Messages in this thread | | | Date | Thu, 5 Apr 2018 16:11:27 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v9 10/10] cpuidle: menu: Avoid selecting shallow states with stopped tick |
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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@infradead.org> 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?
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