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Sender: Tejun Heo <htejun@gmail.com>
Date:   Mon, 10 Jul 2023 15:00:07 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Shuah Khan <shuah@kernel.org>, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kselftest@vger.kernel.org,
        Juri Lelli <juri.lelli@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Mrunal Patel <mpatel@redhat.com>,
        Ryan Phillips <rphillips@redhat.com>,
        Brent Rowsell <browsell@redhat.com>,
        Peter Hunt <pehunt@redhat.com>, Phil Auld <pauld@redhat.com>
Subject: Re: [PATCH v4 0/9] cgroup/cpuset: Support remote partitions
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Hello,

On Mon, Jul 10, 2023 at 08:33:11PM -0400, Waiman Long wrote:
> I would like to clarify that withdrawal of CPUs from cpuset.cpus.exclusive
> is always allowed. It is the addition of CPUs not presents in cpuset.cpus
> that will be rejected. The invariant is that cpuset.cpus.exclusive must
> always be a subset of cpuset.cpus. Any change that violates this rule is not
> allowed. Alternately I can silently dropped the offending CPUs without
> returning an error, but that may surprise users.

Right, that'd be confusing.

> BTW, withdrawal of CPUs from cpuset.cpus will also withdraw them from
> cpuset.cpus.exclusive, if present. This allows the partition code to use
> cpuset.cpus.exclusive directly to determine the allowable exclusive CPUs
> without doing an intersection with cpuset.cpus each time it is used.

This is kinda confusing too, I think. Changing cpuset.cpus in an ancestor
doesn't affect the contents of the descendants' cpuset.cpus files but would
directly modify the contents of their cpuset.cpus.exclusive files.

There's some inherent friction because cpuset.cpus separates configuration
(cpuset.cpus) and the current state (cpuset.cpus.effective) while
cpuset.cpus.exclusive is trying to do both in the same interface file. When
the two behavior modes collide, it becomes rather confusing. Do you think
it'd make sense to make cpus.exclusive follow the same pattern as
cpuset.cpus?

Thanks.

-- 
tejun