Messages in this thread Patch in this message | | | Date | Tue, 22 Mar 2022 21:55:05 -0400 | Subject | Re: [PATCH-mm v3] mm/list_lru: Optimize memcg_reparent_list_lru_node() | From | Waiman Long <> |
| |
On 3/22/22 21:06, Muchun Song wrote: > On Wed, Mar 9, 2022 at 10:40 PM Waiman Long <longman@redhat.com> 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. > Hi Waiman, > > Sorry for the late reply. Quick question: what if there is an inflight > list_lru_add()? How about the following race? > > CPU0: CPU1: > list_lru_add() > spin_lock(&nlru->lock) > l = list_lru_from_kmem(memcg) > memcg_reparent_objcgs(memcg) > memcg_reparent_list_lrus(memcg) > memcg_reparent_list_lru() > memcg_reparent_list_lru_node() > if (!READ_ONCE(nlru->nr_items)) > // Miss reparenting > return > // Assume 0->1 > l->nr_items++ > // Assume 0->1 > nlru->nr_items++ > > IIUC, we use nlru->lock to serialise this scenario.
I guess this race is theoretically possible but very unlikely since it means a very long pause between list_lru_from_kmem() and the increment of nr_items.
How about the following changes to make sure that this race can't happen?
diff --git a/mm/list_lru.c b/mm/list_lru.c index c669d87001a6..c31a0a8ad4e7 100644 --- a/mm/list_lru.c +++ b/mm/list_lru.c @@ -395,9 +395,10 @@ static void memcg_reparent_list_lru_node(struct list_lru *lru, int nid, struct list_lru_one *src, *dst;
/* - * If there is no lru entry in this nlru, we can skip it immediately. + * If there is no lru entry in this nlru and the nlru->lock is free, + * we can skip it immediately. */ - if (!READ_ONCE(nlru->nr_items)) + if (!READ_ONCE(nlru->nr_items) && !spin_is_locked(&nlru->lock)) return;
Cheers, Longman
| |