tcp: properly handle stretch acks in slow start

Slow start now increases cwnd by 1 if an ACK acknowledges some packets,
regardless the number of packets. Consequently slow start performance
is highly dependent on the degree of the stretch ACKs caused by
receiver or network ACK compression mechanisms (e.g., delayed-ACK,
GRO, etc).  But slow start algorithm is to send twice the amount of
packets of packets left so it should process a stretch ACK of degree
N as if N ACKs of degree 1, then exits when cwnd exceeds ssthresh. A
follow up patch will use the remainder of the N (if greater than 1)
to adjust cwnd in the congestion avoidance phase.

In addition this patch retires the experimental limited slow start
(LSS) feature. LSS has multiple drawbacks but questionable benefit. The
fractional cwnd increase in LSS requires a loop in slow start even
though it's rarely used. Configuring such an increase step via a global
sysctl on different BDPS seems hard. Finally and most importantly the
slow start overshoot concern is now better covered by the Hybrid slow
start (hystart) enabled by default.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 4 deletions

diff --git a/net/ipv4/tcp_veno.c b/net/ipv4/tcp_veno.c
index ac43cd747bce..326475a94865 100644
--- a/net/ipv4/tcp_veno.c
+++ b/net/ipv4/tcp_veno.c
@@ -114,13 +114,14 @@ static void tcp_veno_cwnd_event(struct sock *sk, enum tcp_ca_event event)
                 tcp_veno_init(sk);
 }
 
-static void tcp_veno_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
+static void tcp_veno_cong_avoid(struct sock *sk, u32 ack, u32 acked,
+                                u32 in_flight)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct veno *veno = inet_csk_ca(sk);
 
         if (!veno->doing_veno_now) {
-                tcp_reno_cong_avoid(sk, ack, in_flight);
+                tcp_reno_cong_avoid(sk, ack, acked, in_flight);
                 return;
         }
 
@@ -133,7 +134,7 @@ static void tcp_veno_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
                 /* We don't have enough rtt samples to do the Veno
                  * calculation, so we'll behave like Reno.
                  */
-                tcp_reno_cong_avoid(sk, ack, in_flight);
+                tcp_reno_cong_avoid(sk, ack, acked, in_flight);
         } else {
                 u64 target_cwnd;
                 u32 rtt;
@@ -152,7 +153,7 @@ static void tcp_veno_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 
                 if (tp->snd_cwnd <= tp->snd_ssthresh) {
                         /* Slow start.  */
-                        tcp_slow_start(tp);
+                        tcp_slow_start(tp, acked);
                 } else {
                         /* Congestion avoidance. */
                         if (veno->diff < beta) {