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

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Message-ID: <20180420144245.GB14391@e108498-lin.cambridge.arm.com>
Date:   Fri, 20 Apr 2018 15:42:45 +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

Hi Leo,

On Wednesday 18 Apr 2018 at 20:15:47 (+0800), Leo Yan wrote:
> Sorry I introduce mess at here to spread my questions in several
> replying, later will try to ask questions in one replying.  Below are
> more questions which it's good to bring up:
> 
> The code for energy computation is quite neat and simple, but I think
> the energy computation mixes two concepts for CPU util: one concept is
> the estimated CPU util which is used to select CPU OPP in schedutil,
> another concept is the raw CPU util according to CPU real running time;
> for example, cpu_util_next() predicts CPU util but this value might be
> much higher than cpu_util(), especially after enabled UTIL_EST feature
> (I have shallow understanding for UTIL_EST so correct me as needed);

I'm not not sure to understand what you mean by higher than cpu_util()
here ... In which case would that happen ?

cpu_util_next() is basically used to figure out what will be the
cpu_util() of CPU A after task p has been enqueued on CPU B (no matter
what A and B are).

> but this patch simply computes CPU capacity and energy with the single
> one CPU utilization value (and it will be an inflated value afte enable
> UTIL_EST).  Is this purposed for simple implementation?
> 
> IMHO, cpu_util_next() can be used to predict CPU capacity, on the other
> hand, should we use the CPU util without UTIL_EST capping for 'sum_util',
> this can be more reasonable to reflect the CPU utilization?

Why would a decayed utilisation be a better estimate of the time that
a task is going to spend on a CPU ?

> 
> Furthermore, if we consider RT thread is running on CPU and connect with
> 'schedutil' governor, the CPU will run at maximum frequency, but we
> cannot say the CPU has 100% utilization.  The RT thread case is not
> handled in this patch.

Right, we don't account for RT tasks in the OPP prediction for now.
Vincent's patches to have a util_avg for RT runqueues could help us
do that I suppose ...

Thanks !
Quentin