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Date:	Tue, 10 Nov 2009 14:12:27 +0900
From:	Kenji Kaneshige <>
To:	Peter Zijlstra <>
	Rusty Russell <>
Subject: Re: Kernel oops in resched_task() with

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 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.

Kenji Kaneshige

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