linux-kernel - Re: [PATCH] sched/fair: Introduce fits

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Message-ID: <20190618082209.xshfus2t626o2dgb@queper01-lin>
Date:   Tue, 18 Jun 2019 09:22:12 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH] sched/fair: Introduce fits_capacity()

On Tuesday 18 Jun 2019 at 13:17:28 (+0530), Viresh Kumar wrote:
> On 18-06-19, 09:26, Rafael J. Wysocki wrote:
> > On Tue, Jun 18, 2019 at 5:12 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > >
> > > +Rafael
> > >
> > > On 17-06-19, 17:02, Peter Zijlstra wrote:
> > > > On Thu, Jun 06, 2019 at 08:22:04AM +0530, Viresh Kumar wrote:
> > > > > Hmm, even if the values are same currently I am not sure if we want
> > > > > the same for ever. I will write a patch for it though, if Peter/Rafael
> > > > > feel the same as you.
> > > >
> > > > Is it really the same variable or just two numbers that happen to be the
> > > > same?
> > >
> > > In both cases we are trying to keep the load under 80% of what can be supported.
> > > But I am not sure of the answer to your question.
> > >
> > > Maybe Rafael knows :)
> > 
> > Which variable?
> 
> Schedutil multiplies the target frequency by 1.25 (20% more capacity eventually)
> to get enough room for more load and similar thing is done in fair.c at several
> places to see if the new task can fit in a runqueue without overloading it.
> 
> Quentin suggested to use common code for this calculation and that is what is
> getting discussed here.

Right, sugov and load balance happen to use the same margin (1.25)
to check if a given util fits in a given capacity, though the thresholds
are hardcoded in different places (see map_util_freq() and
capacity_margin). So my suggestion was to unify the capacity_margin code
for frequency selection and CPU selection, for clarity and consistency.

But again, this is a small thing and FWIW Viresh's patch LGTM as-is so
no objection from my end if you guys would like to merge it.

Thanks,
Quentin