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>

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 5ca230ed526ae02711e8d2a409b91664b73390f2..1e547138f4fb7f5c47d15990954d4d135f465f73 100644 (file)
--- 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 8a18aeca7ab07480844946120f51a0555699b4c3..4d720aa09a4c67f0f068ad7cbf72b306a65a3ee2 100644 (file)
--- 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 31ea5af49f2dc8a6f95f3f8c24065369765b8987..8fc97f4d8a6b2f8e39cabf6c9b3e6cdae294a5f5 100644 (file)
--- 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)