Messages in this thread | | | Subject | Re: [RFC PATCH 0/3] Make deferred split shrinker memcg aware | From | Yang Shi <> | Date | Wed, 29 May 2019 10:34:24 +0800 |
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On 5/29/19 9:22 AM, David Rientjes wrote: > On Tue, 28 May 2019, Yang Shi wrote: > >> I got some reports from our internal application team about memcg OOM. >> Even though the application has been killed by oom killer, there are >> still a lot THPs reside, page reclaim doesn't reclaim them at all. >> >> Some investigation shows they are on deferred split queue, memcg direct >> reclaim can't shrink them since THP deferred split shrinker is not memcg >> aware, this may cause premature OOM in memcg. The issue can be >> reproduced easily by the below test: >> > Right, we've also encountered this. I talked to Kirill about it a week or > so ago where the suggestion was to split all compound pages on the > deferred split queues under the presence of even memory pressure. > > That breaks cgroup isolation and perhaps unfairly penalizes workloads that > are running attached to other memcg hierarchies that are not under > pressure because their compound pages are now split as a side effect. > There is a benefit to keeping these compound pages around while not under > memory pressure if all pages are subsequently mapped again.
Yes, I do agree. I tried other approaches too, it sounds making deferred split queue per memcg is the optimal one.
> >> $ cgcreate -g memory:thp >> $ echo 4G > /sys/fs/cgroup/memory/thp/memory/limit_in_bytes >> $ cgexec -g memory:thp ./transhuge-stress 4000 >> >> transhuge-stress comes from kernel selftest. >> >> It is easy to hit OOM, but there are still a lot THP on the deferred split >> queue, memcg direct reclaim can't touch them since the deferred split >> shrinker is not memcg aware. >> > Yes, we have seen this on at least 4.15 as well. > >> Convert deferred split shrinker memcg aware by introducing per memcg deferred >> split queue. The THP should be on either per node or per memcg deferred >> split queue if it belongs to a memcg. When the page is immigrated to the >> other memcg, it will be immigrated to the target memcg's deferred split queue >> too. >> >> And, move deleting THP from deferred split queue in page free before memcg >> uncharge so that the page's memcg information is available. >> >> Reuse the second tail page's deferred_list for per memcg list since the same >> THP can't be on multiple deferred split queues at the same time. >> >> Remove THP specific destructor since it is not used anymore with memcg aware >> THP shrinker (Please see the commit log of patch 2/3 for the details). >> >> Make deferred split shrinker not depend on memcg kmem since it is not slab. >> It doesn't make sense to not shrink THP even though memcg kmem is disabled. >> >> With the above change the test demonstrated above doesn't trigger OOM anymore >> even though with cgroup.memory=nokmem. >> > I'm curious if your internal applications team is also asking for > statistics on how much memory can be freed if the deferred split queues > can be shrunk? We have applications that monitor their own memory usage
No, but this reminds me. The THPs on deferred split queue should be accounted into available memory too.
> through memcg stats or usage and proactively try to reduce that usage when > it is growing too large. The deferred split queues have significantly > increased both memcg usage and rss when they've upgraded kernels. > > How are your applications monitoring how much memory from deferred split > queues can be freed on memory pressure? Any thoughts on providing it as a > memcg stat?
I don't think they have such monitor. I saw rss_huge is abormal in memcg stat even after the application is killed by oom, so I realized the deferred split queue may play a role here.
The memcg stat doesn't have counters for available memory as global vmstat. It may be better to have such statistics, or extending reclaimable "slab" to shrinkable/reclaimable "memory".
> > Thanks!
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