Trying to make all the controllers uniform in terms of their treatment
of cgroup hiearchy sounds like a good thing to do.

Agreed.

Apart from nesting cgroups, there're other inconsistencies.

- Some controllers disallow more than one cgroup layer. That's the new
net_prio controller, and I don't know why it's made so, but I guess
it's fine to eliminate this restriction.

- Some controllers move resource charges when a task is moved to
a different cgroup, but some don't?

- Some controllers disallow task attaching under some circumstances.
So if we have a single hierarchy with all subsystems, the chance
that attaching a task to a cgroup fails may be bigger.

Once that is done,
one can probably see if it is worth to put all the controllers in a
single hierarchy.