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

From: Preeti U Murthy <[email-masked]>
Date: Thu, 02 Apr 2015 17:14:30 +0530

On 04/02/2015 05:01 PM, Ingo Molnar wrote:
> 
> * 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?

We will need to move this function to the clockevents_notify() call
under CPU_DOWN_PREPARE. But I see that Tglx wanted to get rid of the
clockevents_notify() function because it is more of a multiplex call and
less of a notification mechanism and get rid of this function explicitly.

Regards
Preeti U Murthy
> 
> Thanks,
> 
> 	Ingo
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