[PATCH v3] mm: memcontrol: optimize per-lruvec stats counter memory usage

[Muchun Song]

The vmstat threshold is 32 (MEMCG_CHARGE_BATCH), Actually the threshold
can be as big as MEMCG_CHARGE_BATCH * PAGE_SIZE. It still fits into s32.
So introducing struct batched_lruvec_stat to optimize memory usage.

The size of struct lruvec_stat is 304 bytes on 64 bits system. As it
is a per-cpu structure. So with this patch, we can save 304 / 2 * ncpu
bytes per-memcg per-node where ncpu is the number of the possible CPU.
If there are c memory cgroup (include dying cgroup) and n NUMA node in
the system. Finally, we can save (152 * ncpu * c * n) bytes.

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
---
Changes in v2 -> v3:
 - Rename per_cpu_lruvec_stat to batched_lruvec_stat. Thanks Shakeel.
 - Update commit log. Thanks Roman.

Changes in v1 -> v2:
 - Update the commit log to point out how many bytes that we can save.

 include/linux/memcontrol.h | 14 ++++++++++++--
 mm/memcontrol.c            | 10 +++++++++-
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 3febf64d1b80..076512e1dc9c 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -92,6 +92,10 @@ struct lruvec_stat {
 	long count[NR_VM_NODE_STAT_ITEMS];
 };
 
+struct batched_lruvec_stat {
+	s32 count[NR_VM_NODE_STAT_ITEMS];
+};
+
 /*
  * Bitmap of shrinker::id corresponding to memcg-aware shrinkers,
  * which have elements charged to this memcg.
@@ -107,11 +111,17 @@ struct memcg_shrinker_map {
 struct mem_cgroup_per_node {
 	struct lruvec		lruvec;
 
-	/* Legacy local VM stats */
+	/*
+	 * Legacy local VM stats. This should be struct lruvec_stat and
+	 * cannot be optimized to struct batched_lruvec_stat. Becasue
+	 * the threshold of the lruvec_stat_cpu can be as big as
+	 * MEMCG_CHARGE_BATCH * PAGE_SIZE. It can fit into s32. But this
+	 * filed has no upper limit.
+	 */
 	struct lruvec_stat __percpu *lruvec_stat_local;
 
 	/* Subtree VM stats (batched updates) */
-	struct lruvec_stat __percpu *lruvec_stat_cpu;
+	struct batched_lruvec_stat __percpu *lruvec_stat_cpu;
 	atomic_long_t		lruvec_stat[NR_VM_NODE_STAT_ITEMS];
 
 	unsigned long		lru_zone_size[MAX_NR_ZONES][NR_LRU_LISTS];
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index eec44918d373..1b01771f2600 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5198,7 +5198,7 @@ static int alloc_mem_cgroup_per_node_info(struct mem_cgroup *memcg, int node)
 		return 1;
 	}
 
-	pn->lruvec_stat_cpu = alloc_percpu_gfp(struct lruvec_stat,
+	pn->lruvec_stat_cpu = alloc_percpu_gfp(struct batched_lruvec_stat,
 					       GFP_KERNEL_ACCOUNT);
 	if (!pn->lruvec_stat_cpu) {
 		free_percpu(pn->lruvec_stat_local);
@@ -7089,6 +7089,14 @@ static int __init mem_cgroup_init(void)
 {
 	int cpu, node;
 
+	/*
+	 * Currently s32 type (can refer to struct batched_lruvec_stat) is
+	 * used for per-memcg-per-cpu caching of per-node statistics. In order
+	 * to work fine, we should make sure that the overfill threshold can't
+	 * exceed S32_MAX / PAGE_SIZE.
+	 */
+	BUILD_BUG_ON(MEMCG_CHARGE_BATCH > S32_MAX / PAGE_SIZE);
+
 	cpuhp_setup_state_nocalls(CPUHP_MM_MEMCQ_DEAD, "mm/memctrl:dead", NULL,
 				  memcg_hotplug_cpu_dead);
 
-- 
2.11.0