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

[Peter Zijlstra]

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?