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

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Date:   Wed, 31 Aug 2016 18:00:02 +0100
From:   Juri Lelli <Juri.Lelli@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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>,
        Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required
 RT CPU capacity

On 31/08/16 18:40, Peter Zijlstra wrote:
> On Wed, Aug 31, 2016 at 06:28:10PM +0200, Thomas Gleixner wrote:
> > > 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.
> > 
> > RT deterministic behaviour is all about meeting the deadlines. If your
> > deadline is relaxed enough that you can meet it even with the lowest cpu
> > frequency then it's perfectly fine to enable cpufreq. The same logic applies
> > to C-States.
> > 
> > There are a lot of RT systems out there which enable both. If cpufreq or
> > c-states cause a deadline violation because the constraints of the system are
> > tight, then people will disable it and we need a knob for both.
> > 
> > Realtime is not as fast as possible. It's as fast as specified.
> 
> Sure, problem is of course that RR/FIFO doesn't specify anything so the
> users are left to prod knobs.
> 
> Another problem is that we have many semi related knobs; we have the
> global RT runtime limit knob, but that doesn't affect cpufreq (maybe it
> should)

Maybe we could create this sort of link when using the cgroup RT
throttling interface as well? It should still then fit well once we
replace the underlying mechanism with DL reservations. And, AFAIK, the
interface is used by Android folks already.

> and cpufreq has knobs to set f_min and f_max, which again are
> unaware of RT anything.
> 
> So before we go do anything, I'd like input on what is needed and how
> things should tie together to make most sense.
>