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 ?

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