[PATCH 5/5] mm: spill memcg percpu stats and events before releasing

[Roman Gushchin <guroan@gmail.com>]

Spill percpu stats and events data to corresponding before releasing
percpu memory.

Although per-cpu stats are never exactly precise, dropping them on
floor regularly may lead to an accumulation of an error. So, it's
safer to sync them before releasing.

To minimize the number of atomic updates, let's sum all stats/events
on all cpus locally, and then make a single update per entry.

Signed-off-by: Roman Gushchin <guro@fb.com>
---
 mm/memcontrol.c | 52 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 52 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 18e863890392..b7eb6fac735e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4612,11 +4612,63 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
 	return 0;
 }
 
+/*
+ * Spill all per-cpu stats and events into atomics.
+ * Try to minimize the number of atomic writes by gathering data from
+ * all cpus locally, and then make one atomic update.
+ * No locking is required, because no one has an access to
+ * the offlined percpu data.
+ */
+static void mem_cgroup_spill_offlined_percpu(struct mem_cgroup *memcg)
+{
+	struct memcg_vmstats_percpu __percpu *vmstats_percpu;
+	struct lruvec_stat __percpu *lruvec_stat_cpu;
+	struct mem_cgroup_per_node *pn;
+	int cpu, i;
+	long x;
+
+	vmstats_percpu = memcg->vmstats_percpu_offlined;
+
+	for (i = 0; i < MEMCG_NR_STAT; i++) {
+		int nid;
+
+		x = 0;
+		for_each_possible_cpu(cpu)
+			x += per_cpu(vmstats_percpu->stat[i], cpu);
+		if (x)
+			atomic_long_add(x, &memcg->vmstats[i]);
+
+		if (i >= NR_VM_NODE_STAT_ITEMS)
+			continue;
+
+		for_each_node(nid) {
+			pn = mem_cgroup_nodeinfo(memcg, nid);
+			lruvec_stat_cpu = pn->lruvec_stat_cpu_offlined;
+
+			x = 0;
+			for_each_possible_cpu(cpu)
+				x += per_cpu(lruvec_stat_cpu->count[i], cpu);
+			if (x)
+				atomic_long_add(x, &pn->lruvec_stat[i]);
+		}
+	}
+
+	for (i = 0; i < NR_VM_EVENT_ITEMS; i++) {
+		x = 0;
+		for_each_possible_cpu(cpu)
+			x += per_cpu(vmstats_percpu->events[i], cpu);
+		if (x)
+			atomic_long_add(x, &memcg->vmevents[i]);
+	}
+}
+
 static void mem_cgroup_free_percpu(struct rcu_head *rcu)
 {
 	struct mem_cgroup *memcg = container_of(rcu, struct mem_cgroup, rcu);
 	int node;
 
+	mem_cgroup_spill_offlined_percpu(memcg);
+
 	for_each_node(node) {
 		struct mem_cgroup_per_node *pn = memcg->nodeinfo[node];
 
-- 
2.20.1