cgroups: Bindable cgroup subsystems

Stephane posted a patchset to add perf_cgroup subsystem, so perf can be used to monitor all threads belonging to a cgroup.

But if you already mounted a cgroup hierarchy but without perf_cgroup and the hierarchy has sub-cgroups, you can't bind perf_cgroup to it, and thus you're not able to use per-cgroup perf feature.

This patchset alleviates the pain, and then a subsytem can be bound/unbound to/from a hierarchy which has sub-cgroups in it.

Some subsystems still can't take advantage of this patchset, memcgroup and cpuset for example.

For cpuset, if a hierarchy has a sub-cgroup and the cgroup has tasks, we can't decide sub-cgroup's cpuset.mems and cpuset.cpus automatically if we try to bind cpuset to this hierarchy.

For memcgroup, memcgroup uses css_get/put(), and due to some complexity, for now bindable subsystems should not use css_get/put().

Usage:

# mount -t cgroup -o cpuset xxx /mnt
# mkdir /mnt/tmp
# echo $$ > /mnt/tmp/tasks

(add cpuacct to the hierarchy)

# mount -o remount,cpuset,cpuacct xxx /mnt

(remove it from the hierarchy)

# mount -o remount,cpuset xxx /mnt

There's another limitation, cpuacct should not be bound to any mounted hierarchy before the above operation. But that's not a problem, as you can remove it from a hierarchy and bind it to another one.

Changelog v2:

- Fix some bugs.
- Spit can_bind flag to bindable and unbindable flags
- Provide a __css_tryget() so a bindable subsystem can pin a cgroup via it.
- ...

---
 Documentation/cgroups/cgroups.txt |   37 +++-
 include/linux/cgroup.h            |   39 +++-
 kernel/cgroup.c                   |  391 +++++++++++++++++++++++++++++++------
 kernel/cgroup_freezer.c           |    1 +
 kernel/sched.c                    |    2 +
 security/device_cgroup.c          |    2 +
 6 files changed, 398 insertions(+), 74 deletions(-)