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

[Roman Gushchin]

On Thu, Oct 04, 2018 at 09:40:57PM +0200, 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?

Just a note: there are now some cgroup core kernel selftests in
tools/testing/selftests/cgroup/test_core.c . It's fairly easy to add new
tests to cover tricky cases like this one. It always nice to have more tests,
and also easier to show and reproduce a problem.

Thanks!