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Date:	Tue, 10 Nov 2009 06:15:37 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Kenji Kaneshige <kaneshige.kenji@...fujitsu.com>,
	stable kernel team <stable@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: Kernel oops in resched_task() with 2.6.31.5


* Kenji Kaneshige <kaneshige.kenji@...fujitsu.com> wrote:

> Kenji Kaneshige wrote:
> >Peter Zijlstra wrote:
> >>On Mon, 2009-11-09 at 21:31 +0900, Kenji Kaneshige wrote:
> >>>Hi,
> >>>
> >>>I frequently encounter the kernel oops attached below in resched_task()
> >>>with 2.6.31.5. This kernel oops happens also with 2.6.32-rc5. I don't
> >>>know about other kernel.
> >>>
> >>>Here is my analysis:
> >>>
> >>>The immediate cause of this kernel oops is that NULL was passed to
> >>>resched_task() from resched_cpu(). From my investigation, this was
> >>>caused as follows:
> >>>
> >>>- trigger_load_balance() caluculated cpu number of idle load balancer
> >>>  using find_new_ilb(), and find_new_ilb() returned *offline* CPU
> >>>  number (16 in my case). Note that I didn't do any CPU hotplug
> >>>  operation. On my system, present, online and offline under
> >>>  /sys/devices/system/cpu/ are
> >>>
> >>>    [kanesige@...alhost ~]$ cat /sys/devices/system/cpu/present
> >>>    0-15
> >>>    [kanesige@...alhost ~]$ cat /sys/devices/system/cpu/online
> >>>    0-15
> >>>    [kanesige@...alhost ~]$ cat /sys/devices/system/cpu/offline
> >>>    16-255
> >>>
> >>>  And nr_cpu_ids is 256.
> >>>
> >>>- resched_cpu() calculated current task by cpu_curr() with offline CPU
> >>>  number.
> >>>
> >>>So this kernel oops seems to be caused by invalid CPU number returned
> >>>from find_new_ilb(). I don't know the find_new_ilb() implementation,
> >>>but I suspect the initialization of cpumasks used by find_new_ilb().
> >>>The patch attached below seems to fix the problem (With this patch,
> >>>the kernel oops doesn't happen). But I don't know if this is the
> >>>correct fix.
> >>
> >>Please send patches against -tip.
> >>
> >>You might find that Rusty has already fixed a similar issue there in
> >>commit: 49557e620339cb134127b5bfbcfecc06b77d0232.
> >>
> >>Now, Rusty's patch does not clear the ilb mask, so maybe it doesn't
> >>fully cover your issue, please test.
> >>
> >
> >Thank you for quick response.
> >
> >I didn't notice Rusty's fix.
> >I'll look at and test it tomorrow.
> >
> 
> I tested Rusty's patch and confirmed it fixes the problem.

Thanks.

-stable team, please cherry-pick this upstream commit for .31.x:

 49557e6: sched: Fix boot crash by zalloc()ing most of the cpu masks

	Ingo
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