Merge branch 'tcp-avoid-sending-too-small-packets'

Eric Dumazet says:

====================
tcp: avoid sending too small packets

tcp_sendmsg() cooks 'large' skbs, that are later split
if needed from tcp_write_xmit().

After a split, the leftover skb size is smaller than the optimal
size, and this causes a performance drop.

In this series, tcp_grow_skb() helper is added to shift
payload from the second skb in the write queue to the first
skb to always send optimal sized skbs.

This increases TSO efficiency, and decreases number of ACK
packets.
====================

Link: https://lore.kernel.org/r/20240418214600.1291486-1-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
@@ -2073,16 +2077,10 @@ static unsigned int tcp_mss_split_point(const struct sock *sk,
 /* Can at least one segment of SKB be sent right now, according to the
  * congestion window rules?  If so, return how many segments are allowed.
  */
-static inline unsigned int tcp_cwnd_test(const struct tcp_sock *tp,
-					 const struct sk_buff *skb)
+static u32 tcp_cwnd_test(const struct tcp_sock *tp)
 {
 	u32 in_flight, cwnd, halfcwnd;
 
-	/* Don't be strict about the congestion window for the final FIN.  */
-	if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) &&
-	    tcp_skb_pcount(skb) == 1)
-		return 1;
-
 	in_flight = tcp_packets_in_flight(tp);
 	cwnd = tcp_snd_cwnd(tp);
 	if (in_flight >= cwnd)
@@ -2103,10 +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;
 }
 
@@ -2686,6 +2683,36 @@ void tcp_chrono_stop(struct sock *sk, const enum tcp_chrono type)
 		tcp_chrono_set(tp, TCP_CHRONO_BUSY);
 }
 
+/* First skb in the write queue is smaller than ideal packet size.
+ * Check if we can move payload from the second skb in the queue.
+ */
+static void tcp_grow_skb(struct sock *sk, struct sk_buff *skb, int amount)
+{
+	struct sk_buff *next_skb = skb->next;
+	unsigned int nlen;
+
+	if (tcp_skb_is_last(sk, skb))
+		return;
+
+	if (!tcp_skb_can_collapse(skb, next_skb))
+		return;
+
+	nlen = min_t(u32, amount, next_skb->len);
+	if (!nlen || !skb_shift(skb, next_skb, nlen))
+		return;
+
+	TCP_SKB_CB(skb)->end_seq += nlen;
+	TCP_SKB_CB(next_skb)->seq += nlen;
+
+	if (!next_skb->len) {
+		TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(next_skb)->end_seq;
+		TCP_SKB_CB(skb)->eor = TCP_SKB_CB(next_skb)->eor;
+		TCP_SKB_CB(skb)->tcp_flags |= TCP_SKB_CB(next_skb)->tcp_flags;
+		tcp_unlink_write_queue(next_skb, sk);
+		tcp_wmem_free_skb(sk, next_skb);
+	}
+}
+
 /* This routine writes packets to the network.  It advances the
  * send_head.  This happens as incoming acks open up the remote
  * window for us.
@@ -2706,10 +2733,9 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct sk_buff *skb;
 	unsigned int tso_segs, sent_pkts;
-	int cwnd_quota;
+	u32 cwnd_quota, max_segs;
 	int result;
 	bool is_cwnd_limited = false, is_rwnd_limited = false;
-	u32 max_segs;
 
 	sent_pkts = 0;
 
@@ -2727,6 +2753,7 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
 	max_segs = tcp_tso_segs(sk, mss_now);
 	while ((skb = tcp_send_head(sk))) {
 		unsigned int limit;
+		int missing_bytes;
 
 		if (unlikely(tp->repair) && tp->repair_queue == TCP_SEND_QUEUE) {
 			/* "skb_mstamp_ns" is used as a start point for the retransmit timer */
@@ -2740,10 +2767,7 @@ 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, skb);
+		cwnd_quota = tcp_cwnd_test(tp);
 		if (!cwnd_quota) {
 			if (push_one == 2)
 				/* Force out a loss probe pkt. */
@@ -2751,6 +2775,12 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
 			else
 				break;
 		}
+		cwnd_quota = min(cwnd_quota, max_segs);
+		missing_bytes = cwnd_quota * mss_now - skb->len;
+		if (missing_bytes > 0)
+			tcp_grow_skb(sk, skb, missing_bytes);
+
+		tso_segs = tcp_set_skb_tso_segs(skb, mss_now);
 
 		if (unlikely(!tcp_snd_wnd_test(tp, skb, mss_now))) {
 			is_rwnd_limited = true;
@@ -2772,9 +2802,7 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
 		limit = mss_now;
 		if (tso_segs > 1 && !tcp_urg_mode(tp))
 			limit = tcp_mss_split_point(sk, skb, mss_now,
-						    min_t(unsigned int,
-							  cwnd_quota,
-							  max_segs),
+						    cwnd_quota,
 						    nonagle);
 
 		if (skb->len > limit &&