linux-kernel - Re: [PATCH v12 9/9] cpuset: Support forced turning off of partition flag

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Message-ID: <20180827164004.GE3978217@devbig004.ftw2.facebook.com>
Date:   Mon, 27 Aug 2018 09:40:04 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Peter Zijlstra <peterz@...radead.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 v12 9/9] cpuset: Support forced turning off of partition
 flag

Hello, Waiman.

On Mon, Aug 27, 2018 at 10:41:24AM -0400, Waiman Long wrote:
> Cpuset allows arbitrary modification of cpu list in "cpuset.cpus"
> even if the requested CPUs won't be granted in "cpuset.cpus.effective"
> as restricted by its parent. However, the validate_change() function
> will inhibit removal of CPUs that have been used in child cpusets.
> 
> Being a partition root, however, limits the kind of cpu list
> modification that is allowed. Adding CPUs is not allowed if the new
> CPUs are not in the parent's effective cpu list that can be put into
> "cpuset.cpus.reserved". In addition, a child partition cannot exhaust
> all the parent's effective CPUs.
> 
> Because of the implicit cpu exclusive nature of the partition root,
> cpu changes that break that cpu exclusivity will not be allowed. Other
> changes that break the conditions of being a partition root is generally
> allowed. However, the partition flag of the cpuset as well those of
> the descendant partitions will be forcefully turned off.

First of all, thanks a lot for your persistence.  I'm not necessarily
against the flag being forced off but wonder whether the same
config/effective approach can be used as in .cpus and .mems.  Can you
elaborate a bit on the choice here?

Thank you.

-- 
tejun