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

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Date:   Fri, 27 Aug 2021 00:24:58 +0800
From:   Tao Zhou <tao.zhou@...ux.dev>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org, tao.zhou@...ux.dev
Cc:     Ingo Molnar <mingo@...hat.com>,
        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>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched: Fix get_push_task() vs migrate_disable()

Hi Sebastian,

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>
> ---
>  kernel/sched/sched.h | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index e205b63d6db07..32a4945730a9b 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2259,6 +2259,9 @@ static inline struct task_struct *get_push_task(struct rq *rq)
>  	if (p->nr_cpus_allowed == 1)
>  		return NULL;
>  
> +	if (p->migration_disabled)
> +		return NULL;

Not much I can restore here..

Is is_migration_disabled(p) be more correct to check migration disable.
And get_push_task() being called in pull_rt_task() has checked migration
disable first and then call get_push_task(). That means this check in
get_push_task() in patch is a second repeatly check.

>  	rq->push_busy = true;
>  	return get_task_struct(p);
>  }
> -- 
> 2.33.0
> 



Thanks,
Tao