linux-kernel - Cgroup v2 thread mode oddity: "domain invalid" cgroup with threaded controller enabled

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Message-ID: <a4912516-8c6b-d8d3-b714-5c03bcfc5e91@gmail.com>
Date:   Thu, 4 Oct 2018 21:40:57 +0200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     mtk.manpages@...il.com,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Cgroup v2 thread mode oddity: "domain invalid" cgroup with threaded
 controller enabled

Hello Tejun,

Suppose we have the following scenario:

x [d]    (pids)
    y [dt]    (pids)
        p [t]
        q [t]
        r [t]
    z [d]

Here, x/y is a "domain threaded root" with a threaded controller
(the 'pids' controller) enabled. (In this scenario, there are no
member processes in any of the cgroups.)

Suppose we now convert x/z to "threaded" type:

     # echo threaded > x/z/cgroup.type

Now we end up in the following state:

x [dt]    (pids)
    y [inv]    (pids)
        p [t]
        q [t]
        r [t]
    z [t]

This seems odd. x/y is now of "domain invalid" type with a controller
enabled! This feels like a violation of the rules, since we can't
in other circumstances do anything with a "domain invalid" cgroup
except convert it to "threaded". In particular, we can't create
child cgroups under a "domain invalid" cgroup, or add member processes
to the cgroup, or *enable controllers in the cgroup*. In fact, when
doing the 

    # echo threaded > x/z/cgroup.type

I had expected a write(2) error because the state of x/y should
(I thought) not be permitted.

Your thoughts?

Thanks,

Michael

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