Subject: Re: [PATCH net-next] net: sock_rps_record_flow() is for connected sockets

From: Paolo Abeni <[email-masked]>
Date: Wed, 07 Dec 2016 08:59:11 +0100

On Tue, 2016-12-06 at 19:32 -0800, Eric Dumazet wrote:
> From: Eric Dumazet <[email-masked]>
> 
> Paolo noticed a cache line miss in UDP recvmsg() to access
> sk_rxhash, sharing a cache line with sk_drops.
> 
> sk_drops might be heavily incremented by cpus handling a flood targeting
> this socket.
> 
> We might place sk_drops on a separate cache line, but lets try
> to avoid wasting 64 bytes per socket just for this, since we have
> other bottlenecks to take care of.
> 
> sock_rps_record_flow() should only access sk_rxhash for connected
> flows.
> 
> Testing sk_state for TCP_ESTABLISHED covers most of the cases for
> connected sockets, for a zero cost, since system calls using
> sock_rps_record_flow() also access sk->sk_prot which is on the
> same cache line.
> 
> A follow up patch will provide a static_key (Jump Label) since most
> hosts do not even use RFS.
> 
> Signed-off-by: Eric Dumazet <[email-masked]>
> Reported-by: Paolo Abeni <[email-masked]>
> ---
>  include/net/sock.h |   12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 6dfe3aa22b970eecfab4d4a0753804b1cc82a200..a7ddab993b496f1f4060f0b41831a161c284df9e 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -913,7 +913,17 @@ static inline void sock_rps_record_flow_hash(__u32 hash)
>  static inline void sock_rps_record_flow(const struct sock *sk)
>  {
>  #ifdef CONFIG_RPS
> -	sock_rps_record_flow_hash(sk->sk_rxhash);
> +	/* Reading sk->sk_rxhash might incur an expensive cache line miss.
> +	 *
> +	 * TCP_ESTABLISHED does cover almost all states where RFS
> +	 * might be useful, and is cheaper [1] than testing :
> +	 *	IPv4: inet_sk(sk)->inet_daddr
> +	 * 	IPv6: ipv6_addr_any(&sk->sk_v6_daddr)
> +	 * OR	an additional socket flag
> +	 * [1] : sk_state and sk_prot are in the same cache line.
> +	 */
> +	if (sk->sk_state == TCP_ESTABLISHED)
> +		sock_rps_record_flow_hash(sk->sk_rxhash);
>  #endif
>  }

Thank you for the very prompt patch!

You made me curious about your other idea on this topic, this what you
initially talked about, right ?

LGTM.

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