linux-kernel - Re: [RFC PATCH 0/5] cgroup/cpuset: A new "isolcpus" paritition

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Date:   Fri, 14 Apr 2023 07:34:05 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Juri Lelli <juri.lelli@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [RFC PATCH 0/5] cgroup/cpuset: A new "isolcpus" paritition

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.

Thanks.

-- 
tejun