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Date:   Fri, 14 Apr 2023 13:38:49 -0400
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Subject: Re: [RFC PATCH 0/5] cgroup/cpuset: A new "isolcpus" paritition
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To:     Tejun Heo <tj@kernel.org>
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>
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From:   Waiman Long <longman@redhat.com>
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On 4/14/23 13:34, Tejun Heo wrote:
> On Fri, Apr 14, 2023 at 01:29:25PM -0400, Waiman Long wrote:
>> On 4/14/23 12:54, Tejun Heo wrote:
>>> On Thu, Apr 13, 2023 at 09:22:19PM -0400, Waiman Long wrote:
>>>> I now have a slightly different idea of how to do that. We already have an
>>>> internal cpumask for partitioning - subparts_cpus. I am thinking about
>>>> exposing it as cpuset.cpus.reserve. The current way of creating
>>>> subpartitions will be called automatic reservation and require a direct
>>>> parent/child partition relationship. But as soon as a user write anything to
>>>> it, it will break automatic reservation and require manual reservation going
>>>> forward.
>>>>
>>>> In that way, we can keep the old behavior, but also support new use cases. I
>>>> am going to work on that.
>>> I'm not sure I fully understand the proposed behavior but it does sound more
>>> quirky.
>> The idea is to use the existing subparts_cpus for cpu reservation instead of
>> adding a new cpumask for that purpose. The current way of partition creation
>> does cpus reservation (setting subparts_cpus) automatically with the
>> constraint that the parent of a partition must be a partition root itself.
>> One way to relax this constraint is to allow a new manual reservation mode
>> where users can set reserve cpus manually and distribute them down the
>> hierarchy before activating a partition to use those cpus.
>>
>> Now the question is how to enable this new manual reservation mode. One way
>> to do it is to enable it whenever the new cpuset.cpus.reserve file is
>> modified. Alternatively, we may enable it by a cgroupfs mount option or a
>> boot command line option.
> It'd probably be best if we can keep the behavior within cgroupfs if
> possible. Would you mind writing up the documentation section describing the
> behavior beforehand? I think things would be clearer if we look at it from
> the interface documentation side.

Sure, will do that. I need some time and so it will be early next week.

Cheers,
Longman