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: c0402760f565 ("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>

1 files changed, 3 insertions, 1 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 60111a0fc201..4858e190f6ac 100644
--- a/net/ipv4/tcp_output.c
+++ b/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