linux-kernel - Re: [PATCH v3 2/3] sched/fair: use util

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Date:   Thu, 25 Jan 2018 20:03:58 +0530
From:   Pavan Kondeti <pkondeti@...eaurora.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...roid.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>
Subject: Re: [PATCH v3 2/3] sched/fair: use util_est in LB and WU paths

On Wed, Jan 24, 2018 at 07:31:38PM +0000, Patrick Bellasi wrote:
> 
> > > +       /*
> > > +        * These are the main cases covered:
> > > +        * - if *p is the only task sleeping on this CPU, then:
> > > +        *      cpu_util (== task_util) > util_est (== 0)
> > > +        *   and thus we return:
> > > +        *      cpu_util_wake = (cpu_util - task_util) = 0
> > > +        *
> > > +        * - if other tasks are SLEEPING on the same CPU, which is just waking
> > > +        *   up, then:
> > > +        *      cpu_util >= task_util
> > > +        *      cpu_util > util_est (== 0)
> > > +        *   and thus we discount *p's blocked utilization to return:
> > > +        *      cpu_util_wake = (cpu_util - task_util) >= 0
> > > +        *
> > > +        * - if other tasks are RUNNABLE on that CPU and
> > > +        *      util_est > cpu_util
> > > +        *   then we use util_est since it returns a more restrictive
> > > +        *   estimation of the spare capacity on that CPU, by just considering
> > > +        *   the expected utilization of tasks already runnable on that CPU.
> > > +        */
> > > +       util_est = cpu_rq(cpu)->cfs.util_est_runnable;
> > > +       util = max(util, util_est);
> > > +
> > > +       return util;
> 
> I should instead clamp util before returning it! ;-)
> 
> > May be a separate patch to remove  the clamping part?
> 
> No, I think we should keep cpu_util_wake clamped to not affect the existing
> call sites. I just need to remove it where not needed (done) and add it where
> needed (will do on the next iteration).

cpu_util_wake() is called only from capacity_spare_wake(). There are no other
callsites. The capacity_spare_wake() is clamping the return value of
cpu_util_wake() to CPU capacity. The clamping is not needed, I think.

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