mm: memcontrol: flush percpu vmstats before releasing memcg

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.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 42a3003 ("mm: memcontrol: fix recursive statistics correctness & scalabilty")
Signed-off-by: Roman Gushchin <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/memcontrol.c

@@ -3260,6 +3260,41 @@ static u64 mem_cgroup_read_u64(struct cgroup_subsys_state *css,
 	}
 }
 
+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]);
+	}
+}
+
 #ifdef CONFIG_MEMCG_KMEM
 static int memcg_online_kmem(struct mem_cgroup *memcg)
 {
@@ -4682,6 +4717,11 @@ 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);