Re: [PATCH 2/2] cgroup: Skip subtree root in cgroup_update_dfl_csses()

[Tejun Heo]

On Thu, Jul 28, 2022 at 04:44:26PM +0200, Michal Koutný wrote:
> On Wed, Jul 27, 2022 at 08:58:15PM -0400, Waiman Long <longman@xxxxxxxxxx> wrote:
> > The cgroup_update_dfl_csses() function updates css associations when a
> > cgroup's subtree_control file is modified. Any changes made to a cgroup's
> > subtree_control file, however, will only affect its descendants but not
> > the cgroup itself. 
> 
> I find this correct.
> 
> > So there is no point in migrating csses associated with that cgroup.
> > We can skip them instead.
> 
> Alone it's not such a big win but it componds with the recent Tejun's
> threadgroup_rwsem elision.

The chance is that if you're writing to a cgroup's subtree_control, that
cgroup is gonna be empty. The only case I can think of that this would make
a difference is w/ threaded cgroups, but it does make sense.

Thanks.

-- 
tejun