netdev - [PATCH net-next 08/11] tcp: skip big rtt sample if receive queue is not empty

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Message-ID: <20250513193919.1089692-9-edumazet@google.com>
Date: Tue, 13 May 2025 19:39:16 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Neal Cardwell <ncardwell@...gle.com>
Cc: Simon Horman <horms@...nel.org>, Kuniyuki Iwashima <kuniyu@...zon.com>, 
	Rick Jones <jonesrick@...gle.com>, Wei Wang <weiwan@...gle.com>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com, Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next 08/11] tcp: skip big rtt sample if receive queue is
 not empty

tcp_rcv_rtt_update() role is to keep an estimation
of RTT (tp->rcv_rtt_est.rtt_us) for receivers.

If an application is too slow to drain the TCP receive
queue, it is better to leave the RTT estimation small,
so that tcp_rcv_space_adjust() does not inflate
tp->rcvq_space.space and sk->sk_rcvbuf.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/ipv4/tcp_input.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 4723d696492517143a2f3c035bfda6b05198a824..8ec92dec321a909abe00203d0097c8bf4df1a240 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -682,6 +682,9 @@ static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep)
 		 */
 		if (win_dep)
 			return;
+		/* Do not use this sample if receive queue is not empty. */
+		if (tp->rcv_nxt != tp->copied_seq)
+			return;
 		new_sample = old_sample - (old_sample >> 3) + sample;
 	}
 
-- 
2.49.0.1045.g170613ef41-goog