Re: WARN_ON_ONCE() hit at kernel/events/core.c:330

[Thomas-Mich Richter]

On 4/4/19 3:03 PM, Peter Zijlstra wrote:
> On Thu, Apr 04, 2019 at 01:09:09PM +0200, Peter Zijlstra wrote:
> 
>> That is not entirely the scenario I talked about, but *groan*.
>>
>> So what I meant was:
>>
>> 	CPU-0							CPU-n
>>
>> 	__schedule()
>> 	  local_irq_disable()
>>
>> 	  ...
>> 	    deactivate_task(prev);
>>
>> 								try_to_wake_up(@p)
>> 								  ...
>> 								  smp_cond_load_acquire(&p->on_cpu, !VAL);
>>
>> 	  <PMI>
>> 	    ..
>> 	    perf_event_disable_inatomic()
>> 	      event->pending_disable = 1;
>> 	      irq_work_queue() /* self-IPI */
>> 	  </PMI>
>>
>> 	  context_switch()
>> 	    prepare_task_switch()
>> 	      perf_event_task_sched_out()
>> 	        // the above chain that clears pending_disable
>>
>> 	    finish_task_switch()
>> 	      finish_task()
>> 	        smp_store_release(prev->on_cpu, 0);
>> 								  /* finally.... */
>> 								// take woken
>> 								// context_switch to @p
>> 	      finish_lock_switch()
>> 	        raw_spin_unlock_irq()
>> 		/* w00t, IRQs enabled, self-IPI time */
>> 	        <self-IPI>
>> 		  perf_pending_event()
>> 		    // event->pending_disable == 0
>> 		</self-IPI>
>>
>>
>> What you're suggesting, is that the time between:
>>
>>   smp_store_release(prev->on_cpu, 0);
>>
>> and
>>
>>   <self-IPI>
>>
>> on CPU-0 is sufficient for CPU-n to context switch to the task, enable
>> the event there, trigger a PMI that calls perf_event_disable_inatomic()
>> _again_ (this would mean irq_work_queue() failing, which we don't check)
>> (and schedule out again, although that's not required).
>>
>> This being virt that might actually be possible if (v)CPU-0 takes a nap
>> I suppose.
>>
>> Let me think about this a little more...
> 
> Does the below cure things? It's not exactly pretty, but it could just
> do the trick.
> 
> ---
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index dfc4bab0b02b..d496e6911442 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2009,8 +2009,8 @@ event_sched_out(struct perf_event *event,
>  	event->pmu->del(event, 0);
>  	event->oncpu = -1;
>  
> -	if (event->pending_disable) {
> -		event->pending_disable = 0;
> +	if (event->pending_disable == smp_processor_id()) {
> +		event->pending_disable = -1;
>  		state = PERF_EVENT_STATE_OFF;
>  	}
>  	perf_event_set_state(event, state);
> @@ -2198,7 +2198,7 @@ EXPORT_SYMBOL_GPL(perf_event_disable);
>  
>  void perf_event_disable_inatomic(struct perf_event *event)
>  {
> -	event->pending_disable = 1;
> +	event->pending_disable = smp_processor_id();
>  	irq_work_queue(&event->pending);
>  }
>  
> @@ -5822,8 +5822,8 @@ static void perf_pending_event(struct irq_work *entry)
>  	 * and we won't recurse 'further'.
>  	 */
>  
> -	if (event->pending_disable) {
> -		event->pending_disable = 0;
> +	if (event->pending_disable == smp_processor_id()) {
> +		event->pending_disable = -1;
>  		perf_event_disable_local(event);
>  	}
>  
> @@ -10236,6 +10236,7 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
>  
>  
>  	init_waitqueue_head(&event->waitq);
> +	event->pending_disable = -1;
>  	init_irq_work(&event->pending, perf_pending_event);
>  
>  	mutex_init(&event->mmap_mutex);
> 

Thanks for the quick reply, will give it a try...


-- 
Thomas Richter, Dept 3252, IBM s390 Linux Development, Boeblingen, Germany
--
Vorsitzender des Aufsichtsrats: Matthias Hartmann
GeschÃftsfÃhrung: Dirk Wittkopp
Sitz der Gesellschaft: BÃblingen / Registergericht: Amtsgericht Stuttgart, HRB 243294