linux-kernel - Re: [PATCH v3 4/6] sched/rt: fast switch to maximum frequency when RT tasks are scheduled

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Date:   Thu, 30 Nov 2017 14:28:59 +0100
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, 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>,
        Todd Kjos <tkjos@...roid.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [PATCH v3 4/6] sched/rt: fast switch to maximum frequency when
 RT tasks are scheduled

Hi,

On 30/11/17 11:47, Patrick Bellasi wrote:
> Currently schedutil updates are triggered for the RT class using a single
> call place, which is part of the rt::update_curr_rt() used in:
> 
> - dequeue_task_rt:
>   but it does not make sense to set the schedutil's SCHED_CPUFREQ_RT in
>   case the next task should not be an RT one
> 
> - put_prev_task_rt:
>   likewise, we set the SCHED_CPUFREQ_RT flag without knowing if required
>   by the next task
> 
> - pick_next_task_rt:
>   likewise, the schedutil's SCHED_CPUFREQ_RT is set in case the prev task
>   was RT, while we don't yet know if the next will be RT
> 
> - task_tick_rt:
>   that's the only really useful call, which can ramp up the frequency in
>   case a RT task started its execution without a chance to order a
>   frequency switch (e.g. because of the schedutil ratelimit)
> 
> Apart from the last call in task_tick_rt, the others are at least useless.
> Thus, although being a simple solution, not all the call sites of that
> update_curr_rt() are interesting to trigger a frequency switch as well as
> some of the most interesting points are not covered by that call.
> For example, a task set to RT has to wait the next tick to get the
> frequency boost.
> 
> This patch fixes these issues by placing explicitly the schedutils
> update calls in the only sensible places, which are:
> - when an RT task wakes up and it's enqueued in a CPU
> - when we actually pick a RT task for execution
> - at each tick time
> - when a task is set to be RT
> 
> Signed-off-by: Patrick Bellasi <patrick.bellasi@....com>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>

Reviewed-by: Juri Lelli <juri.lelli@...hat.com>

Best,

Juri