linux-kernel - Re: [PATCH v6 09/16] sched/cpufreq: uclamp: Add utilization clamping for RT tasks

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Date:   Thu, 24 Jan 2019 16:05:18 +0000
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-api@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v6 09/16] sched/cpufreq: uclamp: Add utilization clamping
 for RT tasks

On 24-Jan 16:15, Peter Zijlstra wrote:
> On Tue, Jan 15, 2019 at 10:15:06AM +0000, Patrick Bellasi wrote:
> > Add uclamp_default_perf, a special set of clamp values to be used
> > for tasks requiring maximum performance, i.e. by default all the non
> > clamped RT tasks.
> 
> Urgh... why though?

Because tasks without a task-specific clamp value assigned are subject
to system default... but those, by default, don't clamp at all:

   uclamp_default = [0..1024]

For RT tasks however, we want to execute then always at max OPP, if
not otherwise requested (e.g. task-specific value). Thus we need a
different set of default clamps:

   uclamp_perf_default = [1024..1024]

-- 
#include <best/regards.h>

Patrick Bellasi