From 405cc51fc1049c73ae03ce3771a2511a7cf8b240 Mon Sep 17 00:00:00 2001 From: Waiman Long Date: Tue, 22 Mar 2022 14:45:44 -0700 Subject: [PATCH] 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@redhat.com Signed-off-by: Waiman Long Reviewed-by: Roman Gushchin Cc: Muchun Song Cc: Michal Hocko Cc: Johannes Weiner Cc: Shakeel Butt Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- 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.