cpuset: implement sane hierarchy behaviors

v2 -> v3:

- remove async hotplug propagation work
- do propagation work in one place for both hotplug and unplug

v1 -> v2:

- add documentation in include/linux/cgroup.h
- drop rcu_read_lock() before calling update_task_nodemask() when iterating cpuset hierarchy

======================================

Currently some cpuset behaviors are not friendly when cpuset is co-mounted with other cgroup controllers.

Now with this patchset if cpuset is mounted with sane_behavior option, it behaves differently:

- Tasks will be kept in empty cpusets when hotplug happens and take masks of ancestors with non-empty cpus/mems, instead of being moved to an ancestor.

- A task can be moved into an empty cpuset, and again it takes masks of ancestors, so the user can drop a task into a newly created cgroup without having to do anything for it.

As tasks can reside in empy cpusets, here're some rules:

- They can be moved to another cpuset, regardless it's empty or not.

- Though it takes masks from ancestors, it takes other configs from the empty cpuset.

- If the ancestors' masks are changed, those tasks will also be updated to take new masks.

Li Zefan (7):
cpuset: let hotplug propagation work wait for task attaching
cpuset: remove async hotplug propagation work
cpuset: record old_mems_allowed in struct cpuset
cpuset: introduce effective_{cpumask|nodemask}_cpuset()
cpuset: allow to keep tasks in empty cpusets
cpuset: allow to move tasks to empty cpusets
cpuset: fix to migrate mm correctly in a corner case

 include/linux/cgroup.h |   7 +
 kernel/cpuset.c        | 390 ++++++++++++++++++++++++++++++++-----------------
 2 files changed, 264 insertions(+), 133 deletions(-)

1.8.0.2