linux-kernel - Re: [PATCH 7/9] sched: Add migrate

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Message-ID: <jhj4knnmk16.mognet@arm.com>
Date:   Thu, 24 Sep 2020 13:33:25 +0100
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
Subject: Re: [PATCH 7/9] sched: Add migrate_disable()


On 24/09/20 13:29, Peter Zijlstra wrote:
> On Thu, Sep 24, 2020 at 12:53:25PM +0100, Valentin Schneider wrote:
>> On 21/09/20 17:36, Peter Zijlstra wrote:
>>
>> [...]
>>
>> > +void migrate_enable(void)
>> > +{
>> > +	if (--current->migration_disabled)
>> > +		return;
>> > +
>> > +	barrier();
>> > +
>> > +	if (p->cpus_ptr == &p->cpus_mask)
>> > +		return;
>>
>> If we get to here this means we're the migrate_enable() invocation that
>> marks the end of the migration_disabled region. How can cpus_ptr already
>> point back to current's cpus_mask?
>
> It might never have been changed away.
>
>
>       migrate_disable()
>         ->migration_disabled++;
>       |	|
>       |	|
>       |	v
>       |	migrate_disable_switch()
>       |	  if (->cpus_ptr == &->cpus_mask)
>       |	    __do_set_cpus_allowed(.new_mask = cpumask_of())
>       |	|
>       |	|
>       v	v
>       migrate_enable()
>         ->migration_disabled--;
>
>
> Only if we've passed through a context switch between migrate_disable()
> and migrate_enable() will the mask have been changed.

Doh, yes indeed. Thanks.