Re: [PATCH] mm: list_lru: hold nlru lock to avoid reading transient negative nr_items

[Kirill Tkhai]

On 01.12.2020 20:15, Yang Shi wrote:
> On Tue, Dec 1, 2020 at 2:25 AM Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
>>
>> On 30.11.2020 23:09, Roman Gushchin wrote:
>>> On Mon, Nov 30, 2020 at 10:45:14AM -0800, Yang Shi wrote:
>>>> When investigating a slab cache bloat problem, significant amount of
>>>> negative dentry cache was seen, but confusingly they neither got shrunk
>>>> by reclaimer (the host has very tight memory) nor be shrunk by dropping
>>>> cache.  The vmcore shows there are over 14M negative dentry objects on lru,
>>>> but tracing result shows they were even not scanned at all.  The further
>>>> investigation shows the memcg's vfs shrinker_map bit is not set.  So the
>>>> reclaimer or dropping cache just skip calling vfs shrinker.  So we have
>>>> to reboot the hosts to get the memory back.
>>>>
>>>> I didn't manage to come up with a reproducer in test environment, and the
>>>> problem can't be reproduced after rebooting.  But it seems there is race
>>>> between shrinker map bit clear and reparenting by code inspection.  The
>>>> hypothesis is elaborated as below.
>>>>
>>>> The memcg hierarchy on our production environment looks like:
>>>>                 root
>>>>                /    \
>>>>           system   user
>>>>
>>>> The main workloads are running under user slice's children, and it creates
>>>> and removes memcg frequently.  So reparenting happens very often under user
>>>> slice, but no task is under user slice directly.
>>>>
>>>> So with the frequent reparenting and tight memory pressure, the below
>>>> hypothetical race condition may happen:
>>>>
>>>>     CPU A                            CPU B                         CPU C
>>>> reparent
>>>>     dst->nr_items == 0
>>>>                                  shrinker:
>>>>                                      total_objects == 0
>>>>     add src->nr_items to dst
>>>>     set_bit
>>>>                                      retrun SHRINK_EMPTY
>>>>                                      clear_bit
>>>>                                                                   list_lru_del()
>>>> reparent again
>>>>     dst->nr_items may go negative
>>>>     due to current list_lru_del()
>>>>     on CPU C
>>>>                                  The second run of shrinker:
>>>>                                      read nr_items without any
>>>>                                      synchronization, so it may
>>>>                                      see intermediate negative
>>>>                                      nr_items then total_objects
>>>>                                      may return 0 conincidently
>>>>
>>>>                                      keep the bit cleared
>>>>     dst->nr_items != 0
>>>>     skip set_bit
>>>>     add scr->nr_item to dst
>>>>
>>>> After this point dst->nr_item may never go zero, so reparenting will not
>>>> set shrinker_map bit anymore.  And since there is no task under user
>>>> slice directly, so no new object will be added to its lru to set the
>>>> shrinker map bit either.  That bit is kept cleared forever.
>>>>
>>>> How does list_lru_del() race with reparenting?  It is because
>>>> reparenting replaces childen's kmemcg_id to parent's without protecting
>>>> from nlru->lock, so list_lru_del() may see parent's kmemcg_id but
>>>> actually deleting items from child's lru, but dec'ing parent's nr_items,
>>>> so the parent's nr_items may go negative as commit
>>>> 2788cf0c401c268b4819c5407493a8769b7007aa ("memcg: reparent list_lrus and
>>>> free kmemcg_id on css offline") says.
>>>>
>>>> Can we move kmemcg_id replacement after reparenting?  No, because the
>>>> race with list_lru_del() may result in negative src->nr_items, but it
>>>> will never be fixed.  So the shrinker may never return SHRINK_EMPTY then
>>>> keep the shrinker map bit set always.  The shrinker will be always
>>>> called for nonsense.
>>>>
>>>> Can we synchronize list_lru_del() and reparenting?  Yes, it could be
>>>> done.  But it seems we need introduce a new lock or use nlru->lock.  But
>>>> it sounds complicated to move kmemcg_id replacement code under nlru->lock.
>>>> And list_lru_del() may be called quite often to exacerbate some hot
>>>> path, i.e. dentry kill.
>>>>
>>>> So, it sounds acceptable to synchronize reading nr_items to avoid seeing
>>>> intermediate negative nr_items given the simplicity and it is typically
>>>> just called by shrinkers when counting the freeable objects.
>>>>
>>>> The patch is tested with some shrinker intensive workloads, no
>>>> noticeable regression is soptted.
>>>
>>> Hi Yang!
>>>
>>> It's really tricky, thank you for digging in! It's a perfect analysis!
>>>
>>> I wonder though, if it's better to just always set the shrinker bit on reparenting
>>> if we do reparent some items? Then we'll avoid adding new synchronization
>>> to the hot path. What do you think?
>>>
>>> --
>>>
>>> @@ -534,7 +534,6 @@ static void memcg_drain_list_lru_node(struct list_lru *lru, int nid,
>>>       struct list_lru_node *nlru = &lru->node[nid];
>>>       int dst_idx = dst_memcg->kmemcg_id;
>>>       struct list_lru_one *src, *dst;
>>> -     bool set;
>>>
>>>       /*
>>>        * Since list_lru_{add,del} may be called under an IRQ-safe lock,
>>> @@ -546,9 +545,8 @@ static void memcg_drain_list_lru_node(struct list_lru *lru, int nid,
>>>       dst = list_lru_from_memcg_idx(nlru, dst_idx);
>>>
>>>       list_splice_init(&src->list, &dst->list);
>>> -     set = (!dst->nr_items && src->nr_items);
>>>       dst->nr_items += src->nr_items;
>>> -     if (set)
>>> +     if (src->nr_items)
>>>               memcg_set_shrinker_bit(dst_memcg, nid, lru_shrinker_id(lru));
>>>       src->nr_items = 0;
>>
>> This looks like a good fix.
>>
>> To make a code more clear, we may also want to group neighbouring lines
>> under the same "if" branch in Yang's v2 resend.
> 
> You mean something like the below (diff based on Roman's proposal)?
> 
> diff --git a/mm/list_lru.c b/mm/list_lru.c
> index 127c2cf9f831..fe230081690b 100644
> --- a/mm/list_lru.c
> +++ b/mm/list_lru.c
> @@ -545,10 +545,12 @@ static void memcg_drain_list_lru_node(struct
> list_lru *lru, int nid,
>         dst = list_lru_from_memcg_idx(nlru, dst_idx);
> 
>         list_splice_init(&src->list, &dst->list);
> -       dst->nr_items += src->nr_items;
> -       if (src->nr_items)
> +
> +       if (src->nr_items) {
> +               dst->nr_items += src->nr_items;
>                 memcg_set_shrinker_bit(dst_memcg, nid, lru_shrinker_id(lru));
> -       src->nr_items = 0;
> +               src->nr_items = 0;
> +       }
> 
>         spin_unlock_irq(&nlru->lock);

Yes.