tcp: cache RTAX_QUICKACK metric in a hot cache line

tcp_in_quickack_mode() is called from input path for small packets.

It calls __sk_dst_get() which reads sk->sk_dst_cache which has been
put in sock_read_tx group (for good reasons).

Then dst_metric(dst, RTAX_QUICKACK) also needs extra cache line misses.

Cache RTAX_QUICKACK in icsk->icsk_ack.dst_quick_ack to no longer pull
these cache lines for the cases a delayed ACK is scheduled.

After this patch TCP receive path does not longer access sock_read_tx
group.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250312083907.1931644-1-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/net/inet_connection_sock.h

@@ -117,7 +117,8 @@ struct inet_connection_sock {
 		#define ATO_BITS 8
 		__u32		  ato:ATO_BITS,	 /* Predicted tick of soft clock	   */
 				  lrcv_flowlabel:20, /* last received ipv6 flowlabel	   */
-				  unused:4;
+				  dst_quick_ack:1, /* cache dst RTAX_QUICKACK		   */
+				  unused:3;
 		unsigned long	  timeout;	 /* Currently scheduled timeout		   */
 		__u32		  lrcvtime;	 /* timestamp of last received data packet */
 		__u16		  last_seg_size; /* Size of last incoming segment	   */

net/core/sock.c

@@ -2565,8 +2565,12 @@ void sk_setup_caps(struct sock *sk, struct dst_entry *dst)
 	u32 max_segs = 1;
 
 	sk->sk_route_caps = dst->dev->features;
-	if (sk_is_tcp(sk))
+	if (sk_is_tcp(sk)) {
+		struct inet_connection_sock *icsk = inet_csk(sk);
+
 		sk->sk_route_caps |= NETIF_F_GSO;
+		icsk->icsk_ack.dst_quick_ack = dst_metric(dst, RTAX_QUICKACK);
+	}
 	if (sk->sk_route_caps & NETIF_F_GSO)
 		sk->sk_route_caps |= NETIF_F_GSO_SOFTWARE;
 	if (unlikely(sk->sk_gso_disabled))

net/ipv4/tcp_input.c

@@ -334,9 +334,8 @@ static void tcp_enter_quickack_mode(struct sock *sk, unsigned int max_quickacks)
 static bool tcp_in_quickack_mode(struct sock *sk)
 {
 	const struct inet_connection_sock *icsk = inet_csk(sk);
-	const struct dst_entry *dst = __sk_dst_get(sk);
 
-	return (dst && dst_metric(dst, RTAX_QUICKACK)) ||
+	return icsk->icsk_ack.dst_quick_ack ||
 		(icsk->icsk_ack.quick && !inet_csk_in_pingpong_mode(sk));
 }