linux-kernel - Re: [PATCH v10 5/9] cpuset: Make sure that domain roots work properly with CPU hotplug

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Date:   Wed, 20 Jun 2018 16:15:38 +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>,
        Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH v10 5/9] cpuset: Make sure that domain roots work
 properly with CPU hotplug

On Mon, Jun 18, 2018 at 12:14:04PM +0800, Waiman Long wrote:
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 5ee5e77..6ef3516 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1626,6 +1626,13 @@ Cpuset Interface Files
>  	2) No CPU that has been distributed to child scheduling domain
>  	   roots is deleted.
>  
> +	When all the CPUs allocated to a scheduling domain are offlined,
> +	that scheduling domain will be temporaily gone and all the
> +	tasks in that scheduling domain will migrate to another one that
> +	belongs to the parent of the scheduling domain root.  When any
> +	of those offlined CPUs is onlined again, a new scheduling domain
> +	will be re-created and the tasks will be migrated back.
> +

You should mention that this is a destructive operation. If any of the
tasks had an affinity smaller than the original cgroup, that will be
gone.