Re: [PATCH][3.10] nohz: Fix lockup on restart from wrong error code

From: Steven Rostedt
Date: Tue May 21 2013 - 20:09:48 EST

On Tue, 2013-05-21 at 22:14 +0200, Frederic Weisbecker wrote:
> 2013/5/21 Steven Rostedt <rostedt@xxxxxxxxxxx>:
> > commit a382bf934449 "nohz: Assign timekeeping duty to a CPU outside the
> > full dynticks range" added a cpu notifier callback that would prevent
> > the time keeping CPU from going offline if the have_nohz_full_mask was
> > set.
> >
> > This also prevents the CPU from going offline on system reboot.
> >
> > Worse yet, the return code was -EINVAL, but the notifier does not
> > recognize error codes, and it must be wrapped by a notifier_from_errno()
> > function. This means that even though the CPU would fail to go down, the
> > notifier would think it succeeded, and the cpu down process would
> > continue.
> >
> > This caused two different problems. One, the migration thread after
> > moving tasks from the CPU would park itself and then a task, namely the
> > reboot task, could migrate onto that CPU. Then the reboot task spins
> > waiting for the cpu to go idle. But because the reboot task happens to
> > be spinning on the cpu its waiting for, the system hangs.
> Can that happen if that CPU is the boot CPU? Note this is the only
> possible timekeeper with the upstream code.

Yep it can happen in upstream (that's all I'm using). In
tick_broadcast_setup_oneshot(), it sets the tick_do_timer_cpu to the
current CPU, which can be something other than the boot CPU. Now that
CPU wont be able to be hot plugged.

> >
> > The other error that happened was that the sched_domain re-setup would
> > get confused, and in get_group() the cpu = cpumask_first() would process
> > a mask that had nothing set, and return cpu > nr_cpu_ids. Later it would
> > reference the per_cpu sg with this CPU and get a bogus pointer and
> > crash.
> Ouch, when are we doing this domain re-setup? I remember we
> repartition the domains after cpu down/up but I don't understand how
> that can interfere with this issue.

I haven't looked hard enough yet, but this problem only appeared when
this bug triggered. By telling the system a CPU is offline, but still
having tasks schedule to it, causes all sorts of weird side effects. I
haven't figured out in detail how this affected the sched domains, but I
don't get the sched domain corruption after fixing this bug.

-- Steve

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