Commit 1c51450f authored by Eric Dumazet's avatar Eric Dumazet Committed by Jakub Kicinski
Browse files

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: default avatarEric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20251013145926.833198-1-edumazet@google.com


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent 6378e25e

include/net/tcp.h

+2 −0
Original line number Diff line number Diff line
@@ -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);

net/ipv4/tcp.c

+27 −4
Original line number Diff line number Diff line
@@ -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 @@ int do_tcp_setsockopt(struct sock *sk, int level, int optname, 
			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:

net/ipv4/tcp_input.c

+2 −2
Original line number Diff line number Diff line
@@ -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)