net-tcp_bbr: v2: record app-limited status of TLP-repaired flight

When sending a TLP retransmit, record whether the outstanding flight
of data is application limited. This is important for congestion
control modules that want to respond to losses repaired by TLP
retransmits. This is important because the following scenarios convey
very different information:
 (1) a packet loss with a small number of packets in flight;
 (2) a packet loss with the maximum amount of data in flight allowed
     by the CC module;

Effort: net-tcp_bbr
Change-Id: Ic8ae567caa4e4bfd5fd82c3d4be12a5d9171655e
Signed-off-by: Alexandre Frade <kernel@xanmod.org>

include/linux/tcp.h

@@ -256,7 +256,8 @@ struct tcp_sock {
 	u8	dup_ack_counter:2,
 		tlp_retrans:1,	/* TLP is a retransmission */
 		fast_ack_mode:2, /* which fast ack mode ? */
-		unused:3;
+		tlp_orig_data_app_limited:1, /* app-limited before TLP rtx? */
+		unused:2;
 	u32	chrono_start;	/* Start time in jiffies of a TCP chrono */
 	u32	chrono_stat[3];	/* Time in jiffies for chrono_stat stats */
 	u8	chrono_type:2,	/* current chronograph type */

net/ipv4/tcp_output.c

@@ -2875,6 +2875,7 @@ void tcp_send_loss_probe(struct sock *sk)
 	if (WARN_ON(!skb || !tcp_skb_pcount(skb)))
 		goto rearm_timer;
 
+	tp->tlp_orig_data_app_limited = TCP_SKB_CB(skb)->tx.is_app_limited;
 	if (__tcp_retransmit_skb(sk, skb, 1))
 		goto rearm_timer;