[142/227] mm/list_lru: optimize memcg_reparent_list_lru_node()

Commit Message

Andrew Morton March 22, 2022, 9:45 p.m. UTC

From: Waiman Long <[e-mail]>
Subject: mm/list_lru: optimize memcg_reparent_list_lru_node()

Since commit 2c80cd57c743 ("mm/list_lru.c: fix list_lru_count_node() to be
race free"), we are tracking the total number of lru entries in a
list_lru_node in its nr_items field.  In the case of
memcg_reparent_list_lru_node(), there is nothing to be done if nr_items is
0.  We don't even need to take the nlru->lock as no new lru entry could be
added by a racing list_lru_add() to the draining src_idx memcg at this
point.

On systems that serve a lot of containers, it is possible that there can
be thousands of list_lru's present due to the fact that each container may
mount its own container specific filesystems.  As a typical container uses
only a few cpus, it is likely that only the list_lru_node that contains
those cpus will be utilized while the rests may be empty.  In other words,
there can be a lot of list_lru_node with 0 nr_items.  By skipping a
lock/unlock operation and loading a cacheline from memcg_lrus, a sizeable
number of cpu cycles can be saved.  That can be substantial if we are
talking about thousands of list_lru_node's with 0 nr_items.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Waiman Long <[e-mail]>
Reviewed-by: Roman Gushchin <[e-mail]>
Cc: Muchun Song <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Shakeel Butt <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/list_lru.c |    6 ++++++
 1 file changed, 6 insertions(+)

Patch

--- a/mm/list_lru.c~mm-list_lru-optimize-memcg_reparent_list_lru_node
+++ a/mm/list_lru.c
@@ -395,6 +395,12 @@  static void memcg_reparent_list_lru_node
 	struct list_lru_one *src, *dst;
 
 	/*
+	 * If there is no lru entry in this nlru, we can skip it immediately.
+	 */
+	if (!READ_ONCE(nlru->nr_items))
+		return;
+
+	/*
 	 * Since list_lru_{add,del} may be called under an IRQ-safe lock,
 	 * we have to use IRQ-safe primitives here to avoid deadlock.
 	 */