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

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Date:	Sun, 29 Jun 2008 00:16:56 +0200
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	"Heiko Carstens" <heiko.carstens@...ibm.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

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);

        }

just in case, all the changes I've used for this test are attached "as is".

p.s. perhaps I won't be able to verify it carefully till tomorrow's
late evening.

-- 
Best regards,
Dmitry Adamushko

View attachment "migration-experiment.patch" of type "text/x-diff" (4206 bytes)