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Date:   Mon, 28 Mar 2022 16:46:39 -0400
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Subject: Re: [PATCH-mm v3] mm/list_lru: Optimize
 memcg_reparent_list_lru_node()
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To:     Roman Gushchin <roman.gushchin@linux.dev>
Cc:     Muchun Song <songmuchun@bytedance.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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From:   Waiman Long <longman@redhat.com>
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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