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Subject[PATCH] mm/list_lru: Optimize memcg_drain_list_lru_node()
Date
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_drain_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.

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

diff --git a/mm/list_lru.c b/mm/list_lru.c
index 0cd5e89ca063..100ca453e885 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -518,6 +518,12 @@ static void memcg_drain_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
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