linux-kernel - Re: [PATCH v3 1/3] sched/fair: add util

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Date:   Wed, 24 Jan 2018 19:16:09 +0000
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux PM <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>,
        Steve Muckle <smuckle@...gle.com>
Subject: Re: [PATCH v3 1/3] sched/fair: add util_est on top of PELT

On 24-Jan 08:40, Joel Fernandes wrote:
> On Tue, Jan 23, 2018 at 10:08 AM, Patrick Bellasi
> <patrick.bellasi@....com> wrote:
> > The util_avg signal computed by PELT is too variable for some use-cases.
> > For example, a big task waking up after a long sleep period will have its
> > utilization almost completely decayed. This introduces some latency before
> > schedutil will be able to pick the best frequency to run a task.
> [...]
> > -static inline unsigned long task_util(struct task_struct *p);
> >  static unsigned long cpu_util_wake(int cpu, struct task_struct *p);
> >
> >  static unsigned long capacity_spare_wake(int cpu, struct task_struct *p)
> > @@ -6262,6 +6337,11 @@ static inline unsigned long task_util(struct task_struct *p)
> >         return p->se.avg.util_avg;
> >  }
> >
> > +static inline unsigned long task_util_est(struct task_struct *p)
> > +{
> > +       return max(p->se.avg.util_est.ewma, p->se.avg.util_est.last);
> > +}
> > +
> >  /*
> >   * cpu_util_wake: Compute cpu utilization with any contributions from
> >   * the waking task p removed.
> > diff --git a/kernel/sched/features.h b/kernel/sched/features.h
> > index 9552fd5854bf..c459a4b61544 100644
> > --- a/kernel/sched/features.h
> > +++ b/kernel/sched/features.h
> > @@ -85,3 +85,8 @@ SCHED_FEAT(ATTACH_AGE_LOAD, true)
> >  SCHED_FEAT(WA_IDLE, true)
> >  SCHED_FEAT(WA_WEIGHT, true)
> >  SCHED_FEAT(WA_BIAS, true)
> > +
> > +/*
> > + * UtilEstimation. Use estimated CPU utilization.
> > + */
> > +SCHED_FEAT(UTIL_EST, false)
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index 2e95505e23c6..0b4d9750a927 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -470,6 +470,7 @@ struct cfs_rq {
> >          * CFS load tracking
> >          */
> >         struct sched_avg avg;
> > +       unsigned long util_est_runnable;
> 
> Since struct sched_avg would now have util_est, cfs_rq gets it too.
> Then can we not try to reuse that struct and avoid having to expand
> cfs_rq more than needed?

Yes, that's possible now... the main issue is that for RQ's we do not
track an EWMA, but still we can use the util_est::last field or maybe
use a union just to use a better name when used from the RQ side.

> I went through previous conversations and couldn't find a reason, if I
> missed something I appreciate if you can explain the rationale.

I've used a separate filed just because SE's util_est was not part of
sched_avg, and missed the opportunity to consolidate better this now
that we moved it. Thanks for pointing this out ;-)

> 
> thanks,
> 
> - Joel

Cheers Patrick

-- 
#include <best/regards.h>

Patrick Bellasi