[Devel] [PATCH vz8 01/19] mm/list_lru.c: combine code under the same define

[Andrey Ryabinin]

From: Kirill Tkhai <ktkhai at virtuozzo.com>

Patch series "Improve shrink_slab() scalability (old complexity was O(n^2), new is O(n))", v8.

This patcheset solves the problem with slow shrink_slab() occuring on
the machines having many shrinkers and memory cgroups (i.e., with many
containers).  The problem is complexity of shrink_slab() is O(n^2) and
it grows too fast with the growth of containers numbers.

Let us have 200 containers, and every container has 10 mounts and 10
cgroups.  All container tasks are isolated, and they don't touch foreign
containers mounts.

In case of global reclaim, a task has to iterate all over the memcgs and
to call all the memcg-aware shrinkers for all of them.  This means, the
task has to visit 200 * 10 = 2000 shrinkers for every memcg, and since
there are 2000 memcgs, the total calls of do_shrink_slab() are 2000 *
2000 = 4000000.

4 million calls are not a number operations, which can takes 1 cpu
cycle.  E.g., super_cache_count() accesses at least two lists, and makes
arifmetical calculations.  Even, if there are no charged objects, we do
these calculations, and replaces cpu caches by read memory.  I observed
nodes spending almost 100% time in kernel, in case of intensive writing
and global reclaim.  The writer consumes pages fast, but it's need to
shrink_slab() before the reclaimer reached shrink pages function (and
frees SWAP_CLUSTER_MAX pages).  Even if there is no writing, the
iterations just waste the time, and slows reclaim down.

Let's see the small test below:

  $echo 1 > /sys/fs/cgroup/memory/memory.use_hierarchy
  $mkdir /sys/fs/cgroup/memory/ct
  $echo 4000M > /sys/fs/cgroup/memory/ct/memory.kmem.limit_in_bytes
  $for i in `seq 0 4000`;
          do mkdir /sys/fs/cgroup/memory/ct/$i;
          echo $$ > /sys/fs/cgroup/memory/ct/$i/cgroup.procs;
          mkdir -p s/$i; mount -t tmpfs $i s/$i; touch s/$i/file;
  done

Then, let's see drop caches time (5 sequential calls):

  $time echo 3 > /proc/sys/vm/drop_caches

  0.00user 13.78system 0:13.78elapsed 99%CPU
  0.00user 5.59system 0:05.60elapsed 99%CPU
  0.00user 5.48system 0:05.48elapsed 99%CPU
  0.00user 8.35system 0:08.35elapsed 99%CPU
  0.00user 8.34system 0:08.35elapsed 99%CPU

The last four calls don't actually shrink anything.  So, the iterations
over slab shrinkers take 5.48 seconds.  Not so good for scalability.

The patchset solves the problem by making shrink_slab() of O(n)
complexity.  There are following functional actions:

1) Assign id to every registered memcg-aware shrinker.

2) Maintain per-memcgroup bitmap of memcg-aware shrinkers, and set a
   shrinker-related bit after the first element is added to lru list
   (also, when removed child memcg elements are reparanted).

3) Split memcg-aware shrinkers and !memcg-aware shrinkers, and call a
   shrinker if its bit is set in memcg's shrinker bitmap.  (Also, there is
   a functionality to clear the bit, after last element is shrinked).

This gives significant performance increase.  The result after patchset
is applied:

  $time echo 3 > /proc/sys/vm/drop_caches

  0.00user 1.10system 0:01.10elapsed 99%CPU
  0.00user 0.00system 0:00.01elapsed 64%CPU
  0.00user 0.01system 0:00.01elapsed 82%CPU
  0.00user 0.00system 0:00.01elapsed 64%CPU
  0.00user 0.01system 0:00.01elapsed 82%CPU

The results show the performance increases at least in 548 times.

So, the patchset makes shrink_slab() of less complexity and improves the
performance in such types of load I pointed.  This will give a profit in
case of !global reclaim case, since there also will be less
do_shrink_slab() calls.

This patch (of 17):

These two pairs of blocks of code are under the same #ifdef #else
#endif.

Link: http://lkml.kernel.org/r/153063052519.1818.9393587113056959488.stgit@localhost.localdomain
Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
Acked-by: Vladimir Davydov <vdavydov.dev at gmail.com>
Tested-by: Shakeel Butt <shakeelb at google.com>
Cc: Al Viro <viro at zeniv.linux.org.uk>
Cc: Johannes Weiner <hannes at cmpxchg.org>
Cc: Michal Hocko <mhocko at kernel.org>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Philippe Ombredanne <pombredanne at nexb.com>
Cc: Sahitya Tummala <stummala at codeaurora.org>
Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Cc: Stephen Rothwell <sfr at canb.auug.org.au>
Cc: Roman Gushchin <guro at fb.com>
Cc: Matthias Kaehlcke <mka at chromium.org>
Cc: Tetsuo Handa <penguin-kernel at I-love.SAKURA.ne.jp>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Waiman Long <longman at redhat.com>
Cc: Minchan Kim <minchan at kernel.org>
Cc: "Huang, Ying" <ying.huang at intel.com>
Cc: Mel Gorman <mgorman at techsingularity.net>
Cc: Josef Bacik <jbacik at fb.com>
Cc: Guenter Roeck <linux at roeck-us.net>
Cc: Matthew Wilcox <willy at infradead.org>
Cc: Li RongQing <lirongqing at baidu.com>
Cc: Andrey Ryabinin <aryabinin at virtuozzo.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(cherry picked from commit e0295238e50f1aa16d4c902c837fd8d17861b698)
Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 mm/list_lru.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/mm/list_lru.c b/mm/list_lru.c
index fcfb6c89ed47..1e3e2f3a2a64 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -29,17 +29,7 @@ static void list_lru_unregister(struct list_lru *lru)
 	list_del(&lru->list);
 	mutex_unlock(&list_lrus_mutex);
 }
-#else
-static void list_lru_register(struct list_lru *lru)
-{
-}
-
-static void list_lru_unregister(struct list_lru *lru)
-{
-}
-#endif /* CONFIG_MEMCG && !CONFIG_SLOB */
 
-#if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)
 static inline bool list_lru_memcg_aware(struct list_lru *lru)
 {
 	/*
@@ -89,6 +79,14 @@ list_lru_from_kmem(struct list_lru_node *nlru, void *ptr)
 	return list_lru_from_memcg_idx(nlru, memcg_cache_id(memcg));
 }
 #else
+static void list_lru_register(struct list_lru *lru)
+{
+}
+
+static void list_lru_unregister(struct list_lru *lru)
+{
+}
+
 static inline bool list_lru_memcg_aware(struct list_lru *lru)
 {
 	return false;
-- 
2.24.1