memcg: implement memory thresholds

It allows to register multiple memory and memsw thresholds and gets
notifications when it crosses.

To register a threshold application need:
- create an eventfd;
- open memory.usage_in_bytes or memory.memsw.usage_in_bytes;
- write string like "<event_fd> <memory.usage_in_bytes> <threshold>" to
  cgroup.event_control.

Application will be notified through eventfd when memory usage crosses
threshold in any direction.

It's applicable for root and non-root cgroup.

It uses stats to track memory usage, simmilar to soft limits. It checks
if we need to send event to userspace on every 100 page in/out. I guess
it's good compromise between performance and accuracy of thresholds.

[[email protected]: coding-style fixes]
[[email protected]: fix documentation merge issue]
Signed-off-by: Kirill A. Shutemov <[email protected]>
Cc: Li Zefan <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Dan Malek <[email protected]>
Cc: Vladislav Buzov <[email protected]>
Cc: Daisuke Nishimura <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Documentation/cgroups/memory.txt

@@ -468,7 +468,24 @@ Note: More type of pages(e.g. file cache, shmem,) will be supported by other
 - All of moving charge operations are done under cgroup_mutex. It's not good
   behavior to hold the mutex too long, so we may need some trick.
 
-9. TODO
+9. Memory thresholds
+
+Memory controler implements memory thresholds using cgroups notification
+API (see cgroups.txt). It allows to register multiple memory and memsw
+thresholds and gets notifications when it crosses.
+
+To register a threshold application need:
+ - create an eventfd using eventfd(2);
+ - open memory.usage_in_bytes or memory.memsw.usage_in_bytes;
+ - write string like "<event_fd> <memory.usage_in_bytes> <threshold>" to
+   cgroup.event_control.
+
+Application will be notified through eventfd when memory usage crosses
+threshold in any direction.
+
+It's applicable for root and non-root cgroup.
+
+10. TODO
 
 1. Add support for accounting huge pages (as a separate controller)
 2. Make per-cgroup scanner reclaim not-shared pages first