tcp: do not slow start when cwnd equals ssthresh

In the original design slow start is only used to raise cwnd
when cwnd is stricly below ssthresh. It makes little sense
to slow start when cwnd == ssthresh: especially
when hystart has set ssthresh in the initial ramp, or after
recovery when cwnd resets to ssthresh. Not doing so will
also help reduce the buffer bloat slightly.

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: Nandita Dukkipati <nanditad@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/net/tcp.h b/include/net/tcp.h
index dba22fc1b065a70b9604872b1460c7b2ea24db1d..364426a2be5a0f7f0a2e6daaf6ce9b9a2f3e3304 100644 (file)
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -991,7 +991,7 @@ static inline unsigned int tcp_packets_in_flight(const struct tcp_sock *tp)
 
 static inline bool tcp_in_slow_start(const struct tcp_sock *tp)
 {
-       return tp->snd_cwnd <= tp->snd_ssthresh;
+       return tp->snd_cwnd < tp->snd_ssthresh;
 }
 
 static inline bool tcp_in_initial_slowstart(const struct tcp_sock *tp)

diff --git a/net/ipv4/tcp_cdg.c b/net/ipv4/tcp_cdg.c
index 8c6fd3d5e40feeb3c0b422d0e697e1a674b4f576..167b6a3e1b9868c88e5553b114556ae312dfb99f 100644 (file)
--- a/net/ipv4/tcp_cdg.c
+++ b/net/ipv4/tcp_cdg.c
@@ -264,7 +264,7 @@ static void tcp_cdg_cong_avoid(struct sock *sk, u32 ack, u32 acked)
        u32 prior_snd_cwnd;
        u32 incr;
 
-       if (tp->snd_cwnd < tp->snd_ssthresh && hystart_detect)
+       if (tcp_in_slow_start(tp) && hystart_detect)
                tcp_cdg_hystart_update(sk);
 
        if (after(ack, ca->rtt_seq) && ca->rtt.v64) {

diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
index 654729a8cb23f724cca59130580dd6a1ab6a8416..a2ed23c595cf185cadbebcdf19e801012a64250a 100644 (file)
--- a/net/ipv4/tcp_cong.c
+++ b/net/ipv4/tcp_cong.c
@@ -365,10 +365,8 @@ int tcp_set_congestion_control(struct sock *sk, const char *name)
  */
 u32 tcp_slow_start(struct tcp_sock *tp, u32 acked)
 {
-       u32 cwnd = tp->snd_cwnd + acked;
+       u32 cwnd = min(tp->snd_cwnd + acked, tp->snd_ssthresh);
 
-       if (cwnd > tp->snd_ssthresh)
-               cwnd = tp->snd_ssthresh + 1;
        acked -= cwnd - tp->snd_cwnd;
        tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp);
 

diff --git a/net/ipv4/tcp_hybla.c b/net/ipv4/tcp_hybla.c
index f963b274f2b0436755ebe8bb5586b1ec9682c336..083831e359df92ca9ba0fe7dd5a7a76fe41a94b0 100644 (file)
--- a/net/ipv4/tcp_hybla.c
+++ b/net/ipv4/tcp_hybla.c
@@ -112,7 +112,7 @@ static void hybla_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 
        rho_fractions = ca->rho_3ls - (ca->rho << 3);
 
-       if (tp->snd_cwnd < tp->snd_ssthresh) {
+       if (tcp_in_slow_start(tp)) {
                /*
                 * slow start
                 *      INC = 2^RHO - 1