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

[Peter Zijlstra]

On Thu, Apr 04, 2019 at 02:02:14PM +0200, 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...
> 
> Arghh... s390 doesn't implement arch_irq_work_raise(), which makes it
> far far worse.
> 
> I have a hack that might've cured it, were it not for that. Let me think
> more still..

Could you educate me on the s390 PMU, afaict only the SF one has a
sampling interrupt (cpumf_measurement_alert), is that NMI-like or a
regular IRQ ?