mm: memcg/slab: optimize objcg stock draining

Imran Khan reported a 16% regression in hackbench results caused by the
commit f2fe7b0 ("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 475d048 ("mm: memcontrol: use per-cpu stocks for socket
memory uncharging").

Link: https://lkml.kernel.org/r/[email protected]
Fixes: f2fe7b0 ("mm: memcg/slab: charge individual slab objects instead of pages")
Signed-off-by: Roman Gushchin <[email protected]>
Reported-by: Imran Khan <[email protected]>
Tested-by: Imran Khan <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Reviewed-by: Michal Koutn <[email protected]>
Cc: Michal Koutný <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/memcontrol.c

@@ -3115,9 +3115,7 @@ 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);
 }
 
 /**