Re: [PATCH-mm v3] mm/list_lru: Optimize memcg_reparent_list_lru_node()

[Muchun Song <songmuchun@bytedance.com>]

On Wed, Mar 30, 2022 at 2:38 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> On Wed, Mar 30, 2022 at 5:53 AM Waiman Long <longman@redhat.com> wrote:
> >
> > On 3/28/22 21:15, Muchun Song wrote:
> > > On Tue, Mar 29, 2022 at 3:12 AM Roman Gushchin <roman.gushchin@linux.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@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:
> > > 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.
> >