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SubjectRe: [PATCH] mm: list_lru: hold nlru lock to avoid reading transient negative nr_items
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On 01.12.2020 20:15, Yang Shi wrote:
> On Tue, Dec 1, 2020 at 2:25 AM Kirill Tkhai <ktkhai@virtuozzo.com> 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.

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