Re: [PATCH v9 10/10] cpuidle: menu: Avoid selecting shallow states with stopped tick

[Peter Zijlstra]

On Thu, Apr 05, 2018 at 04:11:27PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 05, 2018 at 03:49:32PM +0200, Rafael J. Wysocki wrote:
> > On Thu, Apr 5, 2018 at 2:47 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > On Wed, Apr 04, 2018 at 10:50:36AM +0200, Rafael J. Wysocki wrote:
> > >> +     if (tick_nohz_tick_stopped()) {
> > >> +             /*
> > >> +              * If the tick is already stopped, the cost of possible short
> > >> +              * idle duration misprediction is much higher, because the CPU
> > >> +              * may be stuck in a shallow idle state for a long time as a
> > >> +              * result of it.  In that case say we might mispredict and try
> > >> +              * to force the CPU into a state for which we would have stopped
> > >> +              * the tick, unless the tick timer is going to expire really
> > >> +              * soon anyway.
> > >
> > > Wait what; the tick was stopped, therefore it _cannot_ expire soon.
> > >
> > > *confused*
> > >
> > > Did you mean s/tick/a/ ?
> > 
> > Yeah, that should be "a timer".
> 
> *phew* ok, that makes a lot more sense ;-)
> 
> My only concern with this is that we can now be overly pessimistic. The
> predictor might know that statistically it's very likely a device
> interrupt will arrive soon, but because the tick is already disabled, we
> don't dare trust it, causing possible excessive latencies.
> 
> Would an alternative be to make @stop_tick be an enum capable of forcing
> the tick back on?
> 
> enum tick_action {
> 	NOHZ_TICK_STOP,
> 	NOHZ_TICK_RETAIN,
> 	NOHZ_TICK_START,
> };
> 
> 	enum tick_action tick_action = NOHZ_TICK_STOP;
> 
> 	state = cpuidle_select(..., &tick_action);
> 
> 	switch (tick_action) {
> 	case NOHZ_TICK_STOP:
> 		tick_nohz_stop_tick();
> 		break;
> 
> 	case NOHZ_TICK_RETAIN:
> 		tick_nozh_retain_tick();
> 		break;
> 
> 	case NOHZ_TICK_START:
> 		tick_nohz_start_tick();
> 		break;
> 	};
> 
> 
> Or something along those lines?

To clarify, RETAIN keeps the status quo, if its off, it stays off, if
its on it stays on.