Re: [PATCH/RFC] timer: fix deadlock on cpu hotplug

[Tejun Heo]

Hello,

On 09/21/2010 04:20 PM, Heiko Carstens wrote:
> For some reason the scheduler decided to throttle RT tasks on the runqueue
> of cpu 5 (rt_throttled = 1). So as long as rt_throttled == 1 we won't see the
> migration thread coming back to execution.
> The only thing that would unthrottle the runqueue would be the rt_period_timer.
> The timer is indeed scheduled, however in the dump I have it has been expired
> for more than four hours.
> The reason is simply that the timer is pending on the offlined cpu 0 and
> therefore would never fire before it gets migrated to an online cpu. Before
> the cpu hotplug mechanisms (cpu hotplug notifier with state CPU_DEAD) would
> migrate the timer to an online cpu stop_machine() must complete ---> deadlock.
> 
> The fix _seems_ to be simple: just migrate timers after __cpu_disable() has
> been called and use the CPU_DYING state. The subtle difference is of course
> that the migration code now gets executed on the cpu that actually just is
> going to disable itself instead of an arbitrary cpu that stays online.

I think this is the second time we're seeing deadlock during cpu down
due to RT throttling and timer problem.  The rather delicate
dependency there makes me somewhat nervous.  If possible, I think it
would be better if we can simply turn the RT throttling off when
cpu_stop kicks in.  It's intended to be a mechanism to monopolize all
CPU cycles to begin with.  Would that be difficult?

Thanks.

-- 
tejun
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