linux-kernel - Re: [PATCH] cpufreq: schedutil: rate limits for SCHED

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Date:   Fri, 9 Feb 2018 11:53:05 +0100
From:   Juri Lelli <juri.lelli@...hat.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Claudio Scordino <claudio@...dence.eu.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Todd Kjos <tkjos@...roid.com>,
        Joel Fernandes <joelaf@...gle.com>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpufreq: schedutil: rate limits for SCHED_DEADLINE

Hi,

On 09/02/18 11:36, Rafael J. Wysocki wrote:
> On Friday, February 9, 2018 9:02:34 AM CET Claudio Scordino wrote:
> > Hi Viresh,
> > 
> > Il 09/02/2018 04:51, Viresh Kumar ha scritto:
> > > On 08-02-18, 18:01, Claudio Scordino wrote:
> > >> When the SCHED_DEADLINE scheduling class increases the CPU utilization,
> > >> we should not wait for the rate limit, otherwise we may miss some deadline.
> > >>
> > >> Tests using rt-app on Exynos5422 have shown reductions of about 10% of deadline
> > >> misses for tasks with low RT periods.
> > >>
> > >> The patch applies on top of the one recently proposed by Peter to drop the
> > >> SCHED_CPUFREQ_* flags.
> > >>
> 
> [cut]
> 
> > 
> > > 
> > > Is it possible to (somehow) check here if the DL tasks will miss
> > > deadline if we continue to run at current frequency? And only ignore
> > > rate-limit if that is the case ?

Isn't it always the case? Utilization associated to DL tasks is given by
what the user said it's needed to meet a task deadlines (admission
control). If that task wakes up and we realize that adding its
utilization contribution is going to require a frequency change, we
should _theoretically_ always do it, or it will be too late. Now, user
might have asked for a bit more than what strictly required (this is
usually the case to compensate for discrepancies between theory and real
world, e.g.  hw transition limits), but I don't think there is a way to
know "how much". :/

Thanks,

- Juri

> > 
> > I need to think further about it.
> 
> That would be my approach FWIW.
> 
> Increasing the frequency beyond what is necessary means wasting energy
> in any case.
> 
> Thanks,
> Rafael
>