| Message ID | 1559047464-59838-1-git-send-email-yang.shi@linux.alibaba.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | Make deferred split shrinker memcg aware | expand |
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. > $ 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 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? Thanks!
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!
On Wed, 29 May 2019, Yang Shi wrote: > > 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. > The approach we went with were to track the actual counts of compound pages on the deferred split queue for each pgdat for each memcg and then invoke the shrinker for memcg reclaim and iterate those not charged to the hierarchy under reclaim. That's suboptimal and was a stop gap measure under time pressure: it's refreshing to see the optimal method being pursued, thanks! > > 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. > Right, and we have also seen this for users of MADV_FREE that have both an increased rss and memcg usage that don't realize that the memory is freed under pressure. I'm thinking that we need some kind of MemAvailable for memcg hierarchies to be the authoritative source of what can be reclaimed under pressure. > > 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. > Exactly the same in my case :) We were likely looking at the exact same issue at the same time. > 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". > Have you considered following how NR_ANON_MAPPED is tracked for each pgdat and using that as an indicator of when the modify a memcg stat to track the amount of memory on a compound page? I think this would be necessary for userspace to know what their true memory usage is.
On 5/30/19 5:07 AM, David Rientjes wrote: > On Wed, 29 May 2019, Yang Shi wrote: > >>> 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. >> > The approach we went with were to track the actual counts of compound > pages on the deferred split queue for each pgdat for each memcg and then > invoke the shrinker for memcg reclaim and iterate those not charged to the > hierarchy under reclaim. That's suboptimal and was a stop gap measure > under time pressure: it's refreshing to see the optimal method being > pursued, thanks! We did the exactly same thing for a temporary hotfix. > >>> 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. >> > Right, and we have also seen this for users of MADV_FREE that have both an > increased rss and memcg usage that don't realize that the memory is freed > under pressure. I'm thinking that we need some kind of MemAvailable for > memcg hierarchies to be the authoritative source of what can be reclaimed > under pressure. It sounds useful. We also need know the available memory in memcg scope in our containers. > >>> 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. >> > Exactly the same in my case :) We were likely looking at the exact same > issue at the same time. Yes, it seems so. :-) >> 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". >> > Have you considered following how NR_ANON_MAPPED is tracked for each pgdat > and using that as an indicator of when the modify a memcg stat to track > the amount of memory on a compound page? I think this would be necessary > for userspace to know what their true memory usage is. No, I haven't. Do you mean minus MADV_FREE and deferred split THP from NR_ANON_MAPPED? It looks they have been decreased from NR_ANON_MAPPED when removing rmap.
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: $ 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. 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. Yang Shi (3): mm: thp: make deferred split shrinker memcg aware mm: thp: remove THP destructor mm: shrinker: make shrinker not depend on memcg kmem include/linux/huge_mm.h | 24 +++++++++ include/linux/memcontrol.h | 6 +++ include/linux/mm.h | 3 -- include/linux/mm_types.h | 7 ++- include/linux/shrinker.h | 3 +- mm/huge_memory.c | 181 ++++++++++++++++++++++++++++++++++++++++++++++++------------------- mm/memcontrol.c | 20 ++++++++ mm/page_alloc.c | 3 -- mm/swap.c | 4 ++ mm/vmscan.c | 27 +++------- 10 files changed, 198 insertions(+), 80 deletions(-)