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From: Roman Gushchin To: Waiman Long Cc: Muchun Song , Andrew Morton , Linux Memory Management List , LKML Subject: Re: [PATCH-mm v3] mm/list_lru: Optimize memcg_reparent_list_lru_node() Message-ID: References: <20220309144000.1470138-1-longman@redhat.com> <2263666d-5eef-b1fe-d5e3-b166a3185263@redhat.com> <5aa687c4-2888-7977-8c1a-d51384e685aa@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5aa687c4-2888-7977-8c1a-d51384e685aa@redhat.com> X-Migadu-Flow: FLOW_OUT X-Migadu-Auth-User: linux.dev Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Mar 28, 2022 at 04:46:39PM -0400, Waiman Long wrote: > 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 wrote: > > > > > On 3/22/22 21:06, Muchun Song wrote: > > > > > > On Wed, Mar 9, 2022 at 10:40 PM 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. > > > > > > 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. Hm, is this such a big problem in the real life? Can you describe the setup? I'm somewhat resistant to an idea of having arch-specific optimizations here without a HUGE reason. > > > 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. I need to think, I'll share any ideas/patches if there will be any. Thanks!