Re: Kernel oops in resched_task() with 2.6.31.5

[Kenji Kaneshige]

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@localhost ~]$ cat /sys/devices/system/cpu/present
> >     0-15
> >     [kanesige@localhost ~]$ cat /sys/devices/system/cpu/online
> >     0-15
> >     [kanesige@localhost ~]$ 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.

Thanks,
Kenji Kaneshige


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