From: Waiman Long <longman@xxxxxxxxxx>
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/20220309144000.1470138-1-longman@xxxxxxxxxx
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
Reviewed-by: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/list_lru.c | 6 ++++++
1 file changed, 6 insertions(+)
--- 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.
*/
_
