mm, memcg: introduce memory.events.local

The memory controller in cgroup v2 exposes memory.events file for each
memcg which shows the number of times events like low, high, max, oom and
oom_kill have happened for the whole tree rooted at that memcg.  Users can
also poll or register notification to monitor the changes in that file.
Any event at any level of the tree rooted at memcg will notify all the
listeners along the path till root_mem_cgroup.  There are existing users
which depend on this behavior.

However there are users which are only interested in the events happening
at a specific level of the memcg tree and not in the events in the
underlying tree rooted at that memcg.  One such use-case is a centralized
resource monitor which can dynamically adjust the limits of the jobs
running on a system.  The jobs can create their sub-hierarchy for their
own sub-tasks.  The centralized monitor is only interested in the events
at the top level memcgs of the jobs as it can then act and adjust the
limits of the jobs.  Using the current memory.events for such centralized
monitor is very inconvenient.  The monitor will keep receiving events
which it is not interested and to find if the received event is
interesting, it has to read memory.event files of the next level and
compare it with the top level one.  So, let's introduce
memory.events.local to the memcg which shows and notify for the events at
the memcg level.

Now, does memory.stat and memory.pressure need their local versions.  IMHO
no due to the no internal process contraint of the cgroup v2.  The
memory.stat file of the top level memcg of a job shows the stats and
vmevents of the whole tree.  The local stats or vmevents of the top level
memcg will only change if there is a process running in that memcg but v2
does not allow that.  Similarly for memory.pressure there will not be any
process in the internal nodes and thus no chance of local pressure.

Link: http://lkml.kernel.org/r/20190527174643.209172-1-shakeelb@google.com
Signed-off-by: Shakeel Butt <shakeelb@google.com>
Reviewed-by: Roman Gushchin <guro@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Chris Down <chris@chrisdown.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

Documentation/admin-guide/cgroup-v2.rst

@@ -1140,6 +1140,11 @@ PAGE_SIZE multiple when read back.
 	otherwise, a value change in this file generates a file
 	modified event.
 
+	Note that all fields in this file are hierarchical and the
+	file modified event can be generated due to an event down the
+	hierarchy. For for the local events at the cgroup level see
+	memory.events.local.
+
 	  low
 		The number of times the cgroup is reclaimed due to
 		high memory pressure even though its usage is under
@@ -1179,6 +1184,11 @@ PAGE_SIZE multiple when read back.
 		The number of processes belonging to this cgroup
 		killed by any kind of OOM killer.
 
+  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
+	generated on this file reflects only the local events.
+
   memory.stat
 	A read-only flat-keyed file which exists on non-root cgroups.
 

include/linux/memcontrol.h

@@ -233,8 +233,9 @@ struct mem_cgroup {
 	/* OOM-Killer disable */
 	int		oom_kill_disable;
 
-	/* memory.events */
+	/* memory.events and memory.events.local */
 	struct cgroup_file events_file;
+	struct cgroup_file events_local_file;
 
 	/* handle for "memory.swap.events" */
 	struct cgroup_file swap_events_file;
@@ -281,6 +282,7 @@ struct mem_cgroup {
 
 	/* memory.events */
 	atomic_long_t		memory_events[MEMCG_NR_MEMORY_EVENTS];
+	atomic_long_t		memory_events_local[MEMCG_NR_MEMORY_EVENTS];
 
 	unsigned long		socket_pressure;
 
@@ -747,6 +749,9 @@ static inline void count_memcg_event_mm(struct mm_struct *mm,
 static inline void memcg_memory_event(struct mem_cgroup *memcg,
 				      enum memcg_memory_event event)
 {
+	atomic_long_inc(&memcg->memory_events_local[event]);
+	cgroup_file_notify(&memcg->events_local_file);
+
 	do {
 		atomic_long_inc(&memcg->memory_events[event]);
 		cgroup_file_notify(&memcg->events_file);

mm/memcontrol.c

@@ -5623,21 +5623,29 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
 	return nbytes;
 }
 
+static void __memory_events_show(struct seq_file *m, atomic_long_t *events)
+{
+	seq_printf(m, "low %lu\n", atomic_long_read(&events[MEMCG_LOW]));
+	seq_printf(m, "high %lu\n", atomic_long_read(&events[MEMCG_HIGH]));
+	seq_printf(m, "max %lu\n", atomic_long_read(&events[MEMCG_MAX]));
+	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]));
+}
+
 static int memory_events_show(struct seq_file *m, void *v)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
 
-	seq_printf(m, "low %lu\n",
-		   atomic_long_read(&memcg->memory_events[MEMCG_LOW]));
-	seq_printf(m, "high %lu\n",
-		   atomic_long_read(&memcg->memory_events[MEMCG_HIGH]));
-	seq_printf(m, "max %lu\n",
-		   atomic_long_read(&memcg->memory_events[MEMCG_MAX]));
-	seq_printf(m, "oom %lu\n",
-		   atomic_long_read(&memcg->memory_events[MEMCG_OOM]));
-	seq_printf(m, "oom_kill %lu\n",
-		   atomic_long_read(&memcg->memory_events[MEMCG_OOM_KILL]));
+	__memory_events_show(m, memcg->memory_events);
+	return 0;
+}
+
+static int memory_events_local_show(struct seq_file *m, void *v)
+{
+	struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
 
+	__memory_events_show(m, memcg->memory_events_local);
 	return 0;
 }
 
@@ -5799,6 +5807,12 @@ static struct cftype memory_files[] = {
 		.file_offset = offsetof(struct mem_cgroup, events_file),
 		.seq_show = memory_events_show,
 	},
+	{
+		.name = "events.local",
+		.flags = CFTYPE_NOT_ON_ROOT,
+		.file_offset = offsetof(struct mem_cgroup, events_local_file),
+		.seq_show = memory_events_local_show,
+	},
 	{
 		.name = "stat",
 		.flags = CFTYPE_NOT_ON_ROOT,