...This makes most sense to me. It won't break any existing software (most likely) while it seems reasonable to leave 'tasks' unprefixed as this is something that any software using any subsystem of cgroup would be using anyway and it is not that much associated with a particulat subsystem.
A compromise might be to keep "tasks" unprefixed, and say that future
names get the "cgroup." prefix; in this case I'd be inclined to add
the prefix to notify_on_release and release_agent on the grounds that
there's much less chance of breaking anyone with those files since (I
suspect) they're much less used.