linux-kernel - Re: [PATCH V2] cpufreq: schedutil: Redefine the rate_limit

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Date:   Fri, 24 Feb 2017 00:36:31 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        eas-dev@...ts.linaro.org
Subject: Re: [PATCH V2] cpufreq: schedutil: Redefine the rate_limit_us tunable

On Tue, Feb 21, 2017 at 5:45 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> The rate_limit_us tunable is intended to reduce the possible overhead
> from running the schedutil governor.  However, that overhead can be
> divided into two separate parts: the governor computations and the
> invocation of the scaling driver to set the CPU frequency.  The latter
> is where the real overhead comes from.  The former is much less
> expensive in terms of execution time and running it every time the
> governor callback is invoked by the scheduler, after rate_limit_us
> interval has passed since the last frequency update, would not be a
> problem.
>
> For this reason, redefine the rate_limit_us tunable so that it means the
> minimum time that has to pass between two consecutive invocations of the
> scaling driver by the schedutil governor (to set the CPU frequency).
>
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>

I'd prefer this to spend some time in linux-next before it goes into
the mainline, so I will queue it up for 4.12 if no one objects by the
end of the next week.

Thanks,
Rafael


> ---
> V1->V2: Update $subject and commit log (Rafael)
>
>  kernel/sched/cpufreq_schedutil.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index fd4659313640..306d97e7b57c 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -92,14 +92,13 @@ static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time,
>  {
>         struct cpufreq_policy *policy = sg_policy->policy;
>
> -       sg_policy->last_freq_update_time = time;
> -
>         if (policy->fast_switch_enabled) {
>                 if (sg_policy->next_freq == next_freq) {
>                         trace_cpu_frequency(policy->cur, smp_processor_id());
>                         return;
>                 }
>                 sg_policy->next_freq = next_freq;
> +               sg_policy->last_freq_update_time = time;
>                 next_freq = cpufreq_driver_fast_switch(policy, next_freq);
>                 if (next_freq == CPUFREQ_ENTRY_INVALID)
>                         return;
> @@ -108,6 +107,7 @@ static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time,
>                 trace_cpu_frequency(next_freq, smp_processor_id());
>         } else if (sg_policy->next_freq != next_freq) {
>                 sg_policy->next_freq = next_freq;
> +               sg_policy->last_freq_update_time = time;
>                 sg_policy->work_in_progress = true;
>                 irq_work_queue(&sg_policy->irq_work);
>         }
> --
> 2.7.1.410.g6faf27b
>