netdev - [PATCH net v4 1/3] tcp_cubic: fix to run bictcp

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Message-Id: <20240817163400.2616134-2-mrzhang97@gmail.com>
Date: Sat, 17 Aug 2024 11:33:58 -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 1/3] tcp_cubic: fix to run bictcp_update() at least once per RTT

The original code bypasses bictcp_update() under certain conditions
to reduce the CPU overhead. Intuitively, when last_cwnd==cwnd,
bictcp_update() is executed 32 times per second. As a result, 
it is possible that bictcp_update() is not executed for several 
RTTs when RTT is short (specifically < 1/32 second = 31 ms and 
last_cwnd==cwnd which may happen in small-BDP networks), 
thus leading to low throughput in these RTTs.

The patched code executes bictcp_update() 32 times per second
if RTT > 31 ms or every RTT if RTT < 31 ms, when last_cwnd==cwnd.

Fixes: df3271f3361b ("[TCP] BIC: CUBIC window growth (2.0)")
Fixes: ac35f562203a ("tcp: bic, cubic: use tcp_jiffies32 instead of tcp_time_stamp")
Signed-off-by: Mingrui Zhang <mrzhang97@...il.com>
Signed-off-by: Lisong Xu <xu@....edu>
---
v3->v4: Replace min() with min_t()
v2->v3: Correct the "Fixes:" footer content
v1->v2: Separate patches

 net/ipv4/tcp_cubic.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index 5dbed91c6178..00da7d592032 100644
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -218,8 +218,12 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
 
 	ca->ack_cnt += acked;	/* count the number of ACKed packets */
 
+	/* Update 32 times per second if RTT > 1/32 second,
+	 * or every RTT if RTT < 1/32 second even when last_cwnd == cwnd
+	 */
 	if (ca->last_cwnd == cwnd &&
-	    (s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32)
+	    (s32)(tcp_jiffies32 - ca->last_time) <=
+	    min_t(s32, HZ / 32, usecs_to_jiffies(ca->delay_min)))
 		return;
 
 	/* The CUBIC function can update ca->cnt at most once per jiffy.
-- 
2.34.1