On 2018/3/6 16:45, Peter Zijlstra wrote: > On Tue, Mar 06, 2018 at 10:15:10AM +0800, Li, Aubrey wrote: >> On 2018/3/5 21:53, Peter Zijlstra wrote: >>> On Mon, Mar 05, 2018 at 02:05:10PM +0100, Rafael J. Wysocki wrote: >>>> On Mon, Mar 5, 2018 at 1:50 PM, Peter Zijlstra <pet...@infradead.org> >>>> wrote: >>>>> On Mon, Mar 05, 2018 at 12:47:23PM +0100, Rafael J. Wysocki wrote: >>> >>>>>> 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'. >> >> I understood the benefit of mapping duration to state number, is duration <-> >> state number mapping a generic solution to all arches? > > Yes, all platforms have a limited set of possible idle states. > >> Back to the user's concern is, "I'm running a latency sensitive application, >> and >> I want idle switching ASAP". So I think the user may not care about what C >> state >> to go into, that is, even if a deeper state has chance to go, the user >> striving >> for a higher workload score may still not want it? > > The user caring about performance very much cares about the actual idle > state too, exit latency for deeper states is horrific and will screw > them up just as much as the whole nohz timer reprogramming does. > > We can basically view the whole nohz thing as an additional entry/exit > latency for the idle state, which is why I don't think its weird to > couple them.
okay, I see your point now. Thanks! > >>>> Maybe just return a "nohz" indicator from cpuidle_select() in addition >>>> to the state index and make the decision in the governor? >>> >>> Much better option than returning a duration :-) >>> >> So what does "nohz = disable and state index = deepest" mean? This >> combination >> does not make sense for performance only purpose? > > I tend to agree with you that the state space allowed by a separate > variable is larger than required, but it's significantly smaller than > preserving 'time' so I can live with it. >
