Messages in this thread | | | Date | Mon, 28 Mar 2022 16:46:39 -0400 | Subject | Re: [PATCH-mm v3] mm/list_lru: Optimize memcg_reparent_list_lru_node() | From | Waiman Long <> |
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On 3/28/22 15:12, Roman Gushchin wrote: > On Sun, Mar 27, 2022 at 08:57:15PM -0400, Waiman Long wrote: >> 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. > Yes, I think it's not enough: > CPU0 CPU1 > READ_ONCE(&nlru->nr_items) -> 0 > spin_lock(&nlru->lock); > nlru->nr_items++; > spin_unlock(&nlru->lock); > && !spin_is_locked(&nlru->lock) -> 0 I have actually thought of that. I am even thinking about reading nr_items again after spin_is_locked(). Still for arches with relaxed memory semantics, when will a memory write by one cpu be propagated to another cpu can be highly variable. It is very hard to prove that it is completely safe.
x86 has a more strict memory semantics and it is the only architecture that I have enough confidence that doing the check without taking a lock can be safe. Perhaps we could use this optimization just for x86 and do it inside locks for the rests.
> Getting back to the original patch, I wonder if instead we can batch reparenting > of lrus so we don't have to grab and release nlru->lock for each reparenting lru.
nlru is actually a sub-structure within a lru. So if there are m lrus and n nodes, we will have m*n nlrus. I don't believe there is anymore batching that can be done.
Cheers, Longman
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