From: Roman Gushchin guro@fb.com Subject: mm: memcg/slab: optimize objcg stock draining

Imran Khan reported a 16% regression in hackbench results caused by the commit f2fe7b09a52b ("mm: memcg/slab: charge individual slab objects instead of pages"). The regression is noticeable in the case of a consequent allocation of several relatively large slab objects, e.g. skb's. As soon as the amount of stocked bytes exceeds PAGE_SIZE, drain_obj_stock() and __memcg_kmem_uncharge() are called, and it leads to a number of atomic operations in page_counter_uncharge().

The corresponding call graph is below (provided by Imran Khan): |__alloc_skb | | | |__kmalloc_reserve.isra.61 | | | | | |__kmalloc_node_track_caller | | | | | | | |slab_pre_alloc_hook.constprop.88 | | | obj_cgroup_charge | | | | | | | | | |__memcg_kmem_charge | | | | | | | | | | | |page_counter_try_charge | | | | | | | | | |refill_obj_stock | | | | | | | | | | | |drain_obj_stock.isra.68 | | | | | | | | | | | | | |__memcg_kmem_uncharge | | | | | | | | | | | | | | | |page_counter_uncharge | | | | | | | | | | | | | | | | | |page_counter_cancel | | | | | | | | | | | |__slab_alloc | | | | | | | | | |___slab_alloc | | | | | | | | |slab_post_alloc_hook

Instead of directly uncharging the accounted kernel memory, it's possible to refill the generic page-sized per-cpu stock instead. It's a much faster operation, especially on a default hierarchy. As a bonus, __memcg_kmem_uncharge_page() will also get faster, so the freeing of page-sized kernel allocations (e.g. large kmallocs) will become faster.

A similar change has been done earlier for the socket memory by the commit 475d0487a2ad ("mm: memcontrol: use per-cpu stocks for socket memory uncharging").

Link: https://lkml.kernel.org/r/20210106042239.2860107-1-guro@fb.com Fixes: f2fe7b09a52b ("mm: memcg/slab: charge individual slab objects instead of pages") Signed-off-by: Roman Gushchin guro@fb.com Reported-by: Imran Khan imran.f.khan@oracle.com Tested-by: Imran Khan imran.f.khan@oracle.com Reviewed-by: Shakeel Butt shakeelb@google.com Reviewed-by: Michal Koutn mkoutny@suse.com Cc: Michal Koutný mkoutny@suse.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/memcontrol.c | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-)

--- a/mm/memcontrol.c~mm-memcg-slab-optimize-objcg-stock-draining +++ a/mm/memcontrol.c @@ -3115,9 +3115,7 @@ void __memcg_kmem_uncharge(struct mem_cg if (!cgroup_subsys_on_dfl(memory_cgrp_subsys)) page_counter_uncharge(&memcg->kmem, nr_pages);

- page_counter_uncharge(&memcg->memory, nr_pages); - if (do_memsw_account()) - page_counter_uncharge(&memcg->memsw, nr_pages); + refill_stock(memcg, nr_pages); }

/** _