Subject: Re: [PATCH V2] clockevents: Fix cpu down race for hrtimer based broadcasting

From: Ingo Molnar <[email-masked]>
Date: Thu, 2 Apr 2015 13:31:41 +0200

* Preeti U Murthy <[email-masked]> wrote:

> On 04/02/2015 04:12 PM, Ingo Molnar wrote:
> > 
> > * Preeti U Murthy <[email-masked]> wrote:
> > 
> >> It was found when doing a hotplug stress test on POWER, that the machine
> >> either hit softlockups or rcu_sched stall warnings.  The issue was
> >> traced to commit 7cba160ad789a powernv/cpuidle: Redesign idle states
> >> management, which exposed the cpu down race with hrtimer based broadcast
> >> mode(Commit 5d1638acb9f6(tick: Introduce hrtimer based broadcast). This
> >> is explained below.
> >>
> >> Assume CPU1 is the CPU which holds the hrtimer broadcasting duty before
> >> it is taken down.
> >>
> >> CPU0					CPU1
> >>
> >> cpu_down()				take_cpu_down()
> >> 					disable_interrupts()
> >>
> >> cpu_die()
> >>
> >>  while(CPU1 != CPU_DEAD) {
> >>   msleep(100);
> >>    switch_to_idle();
> >>     stop_cpu_timer();
> >>      schedule_broadcast();
> >>  }
> >>
> >> tick_cleanup_cpu_dead()
> >> 	take_over_broadcast()
> >>
> >> So after CPU1 disabled interrupts it cannot handle the broadcast hrtimer
> >> anymore, so CPU0 will be stuck forever.
> >>
> >> Fix this by explicitly taking over broadcast duty before cpu_die().
> >> This is a temporary workaround. What we really want is a callback in the
> >> clockevent device which allows us to do that from the dying CPU by
> >> pushing the hrtimer onto a different cpu. That might involve an IPI and
> >> is definitely more complex than this immediate fix.
> > 
> > So why not use a suitable CPU_DOWN* notifier for this, instead of open 
> > coding it all into a random place in the hotplug machinery?
> 
> This is because each of them is unsuitable for a reason:
> 
> 1. CPU_DOWN_PREPARE stage allows for a fail. The cpu in question may not
> successfully go down. So we may pull the hrtimer unnecessarily.

Failure is really rare - and as long as things will continue to work 
afterwards it's not a problem to pull the hrtimer to this CPU. Right?

Thanks,

	Ingo