linux-kernel - Re: [PATCH v12 3/6] sched/core: uclamp: Propagate system defaults to root group

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Date:   Thu, 25 Jul 2019 13:41:37 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     cgroups@...r.kernel.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Alessio Balsini <balsini@...roid.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Quentin Perret <quentin.perret@....com>,
        Joel Fernandes <joelaf@...gle.com>,
        Paul Turner <pjt@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Todd Kjos <tkjos@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Tejun Heo <tj@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v12 3/6] sched/core: uclamp: Propagate system defaults to
 root group

On Thu, Jul 18, 2019 at 07:17:45PM +0100, Patrick Bellasi <patrick.bellasi@....com> wrote:
> The clamp values are not tunable at the level of the root task group.
> That's for two main reasons:
> 
>  - the root group represents "system resources" which are always
>    entirely available from the cgroup standpoint.
> 
>  - when tuning/restricting "system resources" makes sense, tuning must
>    be done using a system wide API which should also be available when
>    control groups are not.
> 
> When a system wide restriction is available, cgroups should be aware of
> its value in order to know exactly how much "system resources" are
> available for the subgroups.
IIUC, the global default would apply in uclamp_eff_get(), so this
propagation isn't strictly necessary in order to apply to tasks (that's
how it works under !CONFIG_UCLAMP_TASK_GROUP).
The reason is that effective value (which isn't exposed currently) in a
group takes into account this global restriction, right?


> @@ -1043,12 +1063,17 @@ int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
> [...]
> +	if (update_root_tg)
> +		uclamp_update_root_tg();
> +
>  	/*
>  	 * Updating all the RUNNABLE task is expensive, keep it simple and do
>  	 * just a lazy update at each next enqueue time.
Since uclamp_update_root_tg() traverses down to
uclamp_update_active_tasks() is this comment half true now?


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