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

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Message-ID: <20180305135322.GV25201@hirez.programming.kicks-ass.net>
Date:   Mon, 5 Mar 2018 14:53:22 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Thomas Ilsche <thomas.ilsche@...dresden.de>,
        Doug Smythies <dsmythies@...us.net>,
        Rik van Riel <riel@...riel.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Mike Galbraith <mgalbraith@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [RFC/RFT][PATCH 4/7] cpuidle: menu: Split idle duration
 prediction from state selection

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@...radead.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'.
> >
> > 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 :-)