net-tcp_bbr: v2: introduce ca_ops->skb_marked_lost() CC module callba…

…ck API

For connections experiencing reordering, RACK can mark packets lost
long after we receive the SACKs/ACKs hinting that the packets were
actually lost.

This means that CC modules cannot easily learn the volume of inflight
data at which packet loss happens by looking at the current inflight
or even the packets in flight when the most recently SACKed packet was
sent. To learn this, CC modules need to know how many packets were in
flight at the time lost packets were sent. This new callback, combined
with TCP_SKB_CB(skb)->tx.in_flight, allows them to learn this.

This also provides a consistent callback that is invoked whether
packets are marked lost upon ACK processing, using the RACK reordering
timer, or at RTO time.

Effort: net-tcp_bbr
Origin-9xx-SHA1: afcbebe3374e4632ac6714d39e4dc8a8455956f4
Change-Id: I54826ab53df636be537e5d3c618a46145d12d51a

include/net/tcp.h

@@ -1085,6 +1085,9 @@ struct tcp_congestion_ops {
 	/* override sysctl_tcp_min_tso_segs */
 	u32 (*min_tso_segs)(struct sock *sk);
 
+	/* react to a specific lost skb (optional) */
+	void (*skb_marked_lost)(struct sock *sk, const struct sk_buff *skb);
+
 	/* call when packets are delivered to update cwnd and pacing rate,
 	 * after all the ca_state processing. (optional)
 	 */

net/ipv4/tcp_input.c

@@ -1079,7 +1079,12 @@ static void tcp_verify_retransmit_hint(struct tcp_sock *tp, struct sk_buff *skb)
  */
 static void tcp_notify_skb_loss_event(struct tcp_sock *tp, const struct sk_buff *skb)
 {
+	struct sock *sk = (struct sock *)tp;
+	const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
+
 	tp->lost += tcp_skb_pcount(skb);
+	if (ca_ops->skb_marked_lost)
+		ca_ops->skb_marked_lost(sk, skb);
 }
 
 void tcp_mark_skb_lost(struct sock *sk, struct sk_buff *skb)