tcp: better handle TCP_TX_DELAY on established flows

Some applications uses TCP_TX_DELAY socket option after TCP flow
is established.

Some metrics need to be updated, otherwise TCP might take time to
adapt to the new (emulated) RTT.

This patch adjusts tp->srtt_us, tp->rtt_min, icsk_rto
and sk->sk_pacing_rate.

This is best effort, and for instance icsk_rto is reset
without taking backoff into account.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20251013145926.833198-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

3 files changed, 31 insertions, 6 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 5ca230ed526a..1e547138f4fb 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -461,6 +461,8 @@ enum skb_drop_reason tcp_child_process(struct sock *parent, struct sock *child,
 void tcp_enter_loss(struct sock *sk);
 void tcp_cwnd_reduction(struct sock *sk, int newly_acked_sacked, int newly_lost, int flag);
 void tcp_clear_retrans(struct tcp_sock *tp);
+void tcp_update_pacing_rate(struct sock *sk);
+void tcp_set_rto(struct sock *sk);
 void tcp_update_metrics(struct sock *sk);
 void tcp_init_metrics(struct sock *sk);
 void tcp_metrics_init(void);
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 8a18aeca7ab0..4d720aa09a4c 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -3583,9 +3583,12 @@ static int tcp_repair_options_est(struct sock *sk, sockptr_t optbuf,
 DEFINE_STATIC_KEY_FALSE(tcp_tx_delay_enabled);
 EXPORT_IPV6_MOD(tcp_tx_delay_enabled);
 
-static void tcp_enable_tx_delay(void)
+static void tcp_enable_tx_delay(struct sock *sk, int val)
 {
-	if (!static_branch_unlikely(&tcp_tx_delay_enabled)) {
+	struct tcp_sock *tp = tcp_sk(sk);
+	s32 delta = (val - tp->tcp_tx_delay) << 3;
+
+	if (val && !static_branch_unlikely(&tcp_tx_delay_enabled)) {
 		static int __tcp_tx_delay_enabled = 0;
 
 		if (cmpxchg(&__tcp_tx_delay_enabled, 0, 1) == 0) {
@@ -3593,6 +3596,22 @@ static void tcp_enable_tx_delay(void)
 			pr_info("TCP_TX_DELAY enabled\n");
 		}
 	}
+	/* If we change tcp_tx_delay on a live flow, adjust tp->srtt_us,
+	 * tp->rtt_min, icsk_rto and sk->sk_pacing_rate.
+	 * This is best effort.
+	 */
+	if (delta && sk->sk_state == TCP_ESTABLISHED) {
+		s64 srtt = (s64)tp->srtt_us + delta;
+
+		tp->srtt_us = clamp_t(s64, srtt, 1, ~0U);
+
+		/* Note: does not deal with non zero icsk_backoff */
+		tcp_set_rto(sk);
+
+		minmax_reset(&tp->rtt_min, tcp_jiffies32, ~0U);
+
+		tcp_update_pacing_rate(sk);
+	}
 }
 
 /* When set indicates to always queue non-full frames.  Later the user clears
@@ -4119,8 +4138,12 @@ ao_parse:
 			tp->recvmsg_inq = val;
 		break;
 	case TCP_TX_DELAY:
-		if (val)
-			tcp_enable_tx_delay();
+		/* tp->srtt_us is u32, and is shifted by 3 */
+		if (val < 0 || val >= (1U << (31 - 3))) {
+			err = -EINVAL;
+			break;
+		}
+		tcp_enable_tx_delay(sk, val);
 		WRITE_ONCE(tp->tcp_tx_delay, val);
 		break;
 	default:
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 31ea5af49f2d..8fc97f4d8a6b 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1095,7 +1095,7 @@ static void tcp_rtt_estimator(struct sock *sk, long mrtt_us)
 	tp->srtt_us = max(1U, srtt);
 }
 
-static void tcp_update_pacing_rate(struct sock *sk)
+void tcp_update_pacing_rate(struct sock *sk)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
 	u64 rate;
@@ -1132,7 +1132,7 @@ static void tcp_update_pacing_rate(struct sock *sk)
 /* Calculate rto without backoff.  This is the second half of Van Jacobson's
  * routine referred to above.
  */
-static void tcp_set_rto(struct sock *sk)
+void tcp_set_rto(struct sock *sk)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
 	/* Old crap is replaced with new one. 8)