On Sun, 23 Nov 2014, Chris Mason wrote:
> On Sun, Nov 23, 2014 at 4:05 PM, Thomas Gleixner <t...@linutronix.de> wrote:
> > On Sun, 23 Nov 2014, Chris Mason wrote:
> > > On Sun, Nov 23, 2014 at 11:32 AM, Borislav Petkov <b...@alien8.de> wrote:
> > > > On Sun, Nov 23, 2014 at 11:16:51AM -0500, Chris Mason wrote:
> > > > > It must be:
> > > > >
> > > > > commit 6e998916dfe327e785e7c2447959b2c1a3ea4930
> > > > > Author: Stanislaw Gruszka <sgrus...@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.
The fork bomb will kick the migration task pretty often into life, so
the probablity of do_task_stat() to hit a running migration thread is
higher than on a normaly loaded machine.
Thanks,
tglx
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