linux-kernel - Re: [PATCH 2/2] slub: remove one code path and reduce lock contention in __slab

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Date:	Fri, 19 Oct 2012 10:20:32 +0300 (EEST)
From:	Pekka Enberg <penberg@...nel.org>
To:	Joonsoo Kim <js1304@...il.com>
cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH 2/2] slub: remove one code path and reduce lock contention
 in __slab_free()

On Thu, 16 Aug 2012, Joonsoo Kim wrote:
> When we try to free object, there is some of case that we need
> to take a node lock. This is the necessary step for preventing a race.
> After taking a lock, then we try to cmpxchg_double_slab().
> But, there is a possible scenario that cmpxchg_double_slab() is failed
> with taking a lock. Following example explains it.
> 
> CPU A               CPU B
> need lock
> ...                 need lock
> ...                 lock!!
> lock..but spin      free success
> spin...             unlock
> lock!!
> free fail
> 
> In this case, retry with taking a lock is occured in CPU A.
> I think that in this case for CPU A,
> "release a lock first, and re-take a lock if necessary" is preferable way.
> 
> There are two reasons for this.
> 
> First, this makes __slab_free()'s logic somehow simple.
> With this patch, 'was_frozen = 1' is "always" handled without taking a lock.
> So we can remove one code path.
> 
> Second, it may reduce lock contention.
> When we do retrying, status of slab is already changed,
> so we don't need a lock anymore in almost every case.
> "release a lock first, and re-take a lock if necessary" policy is
> helpful to this.
> 
> Signed-off-by: Joonsoo Kim <js1304@...il.com>
> Cc: Christoph Lameter <cl@...ux.com>
> Acked-by: Christoph Lameter <cl@...ux.com>

Applied, thanks!
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