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Message-ID: <9e184cff-263a-d83a-0fc9-0ac7d453aa2a@redhat.com>
Date:   Mon, 28 Mar 2022 17:20:16 -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>
References: <20220309144000.1470138-1-longman@redhat.com>
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From:   Waiman Long <longman@redhat.com>
In-Reply-To: <YkIkvoCgWr8aZeRx@carbon.dhcp.thefacebook.com>
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On 3/28/22 17:12, Roman Gushchin wrote:
> 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 <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.
> 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.

I am just throwing this idea out for discussion. It does not mean that I 
want to do an arch specific patch unless there is performance data to 
indicate a substantial gain in performance in some use cases.

Cheers,
Longman