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

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Date:   Wed, 3 May 2023 00:27:24 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Tejun Heo <tj@...nel.org>, 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 Tue, May 02, 2023 at 05:26:17PM -0400, Waiman Long <longman@...hat.com> wrote:
> In the new scheme, the available cpus are still directly passed down to a
> descendant cgroup. However, isolated CPUs (or more generally CPUs dedicated
> to a partition) have to be exclusive. So what the cpuset.cpus.reserve does
> is to identify those exclusive CPUs that can be excluded from the
> effective_cpus of the parent cgroups before they are claimed by a child
> partition. Currently this is done automatically when a child partition is
> created off a parent partition root. The new scheme will break it into 2
> separate steps without the requirement that the parent of a partition has to
> be a partition root itself.

new scheme
  1st step:
  echo C >p/cpuset.cpus.reserve
  # p/cpuset.cpus.effective == A-C (1)
  2nd step (claim):
  echo C' >p/c/cpuset.cpus # C'⊆C
  echo root >p/c/cpuset.cpus.partition

current scheme
  1st step (configure):
  echo C >p/c/cpuset.cpus
  2nd step (reserve & claim):
  echo root >p/c/cpuset.cpus.partition
  # p/cpuset.cpus.effective == A-C (2)

As long as p/c is unpopulated, (1) and (2) are equal situations.
Why is the (different) two step procedure needed?

Also the relaxation of requirement of a parent being a partition
confuses me -- if the parent is not a partition, i.e. it has no
exclusive ownership of CPUs but it can still "give" it to children -- is
child partition meant to be exclusive? (IOW can parent siblings reserve
some same CPUs?)

Thanks,
Michal