netdev - Re: [PATCH net-next] net: add a prefetch in socket backlog processing

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Date:	Mon, 30 Apr 2012 20:24:23 -0700
From:	Joe Perches <joe@...ches.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] net: add a prefetch in socket backlog
 processing

On Tue, 2012-05-01 at 04:07 +0200, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> TCP or UDP stacks have big enough latencies that prefetching next
> pointer is worth it.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  net/core/sock.c |    1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 836bca6..1a88351 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1700,6 +1700,7 @@ static void __release_sock(struct sock *sk)
>  		do {
>  			struct sk_buff *next = skb->next;
>  
> +			prefetch(next);
>  			WARN_ON_ONCE(skb_dst_is_noref(skb));
>  			skb->next = NULL;
>  			sk_backlog_rcv(sk, skb);

Hi Eric.

Why should next be "prefetch"ed when
two lines below it's set to null and
the only use is as a pointer not as
an apparently undereferenced pointer?


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