[PATCH] mm: memcontrol: flush percpu vmevents before releasing memcg

[Roman Gushchin <guro@fb.com>]

Similar to vmstats, percpu caching of local vmevents leads to an
accumulation of errors on non-leaf levels. This happens because
some leftovers may remain in percpu caches, so that they are
never propagated up by the cgroup tree and just disappear into
nonexistence with on releasing of the memory cgroup.

To fix this issue let's accumulate and propagate percpu vmevents
values before releasing the memory cgroup similar to what we're
doing with vmstats.

Since on cpu hotplug we do flush percpu vmstats anyway, we can
iterate only over online cpus.

Fixes: 42a300353577 ("mm: memcontrol: fix recursive statistics correctness & scalabilty")
Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/memcontrol.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6d2427abcc0c..249187907339 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3459,6 +3459,25 @@ static void memcg_flush_percpu_vmstats(struct mem_cgroup *memcg, bool slab_only)
 	}
 }
 
+static void memcg_flush_percpu_vmevents(struct mem_cgroup *memcg)
+{
+	unsigned long events[NR_VM_EVENT_ITEMS];
+	struct mem_cgroup *mi;
+	int cpu, i;
+
+	for (i = 0; i < NR_VM_EVENT_ITEMS; i++)
+		events[i] = 0;
+
+	for_each_online_cpu(cpu)
+		for (i = 0; i < NR_VM_EVENT_ITEMS; i++)
+			events[i] += raw_cpu_read(
+				memcg->vmstats_percpu->events[i]);
+
+	for (mi = memcg; mi; mi = parent_mem_cgroup(mi))
+		for (i = 0; i < NR_VM_EVENT_ITEMS; i++)
+			atomic_long_add(events[i], &mi->vmevents[i]);
+}
+
 static void memcg_offline_kmem(struct mem_cgroup *memcg)
 {
 	struct cgroup_subsys_state *css;
@@ -4860,10 +4879,11 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg)
 	int node;
 
 	/*
-	 * Flush percpu vmstats to guarantee the value correctness
+	 * Flush percpu vmstats and vmevents to guarantee the value correctness
 	 * on parent's and all ancestor levels.
 	 */
 	memcg_flush_percpu_vmstats(memcg, false);
+	memcg_flush_percpu_vmevents(memcg);
 	for_each_node(node)
 		free_mem_cgroup_per_node_info(memcg, node);
 	free_percpu(memcg->vmstats_percpu);
-- 
2.21.0