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> > 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 never get reclaimed, resulting in dead cgroups staying around forever. > > But this problem pertains to all types of objects, not just the cgroup > cache, yes? Well, of course, but there is a dramatic difference in size. Most of these objects are taking few hundreds bytes (or less), while a memcg can take few hundred kilobytes on a modern multi-CPU machine. Mostly due to per-cpu stats and events counters. > > > 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 them at all, the dying cgroup can't go away. > > Most likely, the parent cgroup hasn't any directly associated 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. > > > > ... > > > > --- a/mm/vmscan.c > > +++ b/mm/vmscan.c > > @@ -476,6 +476,17 @@ 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 even on > > + * small cgroups. This is necessary because some of > > + * belonging objects can hold a reference to a dying > > + * child cgroup. If we don't scan them, the dying > > + * cgroup can't go away unless the memory pressure > > + * (and the scanning priority) raise significantly. > > + */ > > + delta = max(delta, min(freeable, batch_size)); > > + > > If so I think the comment should be cast in more general terms. Maybe > with a final sentence "the cgroup cache is one such case". So, I think that we have to leave explicitly explained memcg refcounting case, but I'll add a line about other cases as well. > > Also, please use all 80 columns in block comments to save a few display > lines. > > And `delta' has type ULL whereas the other two are longs. We'll > presumably hit warnings here, preventable with max_t. > Let me fix this in v3. Thank you!