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..
