netdev - [PATCH net v2] tcp_cubic: fix incorrect HyStart round start detection

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Message-ID: <20250115010450.2472-1-ma.arghavani@yahoo.com>
Date: Wed, 15 Jan 2025 01:04:50 +0000
From: Mahdi Arghavani <ma.arghavani@...oo.com>
To: netdev@...r.kernel.org
Cc: ncardwell@...gle.com,
	edumazet@...gle.com,
	haibo.zhang@...go.ac.nz,
	david.eyers@...go.ac.nz,
	abbas.arghavani@....se,
	Mahdi Arghavani <ma.arghavani@...oo.com>
Subject: [PATCH net v2] tcp_cubic: fix incorrect HyStart round start detection

I noticed that HyStart incorrectly marks the start of rounds,
resulting in inaccurate measurements of ACK train lengths.
Since HyStart relies on ACK train lengths as one of two thresholds
to terminate exponential cwnd growth during Slow-Start, this
inaccuracy renders that threshold ineffective, potentially degrading
TCP performance.

The issue arises because the changes introduced in commit 4e1fddc98d25
("tcp_cubic: fix spurious Hystart ACK train detections for not-cwnd-limited flows")
moved the caller of the `bictcp_hystart_reset` function inside the `hystart_update` function.
This modification added an additional condition for triggering the caller,
requiring that (tcp_snd_cwnd(tp) >= hystart_low_window) must also
be satisfied before invoking `bictcp_hystart_reset`.

This fix ensures that `bictcp_hystart_reset` is correctly called
at the start of a new round, regardless of the congestion window size.
This is achieved by moving the condition
(tcp_snd_cwnd(tp) >= hystart_low_window)
from before calling `bictcp_hystart_reset` to after it.

Fixes: 4e1fddc98d25 ("tcp_cubic: fix spurious Hystart ACK train detections for not-cwnd-limited flows")
Signed-off-by: Mahdi Arghavani <ma.arghavani@...oo.com>
Cc: Neal Cardwell <ncardwell@...gle.com>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Haibo Zhang <haibo.zhang@...go.ac.nz>
Cc: David Eyers <david.eyers@...go.ac.nz>
Cc: Abbas Arghavani <abbas.arghavani@....se>
---
 net/ipv4/tcp_cubic.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index 5dbed91c6178..76c23675ae50 100644
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -392,6 +392,10 @@ static void hystart_update(struct sock *sk, u32 delay)
 	if (after(tp->snd_una, ca->end_seq))
 		bictcp_hystart_reset(sk);

+	/* hystart triggers when cwnd is larger than some threshold */
+	if (tcp_snd_cwnd(tp) < hystart_low_window)
+		return;
+
 	if (hystart_detect & HYSTART_ACK_TRAIN) {
 		u32 now = bictcp_clock_us(sk);

@@ -467,9 +471,7 @@ __bpf_kfunc static void cubictcp_acked(struct sock *sk, const struct ack_sample
 	if (ca->delay_min == 0 || ca->delay_min > delay)
 		ca->delay_min = delay;

-	/* hystart triggers when cwnd is larger than some threshold */
-	if (!ca->found && tcp_in_slow_start(tp) && hystart &&
-	    tcp_snd_cwnd(tp) >= hystart_low_window)
+	if (!ca->found && tcp_in_slow_start(tp) && hystart)
 		hystart_update(sk, delay);
 }

-- 
2.45.2