From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Li, Aubrey" Subject: Re: [RFC/RFT][PATCH 4/7] cpuidle: menu: Split idle duration prediction from state selection Date: Tue, 6 Mar 2018 22:07:01 +0800 Message-ID: <73d0ffca-7496-12f3-e1b7-a96494507817@linux.intel.com> References: <1657351.s4RTvEoqBQ@aspire.rjw.lan> <2332986.m9oRvTSu8E@aspire.rjw.lan> <20180305113803.GO25201@hirez.programming.kicks-ass.net> <20180305125012.GA25181@hirez.programming.kicks-ass.net> <20180305135322.GV25201@hirez.programming.kicks-ass.net> <43825f88-762e-11e5-d70a-45152b95599d@linux.intel.com> <20180306084536.GZ25201@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20180306084536.GZ25201@hirez.programming.kicks-ass.net> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Peter Zijlstra Cc: "Rafael J. Wysocki" , "Rafael J. Wysocki" , Thomas Gleixner , Frederic Weisbecker , Paul McKenney , Thomas Ilsche , Doug Smythies , Rik van Riel , Mike Galbraith , LKML , Linux PM List-Id: linux-pm@vger.kernel.org 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 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. >