[PATCH 1/2] mm: memcontrol: flush percpu vmstats before releasing memcg

[Roman Gushchin <guro@fb.com>]

Percpu caching of local vmstats with the conditional propagation
by the cgroup tree leads to an accumulation of errors on non-leaf
levels.

Let's imagine two nested memory cgroups A and A/B. Say, a process
belonging to A/B allocates 100 pagecache pages on the CPU 0.
The percpu cache will spill 3 times, so that 32*3=96 pages will be
accounted to A/B and A atomic vmstat counters, 4 pages will remain
in the percpu cache.

Imagine A/B is nearby memory.max, so that every following allocation
triggers a direct reclaim on the local CPU. Say, each such attempt
will free 16 pages on a new cpu. That means every percpu cache will
have -16 pages, except the first one, which will have 4 - 16 = -12.
A/B and A atomic counters will not be touched at all.

Now a user removes A/B. All percpu caches are freed and corresponding
vmstat numbers are forgotten. A has 96 pages more than expected.

As memory cgroups are created and destroyed, errors do accumulate.
Even 1-2 pages differences can accumulate into large numbers.

To fix this issue let's accumulate and propagate percpu vmstat
values before releasing the memory cgroup. At this point these
numbers are stable and cannot be changed.

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 | 40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 3e821f34399f..348f685ab94b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3412,6 +3412,41 @@ static int memcg_online_kmem(struct mem_cgroup *memcg)
 	return 0;
 }
 
+static void memcg_flush_percpu_vmstats(struct mem_cgroup *memcg)
+{
+	unsigned long stat[MEMCG_NR_STAT];
+	struct mem_cgroup *mi;
+	int node, cpu, i;
+
+	for (i = 0; i < MEMCG_NR_STAT; i++)
+		stat[i] = 0;
+
+	for_each_online_cpu(cpu)
+		for (i = 0; i < MEMCG_NR_STAT; i++)
+			stat[i] += raw_cpu_read(memcg->vmstats_percpu->stat[i]);
+
+	for (mi = memcg; mi; mi = parent_mem_cgroup(mi))
+		for (i = 0; i < MEMCG_NR_STAT; i++)
+			atomic_long_add(stat[i], &mi->vmstats[i]);
+
+	for_each_node(node) {
+		struct mem_cgroup_per_node *pn = memcg->nodeinfo[node];
+		struct mem_cgroup_per_node *pi;
+
+		for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++)
+			stat[i] = 0;
+
+		for_each_online_cpu(cpu)
+			for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++)
+				stat[i] += raw_cpu_read(
+					pn->lruvec_stat_cpu->count[i]);
+
+		for (pi = pn; pi; pi = parent_nodeinfo(pi, node))
+			for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++)
+				atomic_long_add(stat[i], &pi->lruvec_stat[i]);
+	}
+}
+
 static void memcg_offline_kmem(struct mem_cgroup *memcg)
 {
 	struct cgroup_subsys_state *css;
@@ -4805,6 +4840,11 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg)
 {
 	int node;
 
+	/*
+	 * Flush percpu vmstats to guarantee the value correctness
+	 * on parent's and all ancestor levels.
+	 */
+	memcg_flush_percpu_vmstats(memcg);
 	for_each_node(node)
 		free_mem_cgroup_per_node_info(memcg, node);
 	free_percpu(memcg->vmstats_percpu);
-- 
2.21.0