Commit b879dcb1 authored by Eric Dumazet's avatar Eric Dumazet Committed by Jakub Kicinski
Browse files

tcp: always seek for minimal rtt in tcp_rcv_rtt_update() 



tcp_rcv_rtt_update() goal is to maintain an estimation of the RTT
in tp->rcv_rtt_est.rtt_us, used by tcp_rcv_space_adjust()

When TCP TS are enabled, tcp_rcv_rtt_update() is using
EWMA to smooth the samples.

Change this to immediately latch the incoming value if it
is lower than tp->rcv_rtt_est.rtt_us, so that tcp_rcv_space_adjust()
does not overshoot tp->rcvq_space.space and sk->sk_rcvbuf.

Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250513193919.1089692-8-edumazet@google.com


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent cd171461

net/ipv4/tcp_input.c

+8 −14
Original line number Diff line number Diff line
@@ -664,10 +664,12 @@ EXPORT_IPV6_MOD(tcp_initialize_rcv_mss); 
 */

static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep)

{

	u32 new_sample = tp->rcv_rtt_est.rtt_us;

	long m = sample;

	u32 new_sample, old_sample = tp->rcv_rtt_est.rtt_us;

	long m = sample << 3;



	if (new_sample != 0) {

	if (old_sample == 0 || m < old_sample) {

		new_sample = m;

	} else {

		/* If we sample in larger samples in the non-timestamp

		 * case, we could grossly overestimate the RTT especially

		 * with chatty applications or bulk transfer apps which
@@ -678,17 +680,9 @@ static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep) 
		 * else with timestamps disabled convergence takes too

		 * long.

		 */

		if (!win_dep) {

			m -= (new_sample >> 3);

			new_sample += m;

		} else {

			m <<= 3;

			if (m < new_sample)

				new_sample = m;

		}

	} else {

		/* No previous measure. */

		new_sample = m << 3;

		if (win_dep)

			return;

		new_sample = old_sample - (old_sample >> 3) + sample;

	}



	tp->rcv_rtt_est.rtt_us = new_sample;