On 8/11/21 2:21 PM, Tejun Heo wrote:

On Wed, Aug 11, 2021 at 02:18:17PM -0400, Waiman Long wrote:

quoted

I don't think that is true. A task can reside anywhere in the cgroup
hierarchy. I have encountered no problem moving tasks around.

Oh, that shouldn't be happening with controllers enabled. Can you please
share a repro?

I have done further testing. Enabling controllers won't prohibit moving 
a task into a parent cgroup as long as the child cgroups have no tasks. 
Once the child cgroup has task, moving another task to the parent is not 
allowed (-EBUSY). Similarly if a parent cgroup has tasks, you can't put 
new tasks into the child cgroup. I don't realize that we have such 
constraints as I usually do my testing with a cgroup hierarchy with no 
tasks initially. Anyway, a new lesson learned.

I will try to see how to address that in the patch, but the additional 
check added is still valid in some special case.

Cheers,
Longman