memcg: add documentation for the memory.numastat API

[akpm@linux-foundation.org: rework text, fit it into 80-cols]
Signed-off-by: Ying Han <yinghan@google.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/cgroups/memory.txt

@@ -70,6 +70,7 @@ Brief summary of control files.
 				 (See sysctl's vm.swappiness)
  memory.move_charge_at_immigrate # set/show controls of moving charges
  memory.oom_control		 # set/show oom controls.
+ memory.numa_stat		 # show the number of memory usage per numa node
 
 1. History
 
@@ -464,6 +465,24 @@ value for efficient access. (Of course, when necessary, it's synchronized.)
 If you want to know more exact memory usage, you should use RSS+CACHE(+SWAP)
 value in memory.stat(see 5.2).
 
+5.6 numa_stat
+
+This is similar to numa_maps but operates on a per-memcg basis.  This is
+useful for providing visibility into the numa locality information within
+an memcg since the pages are allowed to be allocated from any physical
+node.  One of the usecases is evaluating application performance by
+combining this information with the application's cpu allocation.
+
+We export "total", "file", "anon" and "unevictable" pages per-node for
+each memcg.  The ouput format of memory.numa_stat is:
+
+total=<total pages> N0=<node 0 pages> N1=<node 1 pages> ...
+file=<total file pages> N0=<node 0 pages> N1=<node 1 pages> ...
+anon=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
+unevictable=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
+
+And we have total = file + anon + unevictable.
+
 6. Hierarchy support
 
 The memory controller supports a deep hierarchy and hierarchical accounting.