linux-kernel - Re: [PATCH] Remove implicit list prefetches for most cases

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Date:	Wed, 08 Sep 2010 08:32:59 -0400
From:	Paul Moore <paul.moore@...com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
	x86@...nel.org, Al Viro <viro@...IV.linux.org.uk>,
	arjan@...radead.org, davem@...emloft.net
Subject: Re: [PATCH] Remove implicit list prefetches for most cases

On Wed, 2010-09-08 at 10:59 +0200, Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> We've had explicit list prefetches in list_for_each and friends
> for quite some time. According to Arjan they were originally
> added for K7 where they were a slight win.
> 
> It's doubtful they help very much today, especially on newer CPUs with
> aggressive prefetching. Most list_for_eachs bodies are quite short and
> the prefetch does not help if it doesn't happen sufficiently in advance
> or when the data is not really cache cold.
> 
> The feedback from CPU designers is that they don't like us using explicit
> prefetches unless there is a very good reason (and list_for_each* alone
> clearly isn't one)
> 
> Also the prefetches cause the list walks to generate bad code,
> increase the number of registers needed.

If prefetch() is generally considered a "Bad Thing", I'm OK with just
removing them from NetLabel; no need to rename and conditionalize.  I
put them in the netlbl_af[4,6]list_*() routines because those routines
were modeled after the normal list routines which had prefetches and I
just assumed someone much smarter had found them to be a win.

-- 
paul moore
linux @ hp


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