Re: Cgroups v2 thread mode "domain invalid" state

From: Tejun Heo
Date: Thu Dec 21 2017 - 19:16:08 EST


On Wed, Dec 20, 2017 at 09:32:04PM +0100, Michael Kerrisk (man-pages) wrote:
> I did find the July 2017 mail thread
> "[PATCH 5/6] cgroup: implement cgroup v2 thread support"
> (
> where this cgroup type was discussed, but I must admit that I still
> don't get it. Instead of converting cgroups under a threaded subtree
> to "domain invalid" type, why can't they just be turned straight into
> "threaded" type?
> I guess another way of asking this question is: what possibilities
> does manually switching each "domain invalid" cgroup to "threaded"
> allow that wouldn't be allowed by immediately making all of the
> cgroups in a threaded subtree "threaded"?

IIRC, there were two reasons. (Writing from memory, so I might not be
completely accurate.)

* Consistency w/ the cgroups right under the root cgroup. Because
they can be both domains and threadroots, we can't switch the
children over to thread mode automatically. Doing that for cgroups
further down in the hierarchy would be really inconsistent.

* Possible extension to threadmode. If we figure out how to do mixed
mode further down in the hierarchy (and if there are actual use
cases which require that), automatically switching would be really

The invalid state while not the most convenient is straight forward
(only the operations which are explicitly asked are performed) and
keeps the door open for future changes.