[PATCH] mm: memcg/slab: optimize objcg stock draining

[Roman Gushchin <guro@fb.com>]

Imran Khan reported a 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").

Signed-off-by: Roman Gushchin <guro@fb.com>
Reported-by: Imran Khan <imran.f.khan@oracle.com>
---
 mm/memcontrol.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 0d74b80fa4de..8148c1df3aff 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3122,9 +3122,7 @@ void __memcg_kmem_uncharge(struct mem_cgroup *memcg, unsigned int nr_pages)
 	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);
 }
 
 /**
-- 
2.26.2