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Date:	Tue, 10 Nov 2009 14:12:27 +0900
From:	Kenji Kaneshige <kaneshige.kenji@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	mingo@...e.hu, 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 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,
Kenji Kaneshige


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