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From:   "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Date:   Tue, 20 Feb 2018 20:26:59 +0100
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Subject: A "domain invalid" cgroup *can* sometimes have member tasks
To:     Tejun Heo <tj@kernel.org>
Cc:     "Serge E. Hallyn" <serge@hallyn.com>,
        lkml <linux-kernel@vger.kernel.org>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@vger.kernel.org>,
        Michael Kerrisk <mtk.manpages@gmail.com>
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Hello Tejun

According to Documentation/cgroup-v2.txt, a "domain invalid" cgroup
can't have member tasks. And indeed this is generally not permitted.

However, someone recently showed me a scenario where a "domain
invalid" cgroup can have member processes. See the following example:

# mkdir -p /sys/fs/cgroup/unified/grp0/grp1
# sleep 1000 &
[1] 10549
# echo 10549 > /sys/fs/cgroup/unified/grp0/grp1/cgroup.procs
# echo threaded > /sys/fs/cgroup/unified/grp0/cgroup.type
# cat /sys/fs/cgroup/unified/grp0/cgroup.type
threaded
# cat /sys/fs/cgroup/unified/grp0/grp1/cgroup.type
domain invalid
# cat /sys/fs/cgroup/unified/grp0/grp1/cgroup.threads
10549

From the above, we see that the cgroup grp0/grp1 is of type "domain
invalid" and has a member thread. This seems to be a violation of the
documented rules, and is I assume a bug, since in the above scenario,
we are denied from adding further tasks to the grp0/grp1 cgroup:

# sleep 2000 &
[2] 10553
# echo 10553 > /sys/fs/cgroup/unified/grp0/grp1/cgroup.procs
sh: echo: write error: Operation not supported

Could you comment please?

Thanks,

Michael

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