Message-ID: <CAKfTPtDSuvX5_GEYQ9HgqdEn-EwiCx7MGHtY73GysQh6m9kZFA@mail.gmail.com>
Date:   Fri, 22 Jun 2018 19:24:24 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.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, 22 Jun 2018 at 17:23, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Jun 22, 2018 at 01:37:13PM +0200, Peter Zijlstra wrote:
> > That is true.. So we could limit the scaling to the case where there is
> > no idle time, something like:
> >
> >       util = sg_cpu->util_cfs;
> >
> >       cap_cfs = (1024 - (sg_cpu->util_rt + ...));
> >       if (util == cap_cfs)
> >               util = sg_cpu->max;
> >
>
> OK, it appears this is more or less what the patches do. And I think
> there's a small risk/hole with this where util ~= cap_cfs but very close
> due to some unaccounted time.
>
> FWIW, when looking, I saw no reason why sugov_get_util() and
> sugov_aggregate_util() were in fact separate functions.

good point

>
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -53,11 +53,7 @@ struct sugov_cpu {
>         unsigned int            iowait_boost_max;
>         u64                     last_update;
>
> -       /* The fields below are only needed when sharing a policy: */
> -       unsigned long           util_cfs;
>         unsigned long           util_dl;
> -       unsigned long           bw_dl;
> -       unsigned long           util_rt;
>         unsigned long           max;
>
>         /* The field below is for single-CPU policies only: */
> @@ -181,44 +177,38 @@ static unsigned int get_next_freq(struct
>         return cpufreq_driver_resolve_freq(policy, freq);
>  }
>
> -static void sugov_get_util(struct sugov_cpu *sg_cpu)
> +static unsigned long sugov_get_util(struct sugov_cpu *sg_cpu)
>  {
>         struct rq *rq = cpu_rq(sg_cpu->cpu);
> +       unsigned long util, max;
>
> -       sg_cpu->max = arch_scale_cpu_capacity(NULL, sg_cpu->cpu);
> -       sg_cpu->util_cfs = cpu_util_cfs(rq);
> -       sg_cpu->util_dl  = cpu_util_dl(rq);
> -       sg_cpu->bw_dl    = cpu_bw_dl(rq);
> -       sg_cpu->util_rt  = cpu_util_rt(rq);
> -}
> -
> -static unsigned long sugov_aggregate_util(struct sugov_cpu *sg_cpu)
> -{
> -       struct rq *rq = cpu_rq(sg_cpu->cpu);
> -       unsigned long util;
> +       sg_cpu->max = max = arch_scale_cpu_capacity(NULL, sg_cpu->cpu);
> +       sg_cpu->util_dl   = cpu_util_dl(rq);
>
>         if (rq->rt.rt_nr_running)
> -               return sg_cpu->max;
> +               return max;
>
> -       util = sg_cpu->util_cfs;
> -       util += sg_cpu->util_rt;
> +       util  = cpu_util_cfs(rq);
> +       util += cpu_util_rt(rq);
>
> -       if ((util + sg_cpu->util_dl) >= sg_cpu->max)
> -               return sg_cpu->max;
> +       /*
> +        * If there is no idle time, we should run at max frequency.
> +        */
> +       if ((util + cpu_util_dl(rq)) >= max)
> +               return max;
>
>         /*
> -        * As there is still idle time on the CPU, we need to compute the
> -        * utilization level of the CPU.
>          * Bandwidth required by DEADLINE must always be granted while, for
>          * FAIR and RT, we use blocked utilization of IDLE CPUs as a mechanism
>          * to gracefully reduce the frequency when no tasks show up for longer
>          * periods of time.
> +        *
> +        * Ideally we would like to set bw_dl as min/guaranteed freq and bw_dl
> +        * + util as requested freq. However, cpufreq is not yet ready for such
> +        * an interface. So, we only do the latter for now.
>          */
>
> -       /* Add DL bandwidth requirement */
> -       util += sg_cpu->bw_dl;
> -
> -       return min(sg_cpu->max, util);
> +       return min(max, cpu_bw_dl(rq) + util);
>  }
>
>  /**
> @@ -396,9 +386,8 @@ static void sugov_update_single(struct u
>
>         busy = sugov_cpu_is_busy(sg_cpu);
>
> -       sugov_get_util(sg_cpu);
> +       util = sugov_get_util(sg_cpu);
>         max = sg_cpu->max;
> -       util = sugov_aggregate_util(sg_cpu);
>         sugov_iowait_apply(sg_cpu, time, &util, &max);
>         next_f = get_next_freq(sg_policy, util, max);
>         /*
> @@ -437,9 +426,8 @@ static unsigned int sugov_next_freq_shar
>                 struct sugov_cpu *j_sg_cpu = &per_cpu(sugov_cpu, j);
>                 unsigned long j_util, j_max;
>
> -               sugov_get_util(j_sg_cpu);
> +               j_util = sugov_get_util(j_sg_cpu);
>                 j_max = j_sg_cpu->max;
> -               j_util = sugov_aggregate_util(j_sg_cpu);
>                 sugov_iowait_apply(j_sg_cpu, time, &j_util, &j_max);
>
>                 if (j_util * max > j_max * util) {
>

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