tcp_bbr: init pacing rate on first RTT sample

Fixes the following behavior: for connections that had no RTT sample
at the time of initializing congestion control, BBR was initializing
the pacing rate to a high nominal rate (based an a guess of RTT=1ms,
in case this is LAN traffic). Then BBR never adjusted the pacing rate
downward upon obtaining an actual RTT sample, if the connection never
filled the pipe (e.g. all sends were small app-limited writes()).

This fix adjusts the pacing rate upon obtaining the first RTT sample.

Fixes: 0f8782ea14 ("tcp_bbr: add BBR congestion control")
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_bbr.c

@@ -112,7 +112,8 @@ struct bbr {
 		cwnd_gain:10,	/* current gain for setting cwnd */
 		full_bw_cnt:3,	/* number of rounds without large bw gains */
 		cycle_idx:3,	/* current index in pacing_gain cycle array */
-		unused_b:6;
+		has_seen_rtt:1, /* have we seen an RTT sample yet? */
+		unused_b:5;
 	u32	prior_cwnd;	/* prior cwnd upon entering loss recovery */
 	u32	full_bw;	/* recent bw, to estimate if pipe is full */
 };
@@ -225,11 +226,13 @@ static u32 bbr_bw_to_pacing_rate(struct sock *sk, u32 bw, int gain)
 static void bbr_init_pacing_rate_from_rtt(struct sock *sk)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
+	struct bbr *bbr = inet_csk_ca(sk);
 	u64 bw;
 	u32 rtt_us;
 
 	if (tp->srtt_us) {		/* any RTT sample yet? */
 		rtt_us = max(tp->srtt_us >> 3, 1U);
+		bbr->has_seen_rtt = 1;
 	} else {			 /* no RTT sample yet */
 		rtt_us = USEC_PER_MSEC;	 /* use nominal default RTT */
 	}
@@ -247,8 +250,12 @@ static void bbr_init_pacing_rate_from_rtt(struct sock *sk)
  */
 static void bbr_set_pacing_rate(struct sock *sk, u32 bw, int gain)
 {
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct bbr *bbr = inet_csk_ca(sk);
 	u32 rate = bbr_bw_to_pacing_rate(sk, bw, gain);
 
+	if (unlikely(!bbr->has_seen_rtt && tp->srtt_us))
+		bbr_init_pacing_rate_from_rtt(sk);
 	if (bbr_full_bw_reached(sk) || rate > sk->sk_pacing_rate)
 		sk->sk_pacing_rate = rate;
 }
@@ -837,6 +844,7 @@ static void bbr_init(struct sock *sk)
 
 	minmax_reset(&bbr->bw, bbr->rtt_cnt, 0);  /* init max bw to 0 */
 
+	bbr->has_seen_rtt = 0;
 	bbr_init_pacing_rate_from_rtt(sk);
 
 	bbr->restore_cwnd = 0;