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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>