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

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Date:   Tue, 28 Jan 2020 15:38:06 +0000
From:   Douglas Raillard <douglas.raillard@....com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     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 1/23/20 9:02 PM, Rafael J. Wysocki wrote:
> 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.

I'll squash that patch with the next one where it's actually used then:
sched/cpufreq: Boost schedutil frequency ramp up

Thanks,
Douglas