Subject: Re: [PATCH net-next] udp: avoid one cache line miss in recvmsg()

From: Paolo Abeni <[email-masked]>
Date: Mon, 21 Nov 2016 22:36:48 +0100

On Fri, 2016-11-18 at 17:18 -0800, Eric Dumazet wrote:
> From: Eric Dumazet <[email-masked]>
> 
> UDP_SKB_CB(skb)->partial_cov is located at offset 66 in skb,
> requesting a cold cache line being read in cpu cache.
> 
> We can avoid this cache line miss for UDP sockets,
> as partial_cov has a meaning only for UDPLite.
> 
> Signed-off-by: Eric Dumazet <[email-masked]>
> ---
>  net/ipv4/udp.c |    3 ++-
>  net/ipv6/udp.c |    3 ++-
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index e1fc0116e8d59d8185670c6e55d1219bde55610d..b949770fdc08398a10f3974505a50b2b4f4b2cf3 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -1389,7 +1389,8 @@ int udp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int noblock,
>  	 * coverage checksum (UDP-Lite), do it before the copy.
>  	 */
>  
> -	if (copied < ulen || UDP_SKB_CB(skb)->partial_cov || peeking) {
> +	if (copied < ulen || peeking ||
> +	    (is_udplite && UDP_SKB_CB(skb)->partial_cov)) {
>  		checksum_valid = !udp_lib_checksum_complete(skb);
>  		if (!checksum_valid)
>  			goto csum_copy_err;
> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> index 4f99417d9b401f2a65c7828e7d6b86d1d6161794..8fd4d89380b86c8630f7fd27ce4e9958497a2b89 100644
> --- a/net/ipv6/udp.c
> +++ b/net/ipv6/udp.c
> @@ -363,7 +363,8 @@ int udpv6_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
>  	 * coverage checksum (UDP-Lite), do it before the copy.
>  	 */
>  
> -	if (copied < ulen || UDP_SKB_CB(skb)->partial_cov || peeking) {
> +	if (copied < ulen || peeking ||
> +	    (is_udplite && UDP_SKB_CB(skb)->partial_cov)) {
>  		checksum_valid = !udp_lib_checksum_complete(skb);
>  		if (!checksum_valid)
>  			goto csum_copy_err;
> 
> 

Nice catch, thank you Eric!

This gives up to 8% speed-up in my performance test (wire speed udp flood
with small packets)

Tested-by: Paolo Abeni <[email-masked]>