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, 3 insertions, 4 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 2d7b4bdc972f..70e55d200610 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -275,7 +275,6 @@ extern int sysctl_tcp_mtu_probing;
 extern int sysctl_tcp_base_mss;
 extern int sysctl_tcp_workaround_signed_windows;
 extern int sysctl_tcp_slow_start_after_idle;
-extern int sysctl_tcp_max_ssthresh;
 extern int sysctl_tcp_thin_linear_timeouts;
 extern int sysctl_tcp_thin_dupack;
 extern int sysctl_tcp_early_retrans;
@@ -797,7 +796,7 @@ struct tcp_congestion_ops {
         /* lower bound for congestion window (optional) */
         u32 (*min_cwnd)(const struct sock *sk);
         /* do new cwnd calculation (required) */
-        void (*cong_avoid)(struct sock *sk, u32 ack, u32 in_flight);
+        void (*cong_avoid)(struct sock *sk, u32 ack, u32 acked, u32 in_flight);
         /* call before changing ca_state (optional) */
         void (*set_state)(struct sock *sk, u8 new_state);
         /* call when cwnd event occurs (optional) */
@@ -824,12 +823,12 @@ void tcp_get_available_congestion_control(char *buf, size_t len);
 void tcp_get_allowed_congestion_control(char *buf, size_t len);
 int tcp_set_allowed_congestion_control(char *allowed);
 int tcp_set_congestion_control(struct sock *sk, const char *name);
-void tcp_slow_start(struct tcp_sock *tp);
+int tcp_slow_start(struct tcp_sock *tp, u32 acked);
 void tcp_cong_avoid_ai(struct tcp_sock *tp, u32 w);
 
 extern struct tcp_congestion_ops tcp_init_congestion_ops;
 u32 tcp_reno_ssthresh(struct sock *sk);
-void tcp_reno_cong_avoid(struct sock *sk, u32 ack, u32 in_flight);
+void tcp_reno_cong_avoid(struct sock *sk, u32 ack, u32 acked, u32 in_flight);
 u32 tcp_reno_min_cwnd(const struct sock *sk);
 extern struct tcp_congestion_ops tcp_reno;