linux-kernel - Re: [RFC v3 0/5] Add capacity capping support to the CPU controller

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Date:   Wed, 12 Apr 2017 16:41:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Paul Turner <pjt@...gle.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        John Stultz <john.stultz@...aro.org>,
        Todd Kjos <tkjos@...roid.com>,
        Tim Murray <timmurray@...gle.com>,
        Andres Oportus <andresoportus@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Juri Lelli <juri.lelli@....com>,
        Chris Redpath <chris.redpath@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [RFC v3 0/5] Add capacity capping support to the CPU controller

On Wed, Apr 12, 2017 at 02:34:48PM +0100, Patrick Bellasi wrote:
> On 12-Apr 14:15, Peter Zijlstra wrote:
> > On Tue, Apr 11, 2017 at 06:58:33PM +0100, Patrick Bellasi wrote:
> > > We should consider also that at the CPUFreq side we already expose
> > > knobs like scaling_{min,max}_freq which are much more platform
> > > dependant than capacity.
> > 
> > So I've always objected to these knobs.
> > 
> > That said; they are a pre-existing user interface so changing them isn't
> > really feasible much.
> > 
> > But at the very least we should integrate them properly. Which for
> > schedutil would mean they have to directly modify the relevant CPU
> > capacity values. Is this currently done? (I think not.)
> 
> AFAICS they are clamping the policy decisions, thus the frequency
> domain... which can be more then one CPU on ARM platforms.

Right, knew that. Must've missed the 's' when typing ;-)

> When you say you would like them to "directly modify the relevant CPU
> capacity values" I really see this as exactly what we do with
> capacity_{min,max}.

What I mean is that when we set max as lower than max-freq, it should
reduce the CPU(s)' capacity. That's something entirely different from what
you're attempting (and not something we do currently afaik).

Also; there's as yet unexplored interaction between these knobs and the
RT bandwidth limits.

But the main point is that these knobs are system wide and do not affect
tasks.