Re: [PATCH] sched: Relax a restriction in sched_rt_can_attach()

[Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>]

On Mon, May 04, 2015 at 05:11:10PM +0800, Zefan Li wrote:

> Some degree of flexibility is provided so that you may disable some controllers
> in a subtree. For example:
>
> root                  ---> child1
> (cpuset,memory,cpu)        (cpuset,memory)
>                       \
>                        \-> child2
>                            (cpu)

Uhm, how does that work? Would a task their effective cgroup be the
first parent that has a controller enabled?

In particular, in your example, if T were part of child1, would its cpu
controller be root?

> I just realized we allow removing/adding controllers from/to cgroups
> while there are tasks in them, which isn't safe unless we eliminate all
> can_attach callbacks. We've done so for some cgroup subsystems, but
> there are still a few of them...

You can't remove can_attach(), we must be able to disallow joining a
cgroup.

If that results in you not being able to change the cgroup setup with
tasks in, so be it -- that seems like a sane restriction anyhow.