linux-kernel - Re: [PATCH v8 0/6] cgroup/cpuset: Add new cpuset partition type & empty effecitve cpus

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Date:   Wed, 10 Nov 2021 19:15:58 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     "Moessbauer, Felix" <felix.moessbauer@...mens.com>
Cc:     "longman@...hat.com" <longman@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
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        "frederic@...nel.org" <frederic@...nel.org>,
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        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
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        "jan.kiszka@...mens.com" <jan.kiszka@...mens.com>,
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Subject: Re: [PATCH v8 0/6] cgroup/cpuset: Add new cpuset partition type &
 empty effecitve cpus

On Wed, Nov 10, 2021 at 03:21:54PM +0000, "Moessbauer, Felix" <felix.moessbauer@...mens.com> wrote:
> 2. Threads can be started on non-rt CPUs and then bound to a specific rt CPU.
> This binding can be specified before thread creation via pthread_create.
> By that, you can make sure that at no point in time a thread has a
> "forbidden" CPU in its affinities.

It should boil down to some clone$version(2) and sched_setaffinity(2)
calls, so strictly speaking even with pthread_create(3) the thread is
shortly running with the parent's affinity.

> With cgroup2, you cannot guarantee the second aspect, as thread
> creation and moving to a cgroup is not an atomic operation.

As suggested by others, CLONE_INTO_CGROUP (into cpuset cgroup) can
actually "hide" the migration into the clone3() call.

> At creation time, you cannot set the final affinity mask (as you
> create it in the non-rt group and there the CPU is not in the
> cpuset.cpus).
> Once you move the thread to the rt cgroup, it has a default mask and
> by that can be executed on other rt cores.

Good point. Perhaps you could work this around by having another level
of (non-root partition) cpuset cgroups for individual CPUs? (Maybe
there's more clever approach, this is just first to come into my mind.)

Michal