On 3/8/22 21:13, Roman Gushchin wrote:
On Tue, Mar 08, 2022 at 08:18:24PM -0500, Waiman Long wrote:
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.
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
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;
This is a per-node counter, not a per-memcg, right?
Right. list_lru_node is a per-node structure inside list_lru.
If so, do we optimize for the case when all lru items belong to one node and
others are empty?
That is actually the case that I am trying to optimize for.
If a system has many containers. It is also likely each container may
mount one or more container specific filesystems. Since a container
likely use just a few cpus, it is highly that only the list_lru_node
that contains those cpus will be utilized while the rests may be empty.
I got the idea of doing this patch when I was looking at a crash dump
related to the list_lru code. That particular crash dump has more than
13k list_lru's and thousands of mount points. I had notice even if
nr_items of a list_lru_node is 0, it still tries to transfer lru entries
from source idx to dest idx. Without doing an lock/unlock and loading a
cacheline from the memcg_lrus, it can save some time. That can be
substantial saving if we are talking about thousands of list_lru's.
Cheers,
Longman
