linux-kernel - Re: [PATCH] slub: do not put a slab to cpu partial list when cpu

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Date:	Wed, 19 Jun 2013 17:52:50 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Wanpeng Li <liwanp@...ux.vnet.ibm.com>
Cc:	Pekka Enberg <penberg@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Matt Mackall <mpm@...enic.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] slub: do not put a slab to cpu partial list when
 cpu_partial is 0

On Wed, Jun 19, 2013 at 04:00:32PM +0800, Wanpeng Li wrote:
> On Wed, Jun 19, 2013 at 03:33:55PM +0900, Joonsoo Kim wrote:
> >In free path, we don't check number of cpu_partial, so one slab can
> >be linked in cpu partial list even if cpu_partial is 0. To prevent this,
> >we should check number of cpu_partial in put_cpu_partial().
> >
> 
> How about skip get_partial entirely? put_cpu_partial is called 
> in two paths, one is during refill cpu partial lists in alloc 
> slow path, the other is in free slow path. And cpu_partial is 0 
> just in debug mode. 
> 
> - alloc slow path, there is unnecessary to call get_partial 
>   since cpu partial lists won't be used in debug mode. 
> - free slow patch, new.inuse won't be true in debug mode 
>   which lead to put_cpu_partial won't be called.
> 

In debug mode, put_cpu_partial() can't be called already on both path.
But, if we assign 0 to cpu_partial via sysfs, put_cpu_partial() will be called
on free slow path. On alloc slow path, it can't be called, because following
test in get_partial_node() is always failed.

available > s->cpu_partial / 2

Thanks.

> Regards,
> Wanpeng Li 
> 
> >Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> >
> >diff --git a/mm/slub.c b/mm/slub.c
> >index 57707f0..7033b4f 100644
> >--- a/mm/slub.c
> >+++ b/mm/slub.c
> >@@ -1955,6 +1955,9 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
> > 	int pages;
> > 	int pobjects;
> >
> >+	if (!s->cpu_partial)
> >+		return;
> >+
> > 	do {
> > 		pages = 0;
> > 		pobjects = 0;
> >-- 
> >1.7.9.5
> >
> >--
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