6.6-stable review patch. If anyone has any objections, please let me know.
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From: Mahdi Arghavani ma.arghavani@yahoo.com
[ Upstream commit 25c1a9ca53db5780757e7f53e688b8f916821baa ]
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 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.
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: 4e1fddc98d25 ("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 Signed-off-by: Sasha Levin sashal@kernel.org --- 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 0fd78ecb67e75..5ff7be13deb6b 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); }