linux-kernel - Re: [BUG] CFS vs cpu hotplug

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Message-ID: <20080710073055.GA7127@osiris.boeblingen.de.ibm.com>
Date:	Thu, 10 Jul 2008 09:30:55 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Dmitry Adamushko <dmitry.adamushko@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, miaox@...fujitsu.com,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Avi Kivity <avi@...ranet.com>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [BUG] CFS vs cpu hotplug

On Thu, Jul 10, 2008 at 12:32:40AM +0200, Dmitry Adamushko wrote:
> 
> hm, while looking at this code again...
> 
> 
> Ingo,
> 
> I think we may have a race between try_to_wake_up() and migrate_live_tasks() -> move_task_off_dead_cpu()
> when the later one may end up looping endlessly.
> 
> 
> Subject: sched: prevent a potentially endless loop in move_task_off_dead_cpu()
> 
> Interrupts are enabled on other CPUs when migration_call(CPU_DEAD, ...) is called so we may get a race
> between try_to_wake_up() and migrate_live_tasks() -> move_task_off_dead_cpu(). The former one may push
> a task out of a dead CPU causing the later one to loop endlessly.

That's exactly what explains a dump I got yesterday. Thanks for fixing! :)

Will apply your patch and let you know if it fixes the problem.
(may take until next week unfortunately).

> Signed-off-by: Dmitry Adamushko <dmitry.adamushko@...il.com>
> 
> ---
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 94ead43..9397b87 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -5621,8 +5621,10 @@ static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
> 
>  	double_rq_lock(rq_src, rq_dest);
>  	/* Already moved. */
> -	if (task_cpu(p) != src_cpu)
> +	if (task_cpu(p) != src_cpu) {
> +		ret = 1;
>  		goto out;
> +	}
>  	/* Affinity changed (again). */
>  	if (!cpu_isset(dest_cpu, p->cpus_allowed))
>  		goto out;
> 
> ---
> 
--
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