linux-kernel - Re: [PATCH] sched: Fix get_push_task() vs migrate

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Date:   Thu, 26 Aug 2021 18:56:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        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>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched: Fix get_push_task() vs migrate_disable()

On Thu, Aug 26, 2021 at 03:37:38PM +0200, Sebastian Andrzej Siewior wrote:
> push_rt_task() attempts to move the currently running task away if the
> next runnable task has migration disabled and therefore is pinned on the
> current CPU.
> 
> The current task is retrieved via get_push_task() which only checks for
> nr_cpus_allowed == 1, but does not check whether the task has migration
> disabled and therefore cannot be moved either. The consequence is a
> pointless invocation of the migration thread which correctly observes
> that the task cannot be moved.
> 
> Return NULL if the task has migration disabled and cannot be moved to
> another CPU.
> 
> Fixes: a7c81556ec4d3 ("sched: Fix migrate_disable() vs rt/dl balancing")
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Thanks!