linux-kernel - Re: [PATH v2] cgroup: add cgroup_favordynmods= command-line option

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Message-ID: <zqgnootbsjyj7wq5ayiallz4m2bz7ad6w67t24peglhc5v6ern@4pfqlshgpnie>
Date:   Thu, 7 Sep 2023 17:57:27 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Luiz Capitulino <luizcap@...zon.com>
Cc:     tj@...nel.org, lizefan.x@...edance.com, hannes@...xchg.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        longman@...hat.com, lcapitulino@...il.com
Subject: Re: [PATH v2] cgroup: add cgroup_favordynmods= command-line option

On Thu, Sep 07, 2023 at 11:16:41AM -0400, Luiz Capitulino <luizcap@...zon.com> wrote:
> Yeah, this works for cgroup v2 but my understanding is that cgroup v1
> doesn't support changing flags in remount, take a look at
> cgroup1_reconfigure().

Ah, didn't notice.
Alhtough -- there seems to be a deeper issue -- the mount option doesn't
have a per-root semantics. There is only a single
cgroup_threadgroup_rwsem afterall.

Even with your cmdline option, you may loose the behavior after
unmounting any of the v1 hierarchies (cgroup_destroy_root()
unconditionally disables it).

Or you could still achieve the result by mounting cgroup2 hierarchy with
favordynmods. (And unmount it, default root is not ever released.)

Maybe it would be the best to have this controllable only via v2
hierarchy (as it's the only documented).
(And maybe v1s should not show the option at all.)

My 0.02€,
Michal

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