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

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Date:   Thu, 29 Oct 2020 16:27:26 +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 23/10/20 11:12, Peter Zijlstra wrote:
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1732,6 +1732,8 @@ void migrate_disable(void)
>               return;
>       }
>
> +	trace_sched_migrate_disable_tp(p);
> +
>       preempt_disable();
>       this_rq()->nr_pinned++;
>       p->migration_disabled = 1;
> @@ -1764,6 +1766,8 @@ void migrate_enable(void)
>       p->migration_disabled = 0;
>       this_rq()->nr_pinned--;
>       preempt_enable();
> +
> +	trace_sched_migrate_enable_tp(p);

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.

>  }
>  EXPORT_SYMBOL_GPL(migrate_enable);