memcg: update memcg_test.txt to describe memory thresholds

Decription of sanity check for memory thresholds.

Signed-off-by: Kirill A.  Shutemov <kirill@shutemov.name>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Paul Menage <menage@google.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Dan Malek <dan@embeddedalley.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/cgroups/memcg_test.txt

@@ -396,3 +396,24 @@ Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y.
 	memory.stat of both A and B.
 	See 8.2 of Documentation/cgroups/memory.txt to see what value should be
 	written to move_charge_at_immigrate.
+
+ 9.10 Memory thresholds
+	Memory controler implements memory thresholds using cgroups notification
+	API. You can use Documentation/cgroups/cgroup_event_listener.c to test
+	it.
+
+	(Shell-A) Create cgroup and run event listener
+	# mkdir /cgroup/A
+	# ./cgroup_event_listener /cgroup/A/memory.usage_in_bytes 5M
+
+	(Shell-B) Add task to cgroup and try to allocate and free memory
+	# echo $$ >/cgroup/A/tasks
+	# a="$(dd if=/dev/zero bs=1M count=10)"
+	# a=
+
+	You will see message from cgroup_event_listener every time you cross
+	the thresholds.
+
+	Use /cgroup/A/memory.memsw.usage_in_bytes to test memsw thresholds.
+
+	It's good idea to test root cgroup as well.