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charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Dec 1, 2020 at 2:25 AM Kirill Tkhai 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