[PATCH 2/4] mm, memcg: introduce MEMCG_PROT_SKIP for memcg zero usage case

[Yafang Shao <laoar.shao@gmail.com>]

If the usage of a memcg is zero, we don't need to do useless work to scan
it. That is a minor optimization.

Cc: Roman Gushchin <guro@fb.com>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 include/linux/memcontrol.h | 1 +
 mm/memcontrol.c            | 2 +-
 mm/vmscan.c                | 6 ++++++
 3 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 612a457..1a315c7 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -54,6 +54,7 @@ enum mem_cgroup_protection {
 	MEMCG_PROT_NONE,
 	MEMCG_PROT_LOW,
 	MEMCG_PROT_MIN,
+	MEMCG_PROT_SKIP,	/* For zero usage case */
 };
 
 struct mem_cgroup_reclaim_cookie {
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c5b5f74..f35fcca 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6292,7 +6292,7 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
 
 	usage = page_counter_read(&memcg->memory);
 	if (!usage)
-		return MEMCG_PROT_NONE;
+		return MEMCG_PROT_SKIP;
 
 	emin = memcg->memory.min;
 	elow = memcg->memory.low;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 5a6445e..3c4c2da 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2677,6 +2677,12 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
 			 * thresholds (see get_scan_count).
 			 */
 			break;
+		case MEMCG_PROT_SKIP:
+			/*
+			 * Skip scanning this memcg if the usage of it is
+			 * zero.
+			 */
+			continue;
 		}
 
 		reclaimed = sc->nr_reclaimed;
-- 
1.8.3.1