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

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Message-ID: <4fd48929-69fa-f13b-0095-1d210068684d@gmail.com>
Date:   Tue, 16 Oct 2018 17:11:41 +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: Re: Cgroup v2 thread mode oddity: "domain invalid" cgroup with
 threaded controller enabled

Hello Tejun,

A ping on the below...

Thanks,

Michael

On 10/04/2018 09:40 PM, Michael Kerrisk (man-pages) wrote:
> 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/