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

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Message-ID: <20181017161958.GT270328@devbig004.ftw2.facebook.com>
Date:   Wed, 17 Oct 2018 09:19:58 -0700
From:   Tejun Heo <tj@...nel.org>
To:     "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:     "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, Michael.

Sorry about the delay.

On Thu, Oct 04, 2018 at 09:40:57PM +0200, Michael Kerrisk (man-pages) wrote:
> 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.

So, both the interim (before turning x/z into threaded) and final
(after) are completely fine - the cgroups are empty and whether
threaded controllers like pids are enabled or not don't really change
things that much.

Maybe it is a bit inconsistent to then deny enabling threaded
controllers on invalid domain cgroups.  We can lift that restriction
but I personally can't see why that'd be clearly better.

Thanks.

-- 
tejun