tcp: make congestion control optionally skip slow start after idle

Congestion control modules that want full control over congestion
control behavior do not want the cwnd modifications controlled by
the sysctl_tcp_slow_start_after_idle code path.
So skip those code paths for CC modules that use the cong_control()
API.
As an example, those cwnd effects are not desired for the BBR congestion
control algorithm.

Fixes: c040276 ("tcp: new CC hook to set sending rate with rate_sample in any CA state")
Signed-off-by: Wei Wang <weiwan@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -1234,10 +1234,12 @@ void tcp_cwnd_restart(struct sock *sk, s32 delta);
 
 static inline void tcp_slow_start_after_idle_check(struct sock *sk)
 {
+	const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
 	struct tcp_sock *tp = tcp_sk(sk);
 	s32 delta;
 
-	if (!sysctl_tcp_slow_start_after_idle || tp->packets_out)
+	if (!sysctl_tcp_slow_start_after_idle || tp->packets_out ||
+	    ca_ops->cong_control)
 		return;
 	delta = tcp_time_stamp - tp->lsndtime;
 	if (delta > inet_csk(sk)->icsk_rto)

net/ipv4/tcp_output.c

@@ -1514,6 +1514,7 @@ static void tcp_cwnd_application_limited(struct sock *sk)
 
 static void tcp_cwnd_validate(struct sock *sk, bool is_cwnd_limited)
 {
+	const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
 	struct tcp_sock *tp = tcp_sk(sk);
 
 	/* Track the maximum number of outstanding packets in each
@@ -1536,7 +1537,8 @@ static void tcp_cwnd_validate(struct sock *sk, bool is_cwnd_limited)
 			tp->snd_cwnd_used = tp->packets_out;
 
 		if (sysctl_tcp_slow_start_after_idle &&
-		    (s32)(tcp_time_stamp - tp->snd_cwnd_stamp) >= inet_csk(sk)->icsk_rto)
+		    (s32)(tcp_time_stamp - tp->snd_cwnd_stamp) >= inet_csk(sk)->icsk_rto &&
+		    !ca_ops->cong_control)
 			tcp_cwnd_application_limited(sk);
 
 		/* The following conditions together indicate the starvation