netdev - Re: [PATCH net-next] ipv4: PKTINFO doesnt need dst reference

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Message-Id: <20111109.163708.2156133928191684256.davem@davemloft.net>
Date:	Wed, 09 Nov 2011 16:37:08 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	bhutchings@...arflare.com, pstaszewski@...are.pl,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] ipv4: PKTINFO doesnt need dst reference

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Wed, 09 Nov 2011 18:24:35 +0100

> [PATCH net-next] ipv4: IP_PKTINFO doesnt need dst reference
> 
> When a socket uses IP_PKTINFO notifications, we currently force a dst
> reference for each received skb. Reader has to access dst to get needed
> information (rt_iif & rt_spec_dst) and must release dst reference.
> 
> We also forced a dst reference if skb was put in socket backlog, even
> without IP_PKTINFO handling. This happens under stress/load.
> 
> We can instead store the needed information in skb->cb[], so that only
> softirq handler really access dst, improving cache hit ratios.
> 
> This removes two atomic operations per packet, and false sharing as
> well.
> 
> On a benchmark using a mono threaded receiver (doing only recvmsg()
> calls), I can reach 720.000 pps instead of 570.000 pps.
> 
> IP_PKTINFO is typically used by DNS servers, and any multihomed aware
> UDP application.
> 
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>

Looks good, if it compiles I'll push it out to net-next :-)
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