thoughts/feedback.

Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
Cc: kernel-team@xxxxxxxxxxx
Reported-by: Zimuzo Ezeozue <zezeozue@xxxxxxxxxx>
Signed-off-by: John Stultz <jstultz@xxxxxxxxxx>
---
 kernel/sched/core.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9116bcc90346..233f06360d6a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3894,6 +3894,7 @@ void sched_ttwu_pending(void *arg)
 		if (WARN_ON_ONCE(task_cpu(p) != cpu_of(rq)))
 			set_task_cpu(p, cpu_of(rq));
 
+		trace_sched_waking(p);
 		ttwu_do_activate(rq, p, p->sched_remote_wakeup ? WF_MIGRATED : 0, &rf);
 	}
 
-- 
2.44.0.rc0.258.g7320e95886-goog