Re: [PATCH net-next-2.6] net: eth_type_trans() should inline skb_pull()

[Eric Dumazet <eric.dumazet@gmail.com>]

Le samedi 01 mai 2010 à 18:15 -0700, David Miller a écrit :
> From: Eric Dumazet <eric.dumazet@gmail.com>
> Date: Sat, 01 May 2010 08:42:25 +0200
> 
> > [PATCH net-next-2.6] net: eth_type_trans() should inline skb_pull()
> > 
> > With RPS, this patch can give a 5 % boost in performance.
> > 
> > Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
> 
> Awesome, but let's do this in a way that allows us to easily annotate
> where inlining makes sense in other places, not just here.
> 
> Something like this, ok?
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 82f5116..746a652 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -1128,6 +1128,11 @@ static inline unsigned char *__skb_pull(struct sk_buff *skb, unsigned int len)
>  	return skb->data += len;
>  }
>  
> +static inline unsigned char *skb_pull_inline(struct sk_buff *skb, unsigned int len)
> +{
> +	return unlikely(len > skb->len) ? NULL : __skb_pull(skb, len);
> +}
> +
>  extern unsigned char *__pskb_pull_tail(struct sk_buff *skb, int delta);
>  
>  static inline unsigned char *__pskb_pull(struct sk_buff *skb, unsigned int len)
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 4218ff4..8b9c109 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -1051,7 +1051,7 @@ EXPORT_SYMBOL(skb_push);
>   */
>  unsigned char *skb_pull(struct sk_buff *skb, unsigned int len)
>  {
> -	return unlikely(len > skb->len) ? NULL : __skb_pull(skb, len);
> +	return skb_pull_inline(skb, len);
>  }
>  EXPORT_SYMBOL(skb_pull);
>  
> diff --git a/net/ethernet/eth.c b/net/ethernet/eth.c
> index 0c0d272..61ec032 100644
> --- a/net/ethernet/eth.c
> +++ b/net/ethernet/eth.c
> @@ -162,7 +162,7 @@ __be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev)
>  
>  	skb->dev = dev;
>  	skb_reset_mac_header(skb);
> -	skb_pull(skb, ETH_HLEN);
> +	skb_pull_inline(skb, ETH_HLEN);
>  	eth = eth_hdr(skb);
>  
>  	if (unlikely(is_multicast_ether_addr(eth->h_dest))) {

Excellent !

Changli privately asked me why we were ignoring cases where skb->len <
ETH_HLEN.
I replied that minimum frame size was 46+12, then he asked me why we
were testing another time :

if (skb->len >= 2 && *(unsigned short *)rawp == 0xFFFF)
	return htons(ETH_P_802_3);


Could we assume all eth_type_trans() must call it with initial skb->len
>= (46 + 12) or not ?
(According to ethernet specs, all frames should have a minimum payload
of 46 bytes)

If not sure, maybe we should issue a WARN_ON_ONCE()

If yes, tests could be removed and we could gain two cycles ;)