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

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Message-ID: <20200924122934.GI2628@hirez.programming.kicks-ass.net>
Date:   Thu, 24 Sep 2020 14:29:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
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 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.