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

[Yang Shi <shy828301@gmail.com>]

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);

>
> Thanks,
> Kirill