tcp: add tcp_in_slow_start helper

Add a helper to test the slow start condition in various congestion
control modules and other places. This is to prepare a slight improvement
in policy as to exactly when to slow start.

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>

include/net/tcp.h

@@ -989,6 +989,11 @@ static inline unsigned int tcp_packets_in_flight(const struct tcp_sock *tp)
 
 #define TCP_INFINITE_SSTHRESH	0x7fffffff
 
+static inline bool tcp_in_slow_start(const struct tcp_sock *tp)
+{
+	return tp->snd_cwnd <= tp->snd_ssthresh;
+}
+
 static inline bool tcp_in_initial_slowstart(const struct tcp_sock *tp)
 {
 	return tp->snd_ssthresh >= TCP_INFINITE_SSTHRESH;
@@ -1065,7 +1070,7 @@ static inline bool tcp_is_cwnd_limited(const struct sock *sk)
 	const struct tcp_sock *tp = tcp_sk(sk);
 
 	/* If in slow start, ensure cwnd grows to twice what was ACKed. */
-	if (tp->snd_cwnd <= tp->snd_ssthresh)
+	if (tcp_in_slow_start(tp))
 		return tp->snd_cwnd < 2 * tp->max_packets_out;
 
 	return tp->is_cwnd_limited;

net/ipv4/tcp_bic.c

@@ -146,7 +146,7 @@ static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 	if (!tcp_is_cwnd_limited(sk))
 		return;
 
-	if (tp->snd_cwnd <= tp->snd_ssthresh)
+	if (tcp_in_slow_start(tp))
 		tcp_slow_start(tp, acked);
 	else {
 		bictcp_update(ca, tp->snd_cwnd);

net/ipv4/tcp_cong.c

@@ -413,7 +413,7 @@ void tcp_reno_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 		return;
 
 	/* In "safe" area, increase. */
-	if (tp->snd_cwnd <= tp->snd_ssthresh) {
+	if (tcp_in_slow_start(tp)) {
 		acked = tcp_slow_start(tp, acked);
 		if (!acked)
 			return;

net/ipv4/tcp_cubic.c

@@ -320,7 +320,7 @@ static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 	if (!tcp_is_cwnd_limited(sk))
 		return;
 
-	if (tp->snd_cwnd <= tp->snd_ssthresh) {
+	if (tcp_in_slow_start(tp)) {
 		if (hystart && after(ack, ca->end_seq))
 			bictcp_hystart_reset(sk);
 		acked = tcp_slow_start(tp, acked);
@@ -439,7 +439,7 @@ static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us)
 		ca->delay_min = delay;
 
 	/* hystart triggers when cwnd is larger than some threshold */
-	if (hystart && tp->snd_cwnd <= tp->snd_ssthresh &&
+	if (hystart && tcp_in_slow_start(tp) &&
 	    tp->snd_cwnd >= hystart_low_window)
 		hystart_update(sk, delay);
 }

net/ipv4/tcp_highspeed.c

@@ -116,7 +116,7 @@ static void hstcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 	if (!tcp_is_cwnd_limited(sk))
 		return;
 
-	if (tp->snd_cwnd <= tp->snd_ssthresh)
+	if (tcp_in_slow_start(tp))
 		tcp_slow_start(tp, acked);
 	else {
 		/* Update AIMD parameters.

net/ipv4/tcp_htcp.c

@@ -236,7 +236,7 @@ static void htcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 	if (!tcp_is_cwnd_limited(sk))
 		return;
 
-	if (tp->snd_cwnd <= tp->snd_ssthresh)
+	if (tcp_in_slow_start(tp))
 		tcp_slow_start(tp, acked);
 	else {
 		/* In dangerous area, increase slowly.

net/ipv4/tcp_illinois.c

@@ -268,7 +268,7 @@ static void tcp_illinois_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 		return;
 
 	/* In slow start */
-	if (tp->snd_cwnd <= tp->snd_ssthresh)
+	if (tcp_in_slow_start(tp))
 		tcp_slow_start(tp, acked);
 
 	else {

net/ipv4/tcp_metrics.c

@@ -461,7 +461,7 @@ void tcp_update_metrics(struct sock *sk)
 				tcp_metric_set(tm, TCP_METRIC_CWND,
 					       tp->snd_cwnd);
 		}
-	} else if (tp->snd_cwnd > tp->snd_ssthresh &&
+	} else if (!tcp_in_slow_start(tp) &&
 		   icsk->icsk_ca_state == TCP_CA_Open) {
 		/* Cong. avoidance phase, cwnd is reliable. */
 		if (!tcp_metric_locked(tm, TCP_METRIC_SSTHRESH))

net/ipv4/tcp_scalable.c

@@ -22,7 +22,7 @@ static void tcp_scalable_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 	if (!tcp_is_cwnd_limited(sk))
 		return;
 
-	if (tp->snd_cwnd <= tp->snd_ssthresh)
+	if (tcp_in_slow_start(tp))
 		tcp_slow_start(tp, acked);
 	else
 		tcp_cong_avoid_ai(tp, min(tp->snd_cwnd, TCP_SCALABLE_AI_CNT),

net/ipv4/tcp_vegas.c

@@ -225,7 +225,7 @@ static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 			 */
 			diff = tp->snd_cwnd * (rtt-vegas->baseRTT) / vegas->baseRTT;
 
-			if (diff > gamma && tp->snd_cwnd <= tp->snd_ssthresh) {
+			if (diff > gamma && tcp_in_slow_start(tp)) {
 				/* Going too fast. Time to slow down
 				 * and switch to congestion avoidance.
 				 */
@@ -240,7 +240,7 @@ static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 				tp->snd_cwnd = min(tp->snd_cwnd, (u32)target_cwnd+1);
 				tp->snd_ssthresh = tcp_vegas_ssthresh(tp);
 
-			} else if (tp->snd_cwnd <= tp->snd_ssthresh) {
+			} else if (tcp_in_slow_start(tp)) {
 				/* Slow start.  */
 				tcp_slow_start(tp, acked);
 			} else {
@@ -281,7 +281,7 @@ static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 		vegas->minRTT = 0x7fffffff;
 	}
 	/* Use normal slow start */
-	else if (tp->snd_cwnd <= tp->snd_ssthresh)
+	else if (tcp_in_slow_start(tp))
 		tcp_slow_start(tp, acked);
 }
 

net/ipv4/tcp_veno.c

@@ -150,7 +150,7 @@ static void tcp_veno_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 
 		veno->diff = (tp->snd_cwnd << V_PARAM_SHIFT) - target_cwnd;
 
-		if (tp->snd_cwnd <= tp->snd_ssthresh) {
+		if (tcp_in_slow_start(tp)) {
 			/* Slow start.  */
 			tcp_slow_start(tp, acked);
 		} else {