linux-kernel - Re: [PATCH v3 07/14] sched/core: uclamp: enforce last task UCLAMP

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Message-ID: <20180816172702.GH2960@e110439-lin>
Date:   Thu, 16 Aug 2018 18:27:02 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paul Turner <pjt@...gle.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 v3 07/14] sched/core: uclamp: enforce last task UCLAMP_MAX

On 16-Aug 19:10, Dietmar Eggemann wrote:
> On 08/16/2018 06:47 PM, Patrick Bellasi wrote:
> >On 16-Aug 17:43, Dietmar Eggemann wrote:
> >>On 08/06/2018 06:39 PM, Patrick Bellasi wrote:
> >>>When a util_max clamped task sleeps, its clamp constraints are removed
> >>>from the CPU. However, the blocked utilization on that CPU can still be
> >>>higher than the max clamp value enforced while that task was running.
> >>>This max clamp removal when a CPU is going to be idle could thus allow
> >>>unwanted CPU frequency increases, right while the task is not running.
> >>
> >>So 'rq->uclamp.flags == UCLAMP_FLAG_IDLE' means CPU is IDLE because
> >>non-clamped tasks are tracked as well ((group_id = 0)).
> >
> >Right, but... with (group_id = 0) you mean that "non-clamped tasks are
> >tracked" in the first clamp group?
> 
> Yes. I was asking myself what will happen if there are only non-clamped
> tasks runnable ...

Non clamped tasks is kind-of ambiguous, since you can have:
a) tasks with util_max = UCLAMP_NOT_VALID (the default for all tasks)
b) tasks with util_max = SCHED_CAPACITY_SCALE as a task specific
    clamp value

They are both technically not clamped but, for case b there should not
be issue, since we will track SCHED_CAPACITY_SCALE as idle hold value.

For case a instead is a bit different, especially when they mix with
tasks with a valid task specific clamp value, as I've just commented
in this posting:

   Message-ID: <20180816172016.GG2960@...0439-lin>

> 
> >
> >>Maybe this is worth mentioning here?
> >
> >Maybe I can explicitely say that we detect that there are not RUNNABLE
> >tasks because all the clamp groups are in UCLAMP_NOT_VALID status.
> 
> Yes, would have helped me the grasp this earlier ...

Right, I'm going to add a bit of text on that.

Cheers Patrick

-- 
#include <best/regards.h>

Patrick Bellasi