linux-kernel - Cgroups v2 thread mode "domain invalid" state

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Message-ID: <CAKgNAkg2u=zt87tKk35vR+tQ7DnkJp9vbj+8n88=_MCcvmH9oQ@mail.gmail.com>
Date:   Wed, 20 Dec 2017 21:32:04 +0100
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, longman@...hat.com,
        lkml <linux-kernel@...r.kernel.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        linux-man <linux-man@...r.kernel.org>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>
Subject: Cgroups v2 thread mode "domain invalid" state

Hello Tejun,

At some point, I'd like to add some details about cgroups v2 "thread
mode" to the cgroup(7) manual page. I think I understand most of the
details by now, but there's one question I still can't work out an
answer to: why does the "domain invalid" cgroup type exist?

I did find the July 2017 mail thread
"[PATCH 5/6] cgroup: implement cgroup v2 thread support"
(https://marc.info/?t=150025735500001&r=1&w=2)
where this cgroup type was discussed, but I must admit that I still
don't get it. Instead of converting cgroups under a threaded subtree
to "domain invalid" type, why can't they just be turned straight into
"threaded" type?

I guess another way of asking this question is: what possibilities
does manually switching each "domain invalid" cgroup to "threaded"
allow that wouldn't be allowed by immediately making all of the
cgroups in a threaded subtree "threaded"?

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/