tcp_cubic: fix incorrect HyStart round start detection

I noticed that HyStart incorrectly marks the start of rounds,
leading to inaccurate measurements of ACK train lengths and
resetting the `ca->sample_cnt` variable. This inaccuracy can impact
HyStart's functionality in terminating exponential cwnd growth during
Slow-Start, potentially degrading TCP performance.

The issue arises because the changes introduced in commit 4e1fddc
("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.

I tested with a client and a server connected through two Linux software routers.
In this setup, the minimum RTT was 150 ms, the bottleneck bandwidth was 50 Mbps,
and the bottleneck buffer size was 1 BDP, calculated as (50M / 1514 / 8) * 0.150 = 619 packets.
I conducted the test twice, transferring data from the server to the client for 1.5 seconds.
Before the patch was applied, HYSTART-DELAY stopped the exponential growth of cwnd when
cwnd = 516, and the bottleneck link was not yet saturated (516 < 619).
After the patch was applied, HYSTART-ACK-TRAIN stopped the exponential growth of cwnd when
cwnd = 632, and the bottleneck link was saturated (632 > 619).
In this test, applying the patch resulted in 300 KB more data delivered.

Fixes: 4e1fddc ("tcp_cubic: fix spurious Hystart ACK train detections for not-cwnd-limited flows")
Signed-off-by: Mahdi Arghavani <ma.arghavani@yahoo.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Haibo Zhang <haibo.zhang@otago.ac.nz>
Cc: David Eyers <david.eyers@otago.ac.nz>
Cc: Abbas Arghavani <abbas.arghavani@mdu.se>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Tested-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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);
 }