tcp: generate CA_EVENT_TX_START on data frames

Issuing a CC TX_START event on control frames like pure ACK
is a waste of time, as a CC should not care.

Following patch needs this change, as we want CUBIC to properly track
idle time at a low cost, with a single TX_START being generated.

Yuchung might slightly refine the condition triggering TX_START
on a followup patch.

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Cc: Jana Iyengar <jri@google.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: Sangtae Ha <sangtae.ha@gmail.com>
Cc: Lawrence Brakmo <lawrence@brakmo.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -164,6 +164,9 @@ static void tcp_event_data_sent(struct tcp_sock *tp,
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	const u32 now = tcp_time_stamp;
 
+	if (tcp_packets_in_flight(tp) == 0)
+		tcp_ca_event(sk, CA_EVENT_TX_START);
+
 	tp->lsndtime = now;
 
 	/* If it is a reply for ato after last received
@@ -940,9 +943,6 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
 							   &md5);
 	tcp_header_size = tcp_options_size + sizeof(struct tcphdr);
 
-	if (tcp_packets_in_flight(tp) == 0)
-		tcp_ca_event(sk, CA_EVENT_TX_START);
-
 	/* if no packet is in qdisc/device queue, then allow XPS to select
 	 * another queue. We can be called from tcp_tsq_handler()
 	 * which holds one reference to sk_wmem_alloc.