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

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Message-ID: <Zv8YiooRdcT-x8iN@google.com>
Date: Thu, 3 Oct 2024 15:19:54 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Tim Chen <tim.c.chen@...ux.intel.com>
Cc: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Athira Rajeev <atrajeev@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Kan Liang <kan.liang@...ux.intel.com>, acme@...hat.com,
	linux-perf-users <linux-perf-users@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] perf sched timehist: Add pre-migration wait time
 option

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