linux-kernel - Re: [PATCH v6 04/11] cpufreq/schedutil: use rt utilization tracking

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Message-ID: <20180622144840.GE2512@hirez.programming.kicks-ass.net>
Date:   Fri, 22 Jun 2018 16:48:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Quentin Perret <quentin.perret@....com>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        viresh kumar <viresh.kumar@...aro.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v6 04/11] cpufreq/schedutil: use rt utilization tracking

On Fri, Jun 22, 2018 at 04:11:59PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 22, 2018 at 03:54:24PM +0200, Vincent Guittot wrote:
> > On Fri, 22 Jun 2018 at 15:26, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > > define f (u,r,n) { return u + ((u/(1-r)) - u) * (u/(1-r))^n; }

> > I'm a bit lost with your example.
> > u = 0.2 (for cfs) and r=0.7 (let say for rt) in your example and idle is 0.1
> > 
> > For rt task, we run 0.7 of the time at f=1 then we will select f=0.4
> > for run cfs task with u=0.2 but u is the utilization at f=1 which
> > means that it will take 250% of normal time to execute at f=0.4 which
> > means 0.5  time instead of 0.2 at f=1 so we are going out of time. In
> > order to have enough time to run r and u we must run at least  f=0.666
> > for cfs = 0.2/(1-0.7). 
> 
> Argh.. that is n=0. So clearly I went off the rails somewhere.

Aah, I think the number I've been computing is a 'corrected' u. Not an
f. It made sure that 0 idle got u=1, but no more.