linux-kernel - Re: [PATCH v2 4/6] cpufreq: schedutil: update CFS util only if used

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Date:   Thu, 6 Jul 2017 22:58:31 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     LKML <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>,
        Juri Lelli <juri.lelli@....com>,
        Andres Oportus <andresoportus@...gle.com>,
        Todd Kjos <tkjos@...roid.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v2 4/6] cpufreq: schedutil: update CFS util only if used

On Tue, Jul 4, 2017 at 10:34 AM, Patrick Bellasi
<patrick.bellasi@....com> wrote:
> Currently the utilization of the FAIR class is collected before locking
> the policy. Although that should not be a big issue for most cases, we
> also don't really know how much latency there can be between the
> utilization reading and its usage.
>
> Let's get the FAIR utilization right before its usage to be better in
> sync with the current status of a CPU.
>
> Signed-off-by: Patrick Bellasi <patrick.bellasi@....com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Cc: Viresh Kumar <viresh.kumar@...aro.org>
> Cc: linux-kernel@...r.kernel.org
> Cc: linux-pm@...r.kernel.org
> ---
>  kernel/sched/cpufreq_schedutil.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 98704d8..df433f1 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -308,10 +308,9 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
>         if (unlikely(current == sg_policy->thread))
>                 return;
>
> -       sugov_get_util(&util, &max);
> -
>         raw_spin_lock(&sg_policy->update_lock);
>
> +       sugov_get_util(&util, &max);
>         sg_cpu->util = util;
>         sg_cpu->max = max;

Is your concern that there will we spinlock contention before calling
sugov_get_util?

If that's the case, with your patch it seems to me such contention
(and hence spinning) itself could cause the utilization to be inflated
- thus calling sugov_get_util after acquiring the lock will not be as
accurate as before. In that case it seems to me its better to let
sugov_get_util be called before acquiring the lock (as before).

thanks,

-Joel