tcp_bbr: centralize code to set gains

Centralize the code that sets gains used for computing cwnd and pacing
rate. This simplifies the code and makes it easier to change the state
machine or (in the future) dynamically change the gain values and
ensure that the correct gain values are always used.
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: Priyaranjan Jha <priyarjha@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_bbr.c

@@ -521,8 +521,6 @@ static void bbr_advance_cycle_phase(struct sock *sk)
 
 	bbr->cycle_idx = (bbr->cycle_idx + 1) & (CYCLE_LEN - 1);
 	bbr->cycle_mstamp = tp->delivered_mstamp;
-	bbr->pacing_gain = bbr->lt_use_bw ? BBR_UNIT :
-					    bbr_pacing_gain[bbr->cycle_idx];
 }
 
 /* Gain cycling: cycle pacing gain to converge to fair share of available bw. */
@@ -540,8 +538,6 @@ static void bbr_reset_startup_mode(struct sock *sk)
 	struct bbr *bbr = inet_csk_ca(sk);
 
 	bbr->mode = BBR_STARTUP;
-	bbr->pacing_gain = bbr_high_gain;
-	bbr->cwnd_gain	 = bbr_high_gain;
 }
 
 static void bbr_reset_probe_bw_mode(struct sock *sk)
@@ -549,8 +545,6 @@ static void bbr_reset_probe_bw_mode(struct sock *sk)
 	struct bbr *bbr = inet_csk_ca(sk);
 
 	bbr->mode = BBR_PROBE_BW;
-	bbr->pacing_gain = BBR_UNIT;
-	bbr->cwnd_gain = bbr_cwnd_gain;
 	bbr->cycle_idx = CYCLE_LEN - 1 - prandom_u32_max(bbr_cycle_rand);
 	bbr_advance_cycle_phase(sk);	/* flip to next phase of gain cycle */
 }
@@ -768,8 +762,6 @@ static void bbr_check_drain(struct sock *sk, const struct rate_sample *rs)
 
 	if (bbr->mode == BBR_STARTUP && bbr_full_bw_reached(sk)) {
 		bbr->mode = BBR_DRAIN;	/* drain queue we created */
-		bbr->pacing_gain = bbr_drain_gain;	/* pace slow to drain */
-		bbr->cwnd_gain = bbr_high_gain;	/* maintain cwnd */
 		tcp_sk(sk)->snd_ssthresh =
 				bbr_target_cwnd(sk, bbr_max_bw(sk), BBR_UNIT);
 	}	/* fall through to check if in-flight is already small: */
@@ -831,8 +823,6 @@ static void bbr_update_min_rtt(struct sock *sk, const struct rate_sample *rs)
 	if (bbr_probe_rtt_mode_ms > 0 && filter_expired &&
 	    !bbr->idle_restart && bbr->mode != BBR_PROBE_RTT) {
 		bbr->mode = BBR_PROBE_RTT;  /* dip, drain queue */
-		bbr->pacing_gain = BBR_UNIT;
-		bbr->cwnd_gain = BBR_UNIT;
 		bbr_save_cwnd(sk);  /* note cwnd so we can restore it */
 		bbr->probe_rtt_done_stamp = 0;
 	}
@@ -860,6 +850,35 @@ static void bbr_update_min_rtt(struct sock *sk, const struct rate_sample *rs)
 		bbr->idle_restart = 0;
 }
 
+static void bbr_update_gains(struct sock *sk)
+{
+	struct bbr *bbr = inet_csk_ca(sk);
+
+	switch (bbr->mode) {
+	case BBR_STARTUP:
+		bbr->pacing_gain = bbr_high_gain;
+		bbr->cwnd_gain	 = bbr_high_gain;
+		break;
+	case BBR_DRAIN:
+		bbr->pacing_gain = bbr_drain_gain;	/* slow, to drain */
+		bbr->cwnd_gain	 = bbr_high_gain;	/* keep cwnd */
+		break;
+	case BBR_PROBE_BW:
+		bbr->pacing_gain = (bbr->lt_use_bw ?
+				    BBR_UNIT :
+				    bbr_pacing_gain[bbr->cycle_idx]);
+		bbr->cwnd_gain	 = bbr_cwnd_gain;
+		break;
+	case BBR_PROBE_RTT:
+		bbr->pacing_gain = BBR_UNIT;
+		bbr->cwnd_gain	 = BBR_UNIT;
+		break;
+	default:
+		WARN_ONCE(1, "BBR bad mode: %u\n", bbr->mode);
+		break;
+	}
+}
+
 static void bbr_update_model(struct sock *sk, const struct rate_sample *rs)
 {
 	bbr_update_bw(sk, rs);
@@ -867,6 +886,7 @@ static void bbr_update_model(struct sock *sk, const struct rate_sample *rs)
 	bbr_check_full_bw_reached(sk, rs);
 	bbr_check_drain(sk, rs);
 	bbr_update_min_rtt(sk, rs);
+	bbr_update_gains(sk);
 }
 
 static void bbr_main(struct sock *sk, const struct rate_sample *rs)