[PATCH-mm v3] mm/list_lru: Optimize memcg_reparent_list_lru_node()

[Waiman Long <longman@redhat.com>]

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.

Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
---
 mm/list_lru.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/list_lru.c b/mm/list_lru.c
index ba76428ceece..c669d87001a6 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -394,6 +394,12 @@ static void memcg_reparent_list_lru_node(struct list_lru *lru, int nid,
 	int dst_idx = dst_memcg->kmemcg_id;
 	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.
-- 
2.27.0