Pull cgroup updates from Tejun Heo:

 - For cpustat, cgroup has a percpu hierarchical stat mechanism which
   propagates up the hierarchy lazily.

   This contains commits to factor out and generalize the mechanism so
   that it can be used for other cgroup stats too.

   The original intention was to update memcg stats to use it but memcg
   went for a different approach, so still the only user is cpustat. The
   factoring out and generalization still make sense and it's likely
   that this can be used for other purposes in the future.

 - cgroup uses kernfs_notify() (which uses fsnotify()) to inform user
   space of certain events. A rate limiting mechanism is added.

 - Other misc changes.

* 'for-4.18' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup:
  cgroup: css_set_lock should nest inside tasklist_lock
  rdmacg: Convert to use match_string() helper
  cgroup: Make cgroup_rstat_updated() ready for root cgroup usage
  cgroup: Add memory barriers to plug cgroup_rstat_updated() race window
  cgroup: Add cgroup_subsys->css_rstat_flush()
  cgroup: Replace cgroup_rstat_mutex with a spinlock
  cgroup: Factor out and expose cgroup_rstat_*() interface functions
  cgroup: Reorganize kernel/cgroup/rstat.c
  cgroup: Distinguish base resource stat implementation from rstat
  cgroup: Rename stat to rstat
  cgroup: Rename kernel/cgroup/stat.c to kernel/cgroup/rstat.c
  cgroup: Limit event generation frequency
  cgroup: Explicitly remove core interface files