linux-kernel - Re: [PATCH v4 17/19] sched: Add migrate

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Message-ID: <jhj5z6sq5kz.mognet@arm.com>
Date:   Thu, 29 Oct 2020 17:56:12 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, mingo@...nel.org, linux-kernel@...r.kernel.org,
        bigeasy@...utronix.de, qais.yousef@....com, swood@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vincent.donnefort@....com,
        tj@...nel.org, ouwen210@...mail.com
Subject: Re: [PATCH v4 17/19] sched: Add migrate_disable() tracepoints


On 29/10/20 17:43, Peter Zijlstra wrote:
> On Thu, Oct 29, 2020 at 04:27:26PM +0000, Valentin Schneider wrote:
>> Don't you want those directly after the ->migration_disabled write?
>> esp. for migrate_enable(), if that preempt_enable() leads to a context
>> switch then the disable->enable deltas won't reflect the kernel view.
>> 
>> That delta may indeed include the time it took to run the stopper and
>> fix the task's affinity on migrate_enable(), but it could include all
>> sorts of other higher-priority tasks.
>
> I can put them in the preempt_disable() section I suppose, but these
> tracers should be looking at task_sched_runtime(), not walltime, and
> then the preemption doesn't matter.
>

True. I was thinking of how to process it downstream, and the first thing
that came to mind was that rd->overutilized flag which we do monitor
fairly closely; however that is system-wide while migrate_disable() is
task-specific.

> Also, a distinct lack of actual users atm.. :/

If you'd rather ditch this one altogether until someone asks for it, that
also works for me.