Messages in this thread |  | | From | "Rafael J. Wysocki" <> | Date | Mon, 5 Mar 2018 14:05:10 +0100 | Subject | Re: [RFC/RFT][PATCH 4/7] cpuidle: menu: Split idle duration prediction from state selection |
| |
On Mon, Mar 5, 2018 at 1:50 PM, Peter Zijlstra <peterz@infradead.org> wrote: > On Mon, Mar 05, 2018 at 12:47:23PM +0100, Rafael J. Wysocki wrote: >> On Mon, Mar 5, 2018 at 12:38 PM, Peter Zijlstra <peterz@infradead.org> wrote: >> > We really should be predicting state not duration. Yes the duration >> > thing is an intermediate value, but I don't think it makes any sense >> > what so ever to preserve that in the predictor. The end result is the >> > idle state, we should aim for that. >> > >> > As per: >> > >> > https://lkml.org/lkml/2017/7/18/615 >> > >> > there are definite advantages to _not_ preserving duration information >> > beyond the state boundaries. >> >> Well, OK >> >> The reason why I need the predicted idle duration is because the >> target residency of the selected state may be below the tick period >> duration and if this is the deepest state available, we still want to >> stop the tick if the predicted idle duration is long. > > Right, so in that case we'd split the deepest state and mark the > resulting smaller state as not disabling the tick and the resulting > larger state as disabling the tick. > > So suppose your deepest state is < TICK_USEC, then we introduce a copy > of that state, modify the boundary to be TICK_USEC and set the 'disable > tick for this state' thing to true. > >> IOW, the target residency of the selected state doesn't tell you how >> much time you should expect to be idle in general. > > Right, but I think that measure isn't of primary relevance. What we want > to know is: 'should I stop the tick' and 'what C state do I go to'. > > In order to answer those questions we need durations as input, but I > don't think we should preserve durations throughout. The scheme from the > above link reduces to N states in order to deal with arbitrary > distributions, only the actual states -- ie boundaries where our answers > changes -- are relevant, anything inside those boundaries would lead to > the exact same answer anyway.
I generally agree here, but I'm not convinced about flagging the states, splitting them and so on.
Maybe just return a "nohz" indicator from cpuidle_select() in addition to the state index and make the decision in the governor?
|  |