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Date: Wed, 30 Mar 2022 15:20:50 +0800
From: Muchun Song <songmuchun@...edance.com>
To: Waiman Long <longman@...hat.com>
Cc: Roman Gushchin <roman.gushchin@...ux.dev>,
Andrew Morton <akpm@...ux-foundation.org>,
Linux Memory Management List <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH-mm v3] mm/list_lru: Optimize memcg_reparent_list_lru_node()
On Wed, Mar 30, 2022 at 2:38 PM Muchun Song <songmuchun@...edance.com> wrote:
>
> On Wed, Mar 30, 2022 at 5:53 AM Waiman Long <longman@...hat.com> wrote:
> >
> > On 3/28/22 21:15, Muchun Song wrote:
> > > On Tue, Mar 29, 2022 at 3:12 AM Roman Gushchin <roman.gushchin@...ux.dev> 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@...hat.com> wrote:
> > >>>>> On 3/22/22 21:06, Muchun Song wrote:
> > >>>>>> On Wed, Mar 9, 2022 at 10:40 PM Waiman Long <longman@...hat.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:
> > > I think it may be enough if we insert a smp_rmb() between those two loads.
> > >
> > >> CPU0 CPU1
> > >> READ_ONCE(&nlru->nr_items) -> 0
> > >> spin_lock(&nlru->lock);
> > >> nlru->nr_items++;
> > > ^^^
> > > |||
> > > The nlr here is not the
> > > same as the one in CPU0,
> > > since CPU0 have done the
> > > memcg reparting. Then
> > > CPU0 will not miss nlru
> > > reparting. If I am wrong, please
> > > correct me. Thanks.
> > >> spin_unlock(&nlru->lock);
> > >> && !spin_is_locked(&nlru->lock) -> 0
> >
> > I just realize that there is another lock/unlock pair in
> > memcg_reparent_objcgs():
> >
> > memcg_reparent_objcgs()
> > spin_lock_irq()
> > memcg reparenting
> > spin_unlock_irq()
> > percpu_ref_kill()
> > spin_lock_irqsave()
> > ...
> > spin_unlock_irqrestore()
> >
> > This lock/unlock pair in percpu_ref_kill() will stop the reordering of
> > read/write before the memcg reparenting. Now I think just adding a
> > spin_is_locked() check with smp_rmb() should be enough. However, I would
> > like to change the ordering like that:
> >
> > if (!spin_is_locked(&nlru->lock)) {
> > smp_rmb();
> > if (!READ_ONCE(nlru->nr_items))
> > return;
> > }
>
> Does the following race still exist?
Ignore this. My bad. I think your approach could work.
>
> CPU0: CPU1:
> spin_is_locked(&nlru->lock)
> 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++
>
> >
> > Otherwise, we will have the problem
> >
> > list_lru_add()
> > spin_lock(&nlru->lock)
> > l = list_lru_from_kmem(memcg)
> > READ_ONCE(nlru->nr_items);
> > // Assume 0->1
> > l->nr_items++
> > // Assume 0->1
> > nlru->nr_items++
> > spin_unlock(&nlru->lock)
> > spin_is_locked()
>
> You are right.
>
> >
> > If spin_is_locked() is before spin_lock() in list_lru_add(),
> > list_lru_from_kmem() is guarantee to get the reparented memcg and so
> > won't added to the reparented lru.
> >
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