Re: [PATCH] mm: list_lru: hold nlru lock to avoid reading transient negative nr_items
From: Yang Shi
Date: Tue Dec 01 2020 - 12:17:01 EST
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);
>
> Thanks,
> Kirill