linux-kernel - Re: [patch 1/2]slub: add slab with one free object to partial list tail

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Date:	Tue, 23 Aug 2011 09:48:18 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm <linux-mm@...ck.org>, lkml <linux-kernel@...r.kernel.org>,
	cl@...ux.com, penberg@...nel.org, "Shi, Alex" <alex.shi@...el.com>,
	"Chen, Tim C" <tim.c.chen@...el.com>
Subject: Re: [patch 1/2]slub: add slab with one free object to partial list
 tail

On Tue, 2011-08-23 at 08:36 +0800, Shaohua Li wrote:
> The slab has just one free object, adding it to partial list head doesn't make
> sense. And it can cause lock contentation. For example,
> 1. CPU takes the slab from partial list
> 2. fetch an object
> 3. switch to another slab
> 4. free an object, then the slab is added to partial list again
> In this way n->list_lock will be heavily contended.
> In fact, Alex had a hackbench regression. 3.1-rc1 performance drops about 70%
> against 3.0. This patch fixes it.
updated the change log a little bit. My apologize to not mention Alex's credit.

Thanks,
Shaohua


The slab has just one free object, adding it to partial list head doesn't make
sense. And it can cause lock contentation. For example,
1. CPU takes the slab from partial list
2. fetch an object
3. switch to another slab
4. free an object, then the slab is added to partial list again
In this way n->list_lock will be heavily contended.
In fact, Alex had a hackbench regression. 3.1-rc1 performance drops about 70%
against 3.0. This patch fixes it. Thanks Alex to bisect the issue to be a slub
regression and collect perf data.

Reported-by: Alex Shi <alex.shi@...el.com>
Signed-off-by: Shaohua Li <shli@...nel.org>
Signed-off-by: Shaohua Li <shaohua.li@...el.com>

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

Index: linux/mm/slub.c
===================================================================
--- linux.orig/mm/slub.c	2011-08-15 09:55:21.000000000 +0800
+++ linux/mm/slub.c	2011-08-23 08:13:54.000000000 +0800
@@ -2377,7 +2377,7 @@ static void __slab_free(struct kmem_cach
 		 */
 		if (unlikely(!prior)) {
 			remove_full(s, page);
-			add_partial(n, page, 0);
+			add_partial(n, page, 1);
 			stat(s, FREE_ADD_PARTIAL);
 		}
 	}


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