netdev - [PATCH net v4 3/3] tcp_cubic: fix to use emulated Reno cwnd one RTT in the future

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Message-Id: <20240817163400.2616134-4-mrzhang97@gmail.com>
Date: Sat, 17 Aug 2024 11:34:00 -0500
From: Mingrui Zhang <mrzhang97@...il.com>
To: edumazet@...gle.com,
	davem@...emloft.net,
	ncardwell@...gle.com,
	netdev@...r.kernel.org
Cc: Mingrui Zhang <mrzhang97@...il.com>,
	Lisong Xu <xu@....edu>
Subject: [PATCH net v4 3/3] tcp_cubic: fix to use emulated Reno cwnd one RTT in the future

The original code estimates RENO snd_cwnd using the estimated 
RENO snd_cwnd at the current time (i.e., tcp_cwnd).

The patched code estimates RENO snd_cwnd using the estimated 
RENO snd_cwnd after one RTT (i.e., tcp_cwnd_next_rtt), 
because ca->cnt is used to increase snd_cwnd for the next RTT.

Fixes: 89b3d9aaf467 ("[TCP] cubic: precompute constants")
Signed-off-by: Mingrui Zhang <mrzhang97@...il.com>
Signed-off-by: Lisong Xu <xu@....edu>
---
v3->v4: Separate declarations and code of tcp_cwnd_next_rtt
v2->v3: Correct the "Fixes:" footer content
v1->v2: Separate patches

 net/ipv4/tcp_cubic.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index 03cfbad37dab..2d7121ca789e 100644
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -304,7 +304,7 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
 tcp_friendliness:
 	/* TCP Friendly */
 	if (tcp_friendliness) {
-		u32 scale = beta_scale;
+		u32 scale = beta_scale, tcp_cwnd_next_rtt;
 
 		if (cwnd < ca->cwnd_prior)
 			delta = (cwnd * scale) >> 3;	/* CUBIC additive increment */
@@ -315,8 +315,11 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
 			ca->tcp_cwnd++;
 		}
 
-		if (ca->tcp_cwnd > cwnd) {	/* if bic is slower than tcp */
-			delta = ca->tcp_cwnd - cwnd;
+		/* Reno cwnd one RTT in the future */
+		tcp_cwnd_next_rtt = ca->tcp_cwnd + (ca->ack_cnt + cwnd) / delta;
+
+		if (tcp_cwnd_next_rtt > cwnd) {  /* if bic is slower than Reno */
+			delta = tcp_cwnd_next_rtt - cwnd;
 			max_cnt = cwnd / delta;
 			if (ca->cnt > max_cnt)
 				ca->cnt = max_cnt;
-- 
2.34.1