On Mon, Nov 30, 2020 at 12:57:47PM -0800, Yang Shi wrote:
> On Mon, Nov 30, 2020 at 12:09 PM Roman Gushchin <guro@xxxxxx> 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.
Also note that since the introduction of the slab reparenting, list_lru_from_kmem()
can return the parent lru.
> > >
> > > 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?
>
> Thanks a lot for the suggestion. I was thinking about the same
> approach too, but I thought src->nr_items may go zero due to
> concurrent list_lru_del() at the first place. But I just rethought the
> whole thing, it seems impossible that dst->nr_items goes negative and
> src->nr_items goes zero at the same time.
Even if it would be possible, it seems less scary: the next reparenting
will likely set the bit. So we'll not get into the permanently bad state.
> list_lru_del() should just
> see either dst or src, it can't manipulate both lists simultaneously.
> So I think your suggestion should work. I will incarnate your
> suggestion in v2.
>
> >
> > --
> >
> > @@ -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;
> >
> >
> > --
> >
> > Btw, it seems that the bug is quite old. I wonder why we haven't seen it before?
> > Any ideas?
>
> It is not new, but not that old from my point of view. The
> shrinker_map thing was introduced since v4.19, I bet pre-v4.19 kernel
> may still dominate in production environment. And, it needs some
> conditions (i.e. nr_inode + nr_dentry == 0 coincidently, and there is
> not task under dst memcg directly, etc) to trigger, so it seems
> unlikely to hit.
>
> And the consequence may be not noticeable to the most people at all.
> We happened to see frequent OOMs on a couple of small machines (32G
> memory w/o swap, but most memory was consumed by anonymous pages)
> recently and they were already up for long time (almost 300 days),
> then the investigation leads to this race condition.
I agree that most users will unlikely notice it.
But https://www.spinics.net/lists/cgroups/msg27295.html looks very similar
and can be caused by the same problem. Once you'll have v2, let's ask
them to test it too.
Thanks!
