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SubjectRe: New crashes walking proc with Saturday's git
On Sun, Nov 23, 2014 at 4:11 PM, Chris Mason <clm@fb.com> wrote:
>
>
> On Sun, Nov 23, 2014 at 4:05 PM, Thomas Gleixner <tglx@linutronix.de>
> wrote:
>> On Sun, 23 Nov 2014, Chris Mason wrote:
>>> On Sun, Nov 23, 2014 at 11:32 AM, Borislav Petkov <bp@alien8.de>
>>> wrote:
>>> > On Sun, Nov 23, 2014 at 11:16:51AM -0500, Chris Mason wrote:
>>> > > It must be:
>>> > >
>>> > > commit 6e998916dfe327e785e7c2447959b2c1a3ea4930
>>> > > Author: Stanislaw Gruszka <sgruszka@redhat.com>
>>> > > Date: Wed Nov 12 16:58:44 2014 +0100
>>> > >
>>> > > sched/cputime: Fix clock_nanosleep()/clock_gettime()
>>> inconsistency
>>> > >
>>> > > I'll do two runs to confirm, but it's the only related patch
>>> between rc5
>>> > > and
>>> > > now.
>>>
>>> I've adding Ingo and Stanislaw to the cc. With
>>> 6e998916dfe327e785e7c2447959b2c1a3ea4930 reverted, I'm no longer
>>> crashing.
>>>
>>> Repeating the stack trace for the new cc list. I see the crash
>>> with atop or
>>> similar walkers of /proc racing against exiting programs. Given
>>> the NULL rip,
>>> this line from the patch is probably broken, but it really feels
>>> like we
>>> should be falling over on p->sched_class and not on the
>>> update_curr func.
>>>
>>> + p->sched_class->update_curr(rq);
>>>
>>> I'm leaving my fork bomb running on two machines with the patch
>>> reverted to
>>> make sure.
>>
>> The sched_class instances which do not have update_curr are stop_task
>> and idle. Patch below.
>>
>> I'm sure nobody thought about the stats read code path here.
>>
>> [ 1053.759741] [<ffffffff81208348>] do_task_stat+0x8b8/0xb00
>>
>> do_task_stat(()
>> thread_group_cputime_adjusted()
>> thread_group_cputime()
>> task_cputime()
>> task_sched_runtime()
>> if (task_current(rq, p) && task_on_rq_queued(p)) {
>> update_rq_clock(rq);
>> p->sched_class->update_curr(rq);
>> }
>>
>> Now if the stats are read for a stomp machine task, aka 'migration/N'
>> and that task is current on its cpu. Ooops.
>>
>> I added the callback for idle tasks as well for completeness sake.
>
> This does make sense, but it doesn't match with the crash being much
> more likely during the fork bomb. The difference is crashing within
> a few hours vs crashing within 5 minutes.
>
> But, maybe I just got lucky. I'll try the patch.

11 minutes later and it's still alive. I'll keep an eye on it and yell
if it falls over.

-chris





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