linux-kernel - Re: [PATCH v11 4/5] sched/core: uclamp: Use TG's clamps to restrict TASK's clamps

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Date:   Tue, 16 Jul 2019 15:34:35 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Michal Koutný <mkoutny@...e.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>,
        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>,
        Alessio Balsini <balsini@...roid.com>
Subject: Re: [PATCH v11 4/5] sched/core: uclamp: Use TG's clamps to restrict
 TASK's clamps

On 15-Jul 18:42, Michal Koutný wrote:
> On Mon, Jul 08, 2019 at 09:43:56AM +0100, Patrick Bellasi <patrick.bellasi@....com> wrote:
> > This mimics what already happens for a task's CPU affinity mask when the
> > task is also in a cpuset, i.e. cgroup attributes are always used to
> > restrict per-task attributes.
> If I am not mistaken when set_schedaffinity(2) call is made that results
> in an empty cpuset, the call fails with EINVAL [1].
> 
> If I track the code correctly, the values passed to sched_setattr(2) are
> checked against the trivial validity (umin <= umax) and later on, they
> are adjusted to match the effective clamping of the containing
> task_group. Is that correct?
> 
> If the user attempted to sched_setattr [a, b], and the effective uclamp
> was [c, d] such that [a, b] ∩ [c, d] = ∅, the set uclamp will be
> silently moved out of their intended range. Wouldn't it be better to
> return with EINVAL too when the intersection is empty (since the user
> supplied range won't be attained)?

You right for the cpuset case, but I don't think we never end up with
a "empty" set in the case of utilization clamping.

We limit clamps hierarchically in such a way that:

  clamp[clamp_id] = min(task::clamp[clamp_id],
                        tg::clamp[clamp_id],
                        system::clamp[clamp_id])

and we ensure, on top of the above that:

  clamp[UCLAMP_MIN] = min(clamp[UCLAMP_MIN], clamp[UCLAMP_MAX])

Since it's all and only about "capping" values, at the very extreme
case you can end up with:

  clamp[UCLAMP_MIN] = clamp[UCLAMP_MAX] = 0

but that's till a valid configuration.

Am I missing something?

Otherwise, I think the changelog sentence you quoted is just
misleading.  I'll remove it from v12 since it does not really clarify
anything more then the rest of the message.

Cheers,
Patrick

-- 
#include <best/regards.h>

Patrick Bellasi