Re: [RFC/RFT][PATCH 4/7] cpuidle: menu: Split idle duration prediction from state selection

[Peter Zijlstra]

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 <peterz@xxxxxxxxxxxxx> 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'.
> >
> > 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.

I think linking them like that makes sense, but I can see room for
discussion...

> 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 :-)