linux-kernel - Re: [RFC PATCH v4 4/6] sched/cpufreq: Introduce sugov_cpu_ramp

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Date:   Thu, 23 Jan 2020 22:02:26 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Douglas Raillard <douglas.raillard@....com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        qperret@...gle.com, Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [RFC PATCH v4 4/6] sched/cpufreq: Introduce sugov_cpu_ramp_boost

On Thu, Jan 23, 2020 at 6:21 PM Douglas Raillard
<douglas.raillard@....com> wrote:
>
>
>
> On 1/23/20 3:55 PM, Rafael J. Wysocki wrote:
> > On Wed, Jan 22, 2020 at 6:36 PM Douglas RAILLARD
> > <douglas.raillard@....com> wrote:
> >>
> >> Use the utilization signals dynamic to detect when the utilization of a
> >> set of tasks starts increasing because of a change in tasks' behavior.
> >> This allows detecting when spending extra power for faster frequency
> >> ramp up response would be beneficial to the reactivity of the system.
> >>
> >> This ramp boost is computed as the difference between util_avg and
> >> util_est_enqueued. This number somehow represents a lower bound of how
> >> much extra utilization this tasks is actually using, compared to our
> >> best current stable knowledge of it (which is util_est_enqueued).
> >>
> >> When the set of runnable tasks changes, the boost is disabled as the
> >> impact of blocked utilization on util_avg will make the delta with
> >> util_est_enqueued not very informative.
> >>
> >> Signed-off-by: Douglas RAILLARD <douglas.raillard@....com>
> >> ---
> >>  kernel/sched/cpufreq_schedutil.c | 43 ++++++++++++++++++++++++++++++++
> >>  1 file changed, 43 insertions(+)
> >>
> >> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> >> index 608963da4916..25a410a1ff6a 100644
> >> --- a/kernel/sched/cpufreq_schedutil.c
> >> +++ b/kernel/sched/cpufreq_schedutil.c
> >> @@ -61,6 +61,10 @@ struct sugov_cpu {
> >>         unsigned long           bw_dl;
> >>         unsigned long           max;
> >>
> >> +       unsigned long           ramp_boost;
> >> +       unsigned long           util_est_enqueued;
> >> +       unsigned long           util_avg;
> >> +
> >>         /* The field below is for single-CPU policies only: */
> >>  #ifdef CONFIG_NO_HZ_COMMON
> >>         unsigned long           saved_idle_calls;
> >> @@ -183,6 +187,42 @@ static void sugov_deferred_update(struct sugov_policy *sg_policy, u64 time,
> >>         }
> >>  }
> >>
> >> +static unsigned long sugov_cpu_ramp_boost(struct sugov_cpu *sg_cpu)
> >> +{
> >> +       return READ_ONCE(sg_cpu->ramp_boost);
> >> +}
> >
> > Where exactly is this function used?
>
> In the next commit where the boost value is actually used to do
> something. The function is introduced here to keep the
> WRITE_ONCE/READ_ONCE pair together.

But ramp_boost itself is not really used in this patch too AFAICS.