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

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Message-ID: <20160831143907.GX10121@twins.programming.kicks-ass.net>
Date:   Wed, 31 Aug 2016 16:39:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steve Muckle <steve.muckle@...aro.org>
Cc:     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>,
        Steve Muckle <smuckle@...aro.org>
Subject: Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required
 RT CPU capacity

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.

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