linux-kernel - Re: [PATCH v3 1/2] sched/cpufreq: always consider blocked FAIR utilization

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Date:   Sun, 27 May 2018 11:50:12 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>
Subject: Re: [PATCH v3 1/2] sched/cpufreq: always consider blocked FAIR utilization

On Thu, May 24, 2018 at 4:10 PM, Patrick Bellasi
<patrick.bellasi@....com> wrote:
> Since the refactoring introduced by:
>
>    commit 8f111bc357aa ("cpufreq/schedutil: Rewrite CPUFREQ_RT support")
>
> we aggregate FAIR utilization only if this class has runnable tasks.
> This was mainly due to avoid the risk to stay on an high frequency just
> because of the blocked utilization of a CPU not being properly decayed
> while the CPU was idle.
>
> However, since:
>
>    commit 31e77c93e432 ("sched/fair: Update blocked load when newly idle")
>
> the FAIR blocked utilization is properly decayed also for IDLE CPUs.
>
> This allows us to use the FAIR blocked utilization as a safe mechanism
> to gracefully reduce the frequency only if no FAIR tasks show up on a
> CPU for a reasonable period of time.
>
> Moreover, we also reduce the frequency drops of CPUs running periodic
> tasks which, depending on the task periodicity and the time required
> for a frequency switch, was increasing the chances to introduce some
> undesirable performance variations.
>
> Reported-by: Vincent Guittot <vincent.guittot@...aro.org>
> Signed-off-by: Patrick Bellasi <patrick.bellasi@....com>
> Acked-by: Viresh Kumar <viresh.kumar@...aro.org>
> Acked-by: Vincent Guittot <vincent.guittot@...aro.org>
> Tested-by: Vincent Guittot <vincent.guittot@...aro.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Cc: Viresh Kumar <viresh.kumar@...aro.org>
> Cc: Joel Fernandes <joelaf@...gle.com>
> Cc: linux-kernel@...r.kernel.org
> Cc: linux-pm@...r.kernel.org

Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

Or please let me know if you want me to apply this one.

> ---
> Changes in v3:
>  - add "Tested-by" and "Acked-by" Vincent tags
>
> Changes in v2:
>  - add "Acked-by" Viresh tag
> ---
>  kernel/sched/cpufreq_schedutil.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index e13df951aca7..28592b62b1d5 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -183,22 +183,21 @@ static void sugov_get_util(struct sugov_cpu *sg_cpu)
>  static unsigned long sugov_aggregate_util(struct sugov_cpu *sg_cpu)
>  {
>         struct rq *rq = cpu_rq(sg_cpu->cpu);
> -       unsigned long util;
>
> -       if (rq->rt.rt_nr_running) {
> -               util = sg_cpu->max;
> -       } else {
> -               util = sg_cpu->util_dl;
> -               if (rq->cfs.h_nr_running)
> -                       util += sg_cpu->util_cfs;
> -       }
> +       if (rq->rt.rt_nr_running)
> +               return sg_cpu->max;
>
>         /*
> +        * Utilization required by DEADLINE must always be granted while, for
> +        * FAIR, 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 util_dl as min/guaranteed freq and
>          * util_cfs + util_dl as requested freq. However, cpufreq is not yet
>          * ready for such an interface. So, we only do the latter for now.
>          */
> -       return min(util, sg_cpu->max);
> +       return min(sg_cpu->max, (sg_cpu->util_dl + sg_cpu->util_cfs));
>  }
>
>  static void sugov_set_iowait_boost(struct sugov_cpu *sg_cpu, u64 time, unsigned int flags)
> --
> 2.15.1
>