@@ -2,7 +2,7 @@
VERSION = 6
PATCHLEVEL = 13
SUBLEVEL = 0
-EXTRAVERSION = -rc6
+EXTRAVERSION = -rc7
NAME = Baby Opossum Posse
# *DOCUMENTATION*
@@ -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);
}
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") move the caller of the `bictcp_hystart_reset` function inside the `hystart_update` function. This modification adds 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`. The proposed 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. Signed-off-by: Mahdi Arghavani <ma.arghavani@yahoo.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> --- Makefile | 2 +- net/ipv4/tcp_cubic.c | 8 +++++--- 2 files changed, 6 insertions(+), 4 deletions(-)