Messages in this thread | | | Date | Sun, 27 Mar 2022 20:57:15 -0400 | Subject | Re: [PATCH-mm v3] mm/list_lru: Optimize memcg_reparent_list_lru_node() | From | Waiman Long <> |
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On 3/22/22 22:12, Muchun Song wrote: > On Wed, Mar 23, 2022 at 9:55 AM Waiman Long <longman@redhat.com> wrote: >> 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. > It is more possible in a VM. > >> 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)) > I think we also should insert a smp_rmb() between those two loads.
Thinking about this some more, I believe that adding spin_is_locked() check will be enough for x86. However, that will likely not be enough for arches with a more relaxed memory semantics. So the safest way to avoid this possible race is to move the check to within the lock critical section, though that comes with a slightly higher overhead for the 0 nr_items case. I will send out a patch to correct that. Thanks for bring this possible race to my attention.
Cheers, Longman
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