linux-kernel - Re: [RFC PATCH v2 4/6] sched/fair: Introduce an energy estimation helper function

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Date:   Wed, 18 Apr 2018 12:06:35 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thara Gopinath <thara.gopinath@...aro.org>,
        linux-pm@...r.kernel.org,
        Morten Rasmussen <morten.rasmussen@....com>,
        Chris Redpath <chris.redpath@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steve Muckle <smuckle@...gle.com>,
        Eduardo Valentin <edubezval@...il.com>
Subject: Re: [RFC PATCH v2 4/6] sched/fair: Introduce an energy estimation
 helper function

On Wednesday 18 Apr 2018 at 17:19:28 (+0800), Leo Yan wrote:
> > > BTW, CPU utilization is decayed value and task_util() is not decayed
> > > value, so 'util - task_util(p)' calculates a smaller value than the
> > > prev CPU pure utilization, right?
> > 
> > task_util() is the raw PELT signal, without UTIL_EST, so I think it's
> > fine to do `util - task_util()`.
> > 
> > > 
> > > Another question is can we reuse the function cpu_util_wake() and
> > > just compenstate task util for dst cpu?
> > 
> > Well it's not that simple. cpu_util_wake() will give you the max between
> > the util_avg and the util_est value, so which task_util() should you add
> > to it ? The util_avg or the uti_est value ?
> 
> If feature 'UTIL_EST' is enabled, then add task's util_est; otherwise
> add task util_avg value.

I don't think this is correct. If UTIL_EST is enabled, cpu_util_wake()
will return the max between the util_avg and the util_est. When you call
it, you don't know what you get. Adding the _task_util_est() to
cpu_util_wake() is wrong if cpu_util_avg > cpu_util_est for example.

> 
> I think cpu_util_wake() has similiar logic with here, it merely returns
> CPU level util; but here needs to accumulate CPU level util + task level
> util.  So seems to me, the logic is:
> 
>   cpu_util_wake() + task_util_wake()

I'm not sure to get that one ... What is task_util_wake() ?

Thanks,
Quentin