linux-kernel - Re: [PATCH] sched: __set_cpus_allowed_ptr(): Check cpus_mask, not cpus

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Message-ID: <20200618080700.cig4x4y7n3thmneu@linutronix.de>
Date:   Thu, 18 Jun 2020 10:07:00 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Scott Wood <swood@...hat.com>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched: __set_cpus_allowed_ptr(): Check cpus_mask, not
 cpus_ptr

On 2020-06-17 17:49:48 [-0500], Scott Wood wrote:
> > Makes sense, but what about the rest of the checks? Further down there is
> > 
> >         /* Can the task run on the task's current CPU? If so, we're done
> > */
> >         if (cpumask_test_cpu(task_cpu(p), new_mask))
> >                 goto out;
> > 
> > If the task is currently migrate disabled and for some stupid reason it
> > gets affined elsewhere, we could try to move it out - which AFAICT we
> > don't
> > want to do because migrate disabled. So I suppose you'd want an extra
> > bailout condition here when the task is migrate disabled.
> > 
> > ISTR in RT you do re-check the affinity and potentially move the task away
> > when re-enabling migration, so that should work out all fine.
> 
> On RT the above test is:
> 
>         /* Can the task run on the task's current CPU? If so, we're done */
>         if (cpumask_test_cpu(task_cpu(p), new_mask) ||
>             p->cpus_ptr != &p->cpus_mask)
>                 goto out;
> 
> ...so we do bail out if we're migrate disabled.

correct. There is a complete migrate_disable() patch in the RT queue
which has to wait. This patch however looked to be independent of that
and could "fix" the pointer part which is already here so I sent it.

> -Scott

Sebastian