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tcp: always seek for minimal rtt in tcp_rcv_rtt_update()

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[ Upstream commit b879dcb1aeeca278eacaac0b1e2425b1c7599f9f ]

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: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250513193919.1089692-8-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Eric Dumazet
2025-05-13 19:39:15 +00:00
committed by Greg Kroah-Hartman
parent 969d61aa16
commit 156b6bac76
1 changed files with 8 additions and 14 deletions
Download Patch File Download Diff File Expand all files Collapse all files

22
net/ipv4/tcp_input.c
View File

@@ -671,10 +671,12 @@ EXPORT_SYMBOL(tcp_initialize_rcv_mss);
*/ */
static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep) static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep)
{ {
u32 new_sample = tp->rcv_rtt_est.rtt_us; u32 new_sample, old_sample = tp->rcv_rtt_est.rtt_us;
long m = sample; 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 /* If we sample in larger samples in the non-timestamp
* case, we could grossly overestimate the RTT especially * case, we could grossly overestimate the RTT especially
* with chatty applications or bulk transfer apps which * with chatty applications or bulk transfer apps which
@@ -685,17 +687,9 @@ static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep)
* else with timestamps disabled convergence takes too * else with timestamps disabled convergence takes too
* long. * long.
*/ */
if (!win_dep) { if (win_dep)
m -= (new_sample >> 3); return;
new_sample += m; new_sample = old_sample - (old_sample >> 3) + sample;
} else {
m <<= 3;
if (m < new_sample)
new_sample = m;
}
} else {
/* No previous measure. */
new_sample = m << 3;
} }
tp->rcv_rtt_est.rtt_us = new_sample; tp->rcv_rtt_est.rtt_us = new_sample;