tcp: fill shinfo->gso_type at last moment

Our goal is to touch skb_shinfo(skb) only when absolutely needed,
to avoid two cache line misses in TCP output path for last skb
that is considered but not sent because of various conditions
(cwnd, tso defer, receiver window, TSQ...)

A packet is GSO only when skb_shinfo(skb)->gso_size is not zero.

We can set skb_shinfo(skb)->gso_type to sk->sk_gso_type even for
non GSO packets.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -1316,16 +1316,12 @@ static bool tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
 	 * code can come after this skb later on it's better to keep
 	 * setting gso_size to something.
 	 */
-	if (!skb_shinfo(prev)->gso_size) {
+	if (!skb_shinfo(prev)->gso_size)
 		skb_shinfo(prev)->gso_size = mss;
-		skb_shinfo(prev)->gso_type = sk->sk_gso_type;
-	}
 
 	/* CHECKME: To clear or not to clear? Mimics normal skb currently */
-	if (tcp_skb_pcount(skb) <= 1) {
+	if (tcp_skb_pcount(skb) <= 1)
 		skb_shinfo(skb)->gso_size = 0;
-		skb_shinfo(skb)->gso_type = 0;
-	}
 
 	/* Difference in this won't matter, both ACKed by the same cumul. ACK */
 	TCP_SKB_CB(prev)->sacked |= (TCP_SKB_CB(skb)->sacked & TCPCB_EVER_RETRANS);

net/ipv4/tcp_output.c

@@ -412,7 +412,6 @@ static void tcp_init_nondata_skb(struct sk_buff *skb, u32 seq, u8 flags)
 
 	tcp_skb_pcount_set(skb, 1);
 	shinfo->gso_size = 0;
-	shinfo->gso_type = 0;
 
 	TCP_SKB_CB(skb)->seq = seq;
 	if (flags & (TCPHDR_SYN | TCPHDR_FIN))
@@ -1003,6 +1002,7 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
 	}
 
 	tcp_options_write((__be32 *)(th + 1), tp, &opts);
+	skb_shinfo(skb)->gso_type = sk->sk_gso_type;
 	if (likely((tcb->tcp_flags & TCPHDR_SYN) == 0))
 		tcp_ecn_send(sk, skb, tcp_header_size);
 
@@ -1080,11 +1080,9 @@ static void tcp_set_skb_tso_segs(const struct sock *sk, struct sk_buff *skb,
 		 */
 		tcp_skb_pcount_set(skb, 1);
 		shinfo->gso_size = 0;
-		shinfo->gso_type = 0;
 	} else {
 		tcp_skb_pcount_set(skb, DIV_ROUND_UP(skb->len, mss_now));
 		shinfo->gso_size = mss_now;
-		shinfo->gso_type = sk->sk_gso_type;
 	}
 }