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 16:33:51 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
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 Thu, Jan 24, 2019 at 12:30:09PM +0000, Patrick Bellasi wrote:
> On 23-Jan 21:11, Peter Zijlstra wrote:

> > Anyway, once you get range composition defined; that should be something
> > like:
> > 
> > 	R_p \Compose_g R_g
> > 
> > Where R_p is the range of task-p, and R_g is the range of the g'th
> > cgroup of p (where you can make an identity between the root cgroup and
> > the system default).
> > 
> > Now; as per the other email; I think the straight forward composition:
> > 
> > struct range compose(struct range a, struct range b)
> > {
> > 	return (range){.min = clamp(a.min, b.min, b.max),
> > 	               .max = clamp(a.max, b.min, b.max), };
> > }
> 
> This composition is done in uclamp_effective_get() but it's
> slightly different, since we want to support a "nice policy" where
> tasks can always ask less then what they have got assigned.

Not sure I follow..

> Thus, from an abstract standpoint, if a task is in a cgroup:
> 
>      task.min <= R_g.min
>      task.max <= R_g.max
> 
> While, for tasks in the root cgroup system default applies and we
> enforece:
> 
>      task.min >= R_0.min
>      task.max <= R_0.max
> 
> ... where the "nice policy" is currently not more supported, but
> perhaps we can/should use the same for system defaults too.

That seems inconsistent at best.

OK, I'll go have another look. I never recognised that function for
doing that.