linux-kernel - Re: [PATCH v3 06/14] sched/cpufreq: uclamp: add utilization clamping for RT tasks

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Date:   Thu, 9 Aug 2018 18:03:57 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     Juri Lelli <juri.lelli@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        viresh kumar <viresh.kumar@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        "Cc: Steve Muckle" <smuckle@...gle.com>, surenb@...gle.com
Subject: Re: [PATCH v3 06/14] sched/cpufreq: uclamp: add utilization clamping
 for RT tasks

Hi Patrick,

On Thu, 9 Aug 2018 at 17:34, Patrick Bellasi <patrick.bellasi@....com> wrote:
>
> On 07-Aug 15:26, Juri Lelli wrote:
> > Hi,
> >
> > On 06/08/18 17:39, Patrick Bellasi wrote:
> >
> > [...]
> >
> > > @@ -223,13 +224,25 @@ static unsigned long sugov_get_util(struct sugov_cpu *sg_cpu)
> > >      * utilization (PELT windows are synchronized) we can directly add them
> > >      * to obtain the CPU's actual utilization.
> > >      *
> > > -    * CFS utilization can be boosted or capped, depending on utilization
> > > -    * clamp constraints configured for currently RUNNABLE tasks.
> > > +    * CFS and RT utilizations can be boosted or capped, depending on
> > > +    * utilization constraints enforce by currently RUNNABLE tasks.
> > > +    * They are individually clamped to ensure fairness across classes,
> > > +    * meaning that CFS always gets (if possible) the (minimum) required
> > > +    * bandwidth on top of that required by higher priority classes.
> >
> > Is this a stale comment written before UCLAMP_SCHED_CLASS was
> > introduced? It seems to apply to the below if branch only.
>
> Yes, you right... I'll update the comment.
>
> > >      */
> > > -   util = cpu_util_cfs(rq);
> > > -   if (util)
> > > -           util = uclamp_util(cpu_of(rq), util);
> > > -   util += cpu_util_rt(rq);
> > > +   util_cfs = cpu_util_cfs(rq);
> > > +   util_rt  = cpu_util_rt(rq);
> > > +   if (sched_feat(UCLAMP_SCHED_CLASS)) {
> > > +           util = 0;
> > > +           if (util_cfs)
> > > +                   util += uclamp_util(cpu_of(rq), util_cfs);
> > > +           if (util_rt)
> > > +                   util += uclamp_util(cpu_of(rq), util_rt);
> > > +   } else {
> > > +           util  = cpu_util_cfs(rq);
> > > +           util += cpu_util_rt(rq);
> > > +           util  = uclamp_util(cpu_of(rq), util);
> > > +   }
>
> Regarding the two policies, do you have any comment?

Does the policy for (sched_feat(UCLAMP_SCHED_CLASS)== true) really
make sense as it is ?
I mean, uclamp_util doesn't make any difference between rt and cfs
tasks when clamping the utilization so why should be add twice the
returned value ?
IMHO, this policy would make sense if there were something like
uclamp_util_rt() and a uclamp_util_cfs()

>
> We had an internal discussion and we found pro/cons for both... but
> I'm not sure keeping the sched_feat is a good solution on the long
> run, i.e. mainline merge ;)
>
> --
> #include <best/regards.h>
>
> Patrick Bellasi