tcp: call tcp_set_skb_tso_segs() from tcp_write_xmit()

tcp_write_xmit() calls tcp_init_tso_segs()
to set gso_size and gso_segs on the packet.

tcp_init_tso_segs() requires the stack to maintain
an up to date tcp_skb_pcount(), and this makes sense
for packets in rtx queue. Not so much for packets
still in the write queue.

In the following patch, we don't want to deal with
tcp_skb_pcount() when moving payload from 2nd
skb to 1st skb in the write queue.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240418214600.1291486-3-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/tcp_output.c

@@ -1502,18 +1502,22 @@ static void tcp_queue_skb(struct sock *sk, struct sk_buff *skb)
 }
 
 /* Initialize TSO segments for a packet. */
-static void tcp_set_skb_tso_segs(struct sk_buff *skb, unsigned int mss_now)
+static int tcp_set_skb_tso_segs(struct sk_buff *skb, unsigned int mss_now)
 {
+	int tso_segs;
+
 	if (skb->len <= mss_now) {
 		/* Avoid the costly divide in the normal
 		 * non-TSO case.
 		 */
-		tcp_skb_pcount_set(skb, 1);
 		TCP_SKB_CB(skb)->tcp_gso_size = 0;
-	} else {
-		tcp_skb_pcount_set(skb, DIV_ROUND_UP(skb->len, mss_now));
-		TCP_SKB_CB(skb)->tcp_gso_size = mss_now;
+		tcp_skb_pcount_set(skb, 1);
+		return 1;
 	}
+	TCP_SKB_CB(skb)->tcp_gso_size = mss_now;
+	tso_segs = DIV_ROUND_UP(skb->len, mss_now);
+	tcp_skb_pcount_set(skb, tso_segs);
+	return tso_segs;
 }
 
 /* Pcount in the middle of the write queue got changed, we need to do various
@@ -2097,10 +2101,9 @@ static int tcp_init_tso_segs(struct sk_buff *skb, unsigned int mss_now)
 {
 	int tso_segs = tcp_skb_pcount(skb);
 
-	if (!tso_segs || (tso_segs > 1 && tcp_skb_mss(skb) != mss_now)) {
-		tcp_set_skb_tso_segs(skb, mss_now);
-		tso_segs = tcp_skb_pcount(skb);
-	}
+	if (!tso_segs || (tso_segs > 1 && tcp_skb_mss(skb) != mss_now))
+		return tcp_set_skb_tso_segs(skb, mss_now);
+
 	return tso_segs;
 }
 
@@ -2733,9 +2736,6 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
 		if (tcp_pacing_check(sk))
 			break;
 
-		tso_segs = tcp_init_tso_segs(skb, mss_now);
-		BUG_ON(!tso_segs);
-
 		cwnd_quota = tcp_cwnd_test(tp);
 		if (!cwnd_quota) {
 			if (push_one == 2)
@@ -2745,6 +2745,8 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
 				break;
 		}
 
+		tso_segs = tcp_set_skb_tso_segs(skb, mss_now);
+
 		if (unlikely(!tcp_snd_wnd_test(tp, skb, mss_now))) {
 			is_rwnd_limited = true;
 			break;