netdev - Re: 2.6.24-rc2: Network commit causes SLUB performance regression with tbench

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Message-Id: <20071114.154616.46285694.davem@davemloft.net>
Date:	Wed, 14 Nov 2007 15:46:16 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	herbert@...dor.apana.org.au
Cc:	nickpiggin@...oo.com.au, clameter@....com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: 2.6.24-rc2: Network commit causes SLUB performance regression
 with tbench

From: Herbert Xu <herbert@...dor.apana.org.au>
Date: Wed, 14 Nov 2007 19:48:44 +0800

> [TCP]: Fix size calculation in sk_stream_alloc_pskb
> 
> We round up the header size in sk_stream_alloc_pskb so that
> TSO packets get zero tail room.  Unfortunately this rounding
> up is not coordinated with the select_size() function used by
> TCP to calculate the second parameter of sk_stream_alloc_pskb.
> 
> As a result, we may allocate more than a page of data in the
> non-TSO case when exactly one page is desired.
> 
> In fact, rounding up the head room is detrimental in the non-TSO
> case because it makes memory that would otherwise be available to
> the payload head room.  TSO doesn't need this either, all it wants
> is the guarantee that there is no tail room.
> 
> So this patch fixes this by adjusting the skb_reserve call so that
> exactly the requested amount (which all callers have calculated in
> a precise way) is made available as tail room.
> 
> Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>

Applied and I'll queue it up for -stable too.
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