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

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Message-ID: <20080630090744.GB6598@osiris.boeblingen.de.ibm.com>
Date:	Mon, 30 Jun 2008 11:07:44 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Dmitry Adamushko <dmitry.adamushko@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Avi Kivity <avi@...ranet.com>, linux-kernel@...r.kernel.org
Subject: Re: [BUG] CFS vs cpu hotplug

On Sun, Jun 29, 2008 at 12:16:56AM +0200, Dmitry Adamushko wrote:
> Hello,
> 
> 
> it seems to be related to migrate_dead_tasks().
> 
> Firstly I added traces to see all tasks being migrated with
> migrate_live_tasks() and migrate_dead_tasks(). On my setup the problem
> pops up (the one with "se == NULL" in the loop of
> pick_next_task_fair()) shortly after the traces indicate that some has
> been migrated with migrate_dead_tasks()). btw., I can reproduce it
> much faster now with just a plain cpu down/up loop.
> 
> [disclaimer] Well, unless I'm really missing something important in
> this late hour [/desclaimer] pick_next_task() is not something
> appropriate for migrate_dead_tasks() :-)
> 
> the following change seems to eliminate the problem on my setup
> (although, I kept it running only for a few minutes to get a few
> messages indicating migrate_dead_tasks() does move tasks and the
> system is still ok)
> 
> [ quick hack ]
> 
> @@ -5887,6 +5907,7 @@ static void migrate_dead_tasks(unsigned int dead_cpu)
>                 next = pick_next_task(rq, rq->curr);
>                 if (!next)
>                         break;
> +               next->sched_class->put_prev_task(rq, next);
>                 migrate_dead(dead_cpu, next);
> 
>         }

Thanks Dmitry! With your patch I cannot reproduce the bug anymore.
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