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

[Peter Zijlstra]

On Fri, Apr 05, 2019 at 12:18:54PM +0200, Thomas-Mich Richter wrote:
> 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.
> > 
> 
> Thanks a lot for the patch, I have built a new kernel and let it run over the week end.
> 
> s390 does not have a PMI, all interrupts (including the measurement interrupts from 
> the PMU) are normal, maskable interrupts.

Please implement arch_irq_work_raise() for s390 though, traditionally
that was only required if you had NMI-like PMis, and IRQ based PMIs
would have to run irq_work_run() at the end of their handler.

The s390-sf handler does not do this, but even it if were to do that, it
wouldn't be sufficient, since you also drain the buffer from
pmu::stop().

Not doing this will get you some 'weird' behavoiur. As in, your signals
could be delivered very late etc.