Messages in this thread |  | | Date | Thu, 5 Apr 2018 16:13:26 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v9 10/10] cpuidle: menu: Avoid selecting shallow states with stopped tick |
| |
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@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?
To clarify, RETAIN keeps the status quo, if its off, it stays off, if its on it stays on.
|  |