From: Roman Gushchin guro@fb.com
[ Upstream commit 172b06c32b949759fe6313abec514bc4f15014f4 ]
9092c71bb724 ("mm: use sc->priority for slab shrink targets") changed the way that the target slab pressure is calculated and made it priority-based:
delta = freeable >> priority; delta *= 4; do_div(delta, shrinker->seeks);
The problem is that on a default priority (which is 12) no pressure is applied at all, if the number of potentially reclaimable objects is less than 4096 (1<<12).
This causes the last objects on slab caches of no longer used cgroups to (almost) never get reclaimed. It's obviously a waste of memory.
It can be especially painful, if these stale objects are holding a reference to a dying cgroup. Slab LRU lists are reparented on memcg offlining, but corresponding objects are still holding a reference to the dying cgroup. If we don't scan these objects, the dying cgroup can't go away. Most likely, the parent cgroup hasn't any directly charged objects, only remaining objects from dying children cgroups. So it can easily hold a reference to hundreds of dying cgroups.
If there are no big spikes in memory pressure, and new memory cgroups are created and destroyed periodically, this causes the number of dying cgroups grow steadily, causing a slow-ish and hard-to-detect memory "leak". It's not a real leak, as the memory can be eventually reclaimed, but it could not happen in a real life at all. I've seen hosts with a steadily climbing number of dying cgroups, which doesn't show any signs of a decline in months, despite the host is loaded with a production workload.
It is an obvious waste of memory, and to prevent it, let's apply a minimal pressure even on small shrinker lists. E.g. if there are freeable objects, let's scan at least min(freeable, scan_batch) objects.
This fix significantly improves a chance of a dying cgroup to be reclaimed, and together with some previous patches stops the steady growth of the dying cgroups number on some of our hosts.
Link: http://lkml.kernel.org/r/20180905230759.12236-1-guro@fb.com Fixes: 9092c71bb724 ("mm: use sc->priority for slab shrink targets") Signed-off-by: Roman Gushchin guro@fb.com Acked-by: Rik van Riel riel@surriel.com Cc: Josef Bacik jbacik@fb.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Shakeel Butt shakeelb@google.com Cc: Michal Hocko mhocko@kernel.org Cc: Vladimir Davydov vdavydov.dev@gmail.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Sasha Levin alexander.levin@microsoft.com --- mm/vmscan.c | 11 +++++++++++ 1 file changed, 11 insertions(+)
diff --git a/mm/vmscan.c b/mm/vmscan.c index 03822f86f288..fc0436407471 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -386,6 +386,17 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl, delta = freeable >> priority; delta *= 4; do_div(delta, shrinker->seeks); + + /* + * Make sure we apply some minimal pressure on default priority + * even on small cgroups. Stale objects are not only consuming memory + * by themselves, but can also hold a reference to a dying cgroup, + * preventing it from being reclaimed. A dying cgroup with all + * corresponding structures like per-cpu stats and kmem caches + * can be really big, so it may lead to a significant waste of memory. + */ + delta = max_t(unsigned long long, delta, min(freeable, batch_size)); + total_scan += delta; if (total_scan < 0) { pr_err("shrink_slab: %pF negative objects to delete nr=%ld\n",