net-tcp_bbr: v2: inform CC module of losses repaired by TLP probe

Before this commit, when there is a packet loss that creates a sequence
hole that is filled by a TLP loss probe, then tcp_process_tlp_ack()
only informs the congestion control (CC) module via a back-to-back entry
and exit of CWR. But some congestion control modules (e.g. BBR) do not
respond to CWR events.

This commit adds a new CA event with which the core TCP stack notifies
the CC module when a loss is repaired by a TLP. This will allow CC
modules that do not use the CWR mechanism to have a custom handler for
such TLP recoveries.

Effort: net-tcp_bbr
Change-Id: Ieba72332b401b329bff5a641d2b2043a3fb8f632
Signed-off-by: Alexandre Frade <kernel@xanmod.org>

include/net/tcp.h

@@ -1010,6 +1010,7 @@ enum tcp_ca_event {
 	CA_EVENT_LOSS,		/* loss timeout */
 	CA_EVENT_ECN_NO_CE,	/* ECT set, but not CE marked */
 	CA_EVENT_ECN_IS_CE,	/* received CE marked IP packet */
+	CA_EVENT_TLP_RECOVERY,	/* a lost segment was repaired by TLP probe */
 };
 
 /* Information about inbound ACK, passed to cong_ops->in_ack_event() */

net/ipv4/tcp_input.c

@@ -3739,6 +3739,7 @@ static void tcp_process_tlp_ack(struct sock *sk, u32 ack, int flag)
 		/* ACK advances: there was a loss, so reduce cwnd. Reset
 		 * tlp_high_seq in tcp_init_cwnd_reduction()
 		 */
+		tcp_ca_event(sk, CA_EVENT_TLP_RECOVERY);
 		tcp_init_cwnd_reduction(sk);
 		tcp_set_ca_state(sk, TCP_CA_CWR);
 		tcp_end_cwnd_reduction(sk);