Re: [PATCH v3] perf sched timehist: Add pre-migration wait time option

[Namhyung Kim <namhyung@kernel.org>]

On Thu, Oct 03, 2024 at 03:04:28PM -0700, Tim Chen wrote:
> On Wed, 2024-10-02 at 22:09 +0530, Madadi Vineeth Reddy wrote:
> > pre-migration wait time is the time that a task unnecessarily spends
> > on the runqueue of a CPU but doesn't get switched-in there. In terms
> > of tracepoints, it is the time between sched:sched_wakeup and
> > sched:sched_migrate_task.
> > 
> > Let's say a task woke up on CPU2, then it got migrated to CPU4 and
> > then it's switched-in to CPU4. So, here pre-migration wait time is
> > time that it was waiting on runqueue of CPU2 after it is woken up.
> > 
> > The general pattern for pre-migration to occur is:
> > sched:sched_wakeup
> > sched:sched_migrate_task
> > sched:sched_switch
> 
> If a task migrate from CPU A to CPU B, but is unlucky that
> someone took CPU B and it has to wait. Then it is yet again migrated to CPU C.
> Do you only compute pre-mig time as 
> t_sched_migrate_task_CPU_A - t_sched_migrate_task_CPU_A?
> 
> The task also spend some pre-mig time on CPU_B that probably
> should be included.  And that time is when it migrates to B
> till it is migrated away from B.  Do you take that into
> account?

Hmm.. right.  The current code updates the migrate time only if it's 0.
But I think it can just always update the time to get the latest one.

Thanks,
Namhyung