linux-kernel - Re: [PATCH v7 3/5] cpuset: Add a root-only cpus.isolated v2 control file

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Message-ID: <20180502140835.GK12180@hirez.programming.kicks-ass.net>
Date:   Wed, 2 May 2018 16:08:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kernel-team@...com, pjt@...gle.com, luto@...capital.net,
        Mike Galbraith <efault@....de>, torvalds@...ux-foundation.org,
        Roman Gushchin <guro@...com>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH v7 3/5] cpuset: Add a root-only cpus.isolated v2 control
 file

On Thu, Apr 19, 2018 at 09:47:02AM -0400, Waiman Long wrote:
> diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
> index c970bd7..8d89dc2 100644
> --- a/Documentation/cgroup-v2.txt
> +++ b/Documentation/cgroup-v2.txt
> @@ -1484,6 +1484,31 @@ Cpuset Interface Files
>  	a subset of "cpuset.cpus".  Its value will be affected by CPU
>  	hotplug events.
>  
> +  cpuset.cpus.isolated
> +	A read-write multiple values file which exists on root cgroup
> +	only.
> +
> +	It lists the CPUs that have been withdrawn from the root cgroup
> +	for load balancing.  These CPUs can still be allocated to child
> +	cpusets with load balancing enabled, if necessary.
> +
> +	If a child cpuset contains only an exclusive set of CPUs that are
> +	a subset of the isolated CPUs and with load balancing enabled,
> +	these CPUs will be load balanced on a separate root domain from
> +	the one in the root cgroup.
> +
> +	Just putting the CPUs into "cpuset.cpus.isolated" will be
> +	enough to disable load balancing on those CPUs as long as they
> +	do not appear in a child cpuset with load balancing enabled.
> +	Fine-grained control of cpu isolation can also be done by
> +	putting these isolated CPUs into child cpusets with load
> +	balancing disabled.
> +
> +	The "cpuset.cpus.isolated" should be set up before child
> +	cpusets are created.  Once child cpusets are present, changes
> +	to "cpuset.cpus.isolated" will not be allowed if the CPUs that
> +	change their states are in any of the child cpusets.
> +

So I see why you did this, but it is _really_ ugly and breaks the
container invariant.

Ideally we'd make the root group less special, not more special.