[PATCH 4.8 020/138] mm/slab: fix kmemcg cache creation delayed issue

[Greg Kroah-Hartman]

4.8-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

commit 86d9f48534e800e4d62cdc1b5aaf539f4c1d47d6 upstream.

There is a bug report that SLAB makes extreme load average due to over
2000 kworker thread.

  https://bugzilla.kernel.org/show_bug.cgi?id=172981

This issue is caused by kmemcg feature that try to create new set of
kmem_caches for each memcg.  Recently, kmem_cache creation is slowed by
synchronize_sched() and futher kmem_cache creation is also delayed since
kmem_cache creation is synchronized by a global slab_mutex lock.  So,
the number of kworker that try to create kmem_cache increases quietly.

synchronize_sched() is for lockless access to node's shared array but
it's not needed when a new kmem_cache is created.  So, this patch rules
out that case.

Fixes: 801faf0db894 ("mm/slab: lockless decision to grow cache")
Link: http://lkml.kernel.org/r/1475734855-4837-1-git-send-email-iamjoonsoo.kim@xxxxxxx
Reported-by: Doug Smythies <dsmythies@xxxxxxxxx>
Tested-by: Doug Smythies <dsmythies@xxxxxxxxx>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 mm/slab.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/slab.c
+++ b/mm/slab.c
@@ -964,7 +964,7 @@ static int setup_kmem_cache_node(struct
 	 * guaranteed to be valid until irq is re-enabled, because it will be
 	 * freed after synchronize_sched().
 	 */
-	if (force_change)
+	if (old_shared && force_change)
 		synchronize_sched();
 
 fail: