Re: [RFC PATCH-cgroup 4/6] cgroup: Introduce subtree root mode

[Tejun Heo]

Hello, Waiman.

On Wed, Jun 14, 2017 at 11:05:35AM -0400, Waiman Long wrote:
> Subtree root mode is a new cgroup mode which applies the following
> restrictions when turned on:
> 
>  1) Controllers are only allowed to be passed to the children in
>     bypass mode except those with the "enable_on_root" flag on.
>  2) Only 1 child cgroup is allowed.
> 
> That lone child can be used as the pseudo root of a container cgroup
> hierarchy.  All the resources, if controlled, are in the parent
> cgroup. There will be no control knobs in the child. That makes it
> look and feel like a root.

I'm not sure not having the control files in the child makes that much
difference.

> That pseudo root is also considered to be mixable and so can become
> root of a mixed threaded subtree. The no internal process constraint
> also does not apply.

Heh, you can't just declare a non-root cgroup to be a mixed root but
if you're special casing this and making the kernel play a masquerade
with special node, you can make cgroup provide a mixed root while
hosting the internal processes in a dedicated leaf cgroup which isn't
visible to the nested root, right?

It's all a game of masquerading tho and doesn't actually enable
anything which isn't possible now.  This would definitely be useful
for testing.

Thanks.

-- 
tejun