linux-kernel - Re: [PATCH v3 09/14] sched/core: uclamp: propagate parent clamps

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Message-ID: <804c7937-2c47-0781-9c53-a8ef8eb04530@arm.com>
Date:   Fri, 17 Aug 2018 15:43:59 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Patrick Bellasi <patrick.bellasi@....com>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Cc:     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 09/14] sched/core: uclamp: propagate parent clamps

On 08/06/2018 06:39 PM, Patrick Bellasi wrote:
> In order to properly support hierarchical resources control, the cgroup
> delegation model requires that attribute writes from a child group never
> fail but still are (potentially) constrained based on parent's assigned
> resources. This requires to properly propagate and aggregate parent
> attributes down to its descendants.

I don't understand the reason mentioned here:

IMHO, a write to a child's (tg1/tg11) cpu.rt_runtime_us can fail if the 
value is restricted by the parents value:

root@...o:/sys/fs/cgroup/cpu# cat cpu.rt_*
1000000
950000
root@...o:/sys/fs/cgroup/cpu# cat tg1/cpu.rt_*
1000000
0
root@...o:/sys/fs/cgroup/cpu# cat tg1/tg11/cpu.rt_*
1000000
0
root@...o:/sys/fs/cgroup/cpu# echo 950000 > tg1/tg11/cpu.rt_runtime_us
-bash: echo: write error: Invalid argument
root@...o:/sys/fs/cgroup/cpu# echo 950000 > tg1/cpu.rt_runtime_us
root@...o:/sys/fs/cgroup/cpu# echo 950000 > tg1/tg11/cpu.rt_runtime_us
root@...o:/sys/fs/cgroup/cpu#

> Let's implement this mechanism by adding a new "effective" clamp value
> for each task group. The effective clamp value is defined as the smaller
> value between the clamp value of a group and the effective clamp value
> of its parent. This represent also the clamp value which is actually
> used to clamp tasks in each task group.
> 
> Since it can be interesting for tasks in a cgroup to know exactly what
> is the currently propagated/enforced configuration, the effective clamp
> values are exposed to user-space by means of a new pair of read-only
> attributes: cpu.util.{min,max}.effective.

I assume here that the cpu.util.{min,max} of the child will not be used 
any more because the 'effective' counterparts are taken instead.

I wonder if this propagation not been provided with only 
cpu.util.{min,max}?

[...]