mm/memcg: add oom_group_kill memory event

Our container agent wants to know when a container exits if it was OOM
killed or not to report to the user.  We use memory.oom.group = 1 to
ensure that OOM kills within the container's cgroup kill everything.
Existing memory.events are insufficient for knowing if this triggered:

1) Our current approach reads memory.events oom_kill and reports the
   container was killed if the value is non-zero. This is erroneous in
   some cases where containers create their children cgroups with
   memory.oom.group=1 as such OOM kills will get counted against the
   container cgroup's oom_kill counter despite not actually OOM killing
   the entire container.

2) Reading memory.events.local will fail to identify OOM kills in leaf
   cgroups (that don't set memory.oom.group) within the container
   cgroup.

This patch adds a new oom_group_kill event when memory.oom.group
triggers to allow userspace to cleanly identify when an entire cgroup is
oom killed.

[[email protected]: changes from Johannes and Chris]
  Link: https://lkml.kernel.org/r/[email protected]

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Dan Schatzberg <[email protected]>
Reviewed-by: Roman Gushchin <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: Chris Down <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Zefan Li <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Alex Shi <[email protected]>
Cc: Wei Yang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Documentation/admin-guide/cgroup-v2.rst

@@ -1268,6 +1268,9 @@ PAGE_SIZE multiple when read back.
 		The number of processes belonging to this cgroup
 		killed by any kind of OOM killer.
 
+          oom_group_kill
+                The number of times a group OOM has occurred.
+
   memory.events.local
 	Similar to memory.events but the fields in the file are local
 	to the cgroup i.e. not hierarchical. The file modified event

include/linux/memcontrol.h

@@ -42,6 +42,7 @@ enum memcg_memory_event {
 	MEMCG_MAX,
 	MEMCG_OOM,
 	MEMCG_OOM_KILL,
+	MEMCG_OOM_GROUP_KILL,
 	MEMCG_SWAP_HIGH,
 	MEMCG_SWAP_MAX,
 	MEMCG_SWAP_FAIL,

mm/memcontrol.c

@@ -6318,6 +6318,8 @@ static void __memory_events_show(struct seq_file *m, atomic_long_t *events)
 	seq_printf(m, "oom %lu\n", atomic_long_read(&events[MEMCG_OOM]));
 	seq_printf(m, "oom_kill %lu\n",
 		   atomic_long_read(&events[MEMCG_OOM_KILL]));
+	seq_printf(m, "oom_group_kill %lu\n",
+		   atomic_long_read(&events[MEMCG_OOM_GROUP_KILL]));
 }
 
 static int memory_events_show(struct seq_file *m, void *v)

mm/oom_kill.c

@@ -994,6 +994,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
 	 * If necessary, kill all tasks in the selected memory cgroup.
 	 */
 	if (oom_group) {
+		memcg_memory_event(oom_group, MEMCG_OOM_GROUP_KILL);
 		mem_cgroup_print_oom_group(oom_group);
 		mem_cgroup_scan_tasks(oom_group, oom_kill_memcg_member,
 				      (void *)message);