linux-kernel - Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required RT CPU capacity

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Message-ID: <20160831150846.GN5599@graphite.smuckle.net>
Date:   Wed, 31 Aug 2016 08:08:46 -0700
From:   Steve Muckle <steve.muckle@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steve Muckle <steve.muckle@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <Juri.Lelli@....com>,
        Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required
 RT CPU capacity

On Wed, Aug 31, 2016 at 04:39:07PM +0200, Peter Zijlstra wrote:
> On Fri, Aug 26, 2016 at 11:40:48AM -0700, Steve Muckle wrote:
> > A policy of going to fmax on any RT activity will be detrimental
> > for power on many platforms. Often RT accounts for only a small amount
> > of CPU activity so sending the CPU frequency to fmax is overkill. Worse
> > still, some platforms may not be able to even complete the CPU frequency
> > change before the RT activity has already completed.
> > 
> > Cpufreq governors have not treated RT activity this way in the past so
> > it is not part of the expected semantics of the RT scheduling class. The
> > DL class offers guarantees about task completion and could be used for
> > this purpose.
> 
> Not entirely true. People have simply disabled cpufreq because of this.
>
> Yes, RR/FIFO are a pain, but they should still be deterministic, and
> variable cpufreq destroys that.

That is the way it's been with cpufreq and many systems (including all
mobile devices) rely on that to not destroy power. RT + variable cpufreq
is not deterministic.

Given we don't have good constraints on RT tasks I don't think we should
try to strengthen the semantics there. Folks should either move to DL if
they want determinism *and* not-sucky power, or continue disabling
cpufreq if they are able to do so.

> I realize that the fmax thing is annoying, but I'm not seeing how rt_avg
> is much better.

Rt_avg is much closer to the current behavior offered by the most
commonly used cpufreq governors since it tracks actual CPU utilization.
Power is not impacted by minimal RT activity and the frequency is raised
if RT activity is high.