linux-kernel - [PATCH] mm/slab: fix kmemcg cache creation delayed issue

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Date:   Thu,  6 Oct 2016 15:20:55 +0900
From:   js1304@...il.com
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Doug Smythies <dsmythies@...us.net>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Joonsoo Kim <iamjoonsoo.kim@....com>,
        stable@...r.kernel.org
Subject: [PATCH] mm/slab: fix kmemcg cache creation delayed issue

From: Joonsoo Kim <iamjoonsoo.kim@....com>

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 quitely.
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")
Cc: stable@...r.kernel.org
Reported-by: Doug Smythies <dsmythies@...us.net>
Tested-by: Doug Smythies <dsmythies@...us.net>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
---
 mm/slab.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/slab.c b/mm/slab.c
index 6508b4d..3c83c29 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -961,7 +961,7 @@ static int setup_kmem_cache_node(struct kmem_cache *cachep,
 	 * 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:
-- 
1.9.1