On Tue, 03 Jul 2018 18:09:26 +0300 Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote: > Imagine a big node with many cpus, memory cgroups and containers. > Let we have 200 containers, every container has 10 mounts, > and 10 cgroups. All container tasks don't touch foreign > containers mounts. If there is intensive pages write, > and global reclaim happens, a writing task has to iterate > over all memcgs to shrink slab, before it's able to go > to shrink_page_list(). > > Iteration over all the memcg slabs is very expensive: > the task has to visit 200 * 10 = 2000 shrinkers > for every memcg, and since there are 2000 memcgs, > the total calls are 2000 * 2000 = 4000000. > > So, the shrinker makes 4 million do_shrink_slab() calls > just to try to isolate SWAP_CLUSTER_MAX pages in one > of the actively writing memcg via shrink_page_list(). > I've observed a node spending almost 100% in kernel, > making useless iteration over already shrinked slab. > > This patch adds bitmap of memcg-aware shrinkers to memcg. > The size of the bitmap depends on bitmap_nr_ids, and during > memcg life it's maintained to be enough to fit bitmap_nr_ids > shrinkers. Every bit in the map is related to corresponding > shrinker id. > > Next patches will maintain set bit only for really charged > memcg. This will allow shrink_slab() to increase its > performance in significant way. See the last patch for > the numbers. > > ... > > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -182,6 +182,11 @@ static int prealloc_memcg_shrinker(struct shrinker *shrinker) > if (id < 0) > goto unlock; > > + if (memcg_expand_shrinker_maps(id)) { > + idr_remove(&shrinker_idr, id); > + goto unlock; > + } > + > if (id >= shrinker_nr_max) > shrinker_nr_max = id + 1; > shrinker->id = id; This function ends up being a rather sad little thing. : static int prealloc_memcg_shrinker(struct shrinker *shrinker) : { : int id, ret = -ENOMEM; : : down_write(&shrinker_rwsem); : id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_KERNEL); : if (id < 0) : goto unlock; : : if (memcg_expand_shrinker_maps(id)) { : idr_remove(&shrinker_idr, id); : goto unlock; : } : : if (id >= shrinker_nr_max) : shrinker_nr_max = id + 1; : shrinker->id = id; : ret = 0; : unlock: : up_write(&shrinker_rwsem); : return ret; : } - there's no need to call memcg_expand_shrinker_maps() unless id >= shrinker_nr_max so why not move the code and avoid calling memcg_expand_shrinker_maps() in most cases. - why aren't we decreasing shrinker_nr_max in unregister_memcg_shrinker()? That's easy to do, avoids pointless work in shrink_slab_memcg() and avoids memory waste in future prealloc_memcg_shrinker() calls. It should be possible to find the highest ID in an IDR tree with a straightforward descent of the underlying radix tree, but I doubt if that has been wired up. Otherwise a simple loop in unregister_memcg_shrinker() would be needed.