tcp: new TCP_INFO stats for RTO events

The 2023 SIGCOMM paper "Improving Network Availability with Protective
ReRoute" has indicated Linux TCP's RTO-triggered txhash rehashing can
effectively reduce application disruption during outages. To better
measure the efficacy of this feature, this patch adds three more
detailed stats during RTO recovery and exports via TCP_INFO.
Applications and monitoring systems can leverage this data to measure
the network path diversity and end-to-end repair latency during network
outages to improve their network infrastructure.

The following counters are added to tcp_sock in order to track RTO
events over the lifetime of a TCP socket.

1. u16 total_rto - Counts the total number of RTO timeouts.
2. u16 total_rto_recoveries - Counts the total number of RTO recoveries.
3. u32 total_rto_time - Counts the total time spent (ms) in RTO
                        recoveries. (time spent in CA_Loss and
                        CA_Recovery states)

To compute total_rto_time, we add a new u32 rto_stamp field to
tcp_sock. rto_stamp records the start timestamp (ms) of the last RTO
recovery (CA_Loss).

Corresponding fields are also added to the tcp_info struct.

Signed-off-by: Aananth V <aananthv@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 15 insertions, 0 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 8d2c91703158..584825ddd0a0 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2088,6 +2088,10 @@ void tcp_clear_retrans(struct tcp_sock *tp)
 	tp->undo_marker = 0;
 	tp->undo_retrans = -1;
 	tp->sacked_out = 0;
+	tp->rto_stamp = 0;
+	tp->total_rto = 0;
+	tp->total_rto_recoveries = 0;
+	tp->total_rto_time = 0;
 }
 
 static inline void tcp_init_undo(struct tcp_sock *tp)
@@ -2825,6 +2829,14 @@ void tcp_enter_recovery(struct sock *sk, bool ece_ack)
 	tcp_set_ca_state(sk, TCP_CA_Recovery);
 }
 
+static void tcp_update_rto_time(struct tcp_sock *tp)
+{
+	if (tp->rto_stamp) {
+		tp->total_rto_time += tcp_time_stamp(tp) - tp->rto_stamp;
+		tp->rto_stamp = 0;
+	}
+}
+
 /* Process an ACK in CA_Loss state. Move to CA_Open if lost data are
  * recovered or spurious. Otherwise retransmits more on partial ACKs.
  */
@@ -3029,6 +3041,8 @@ static void tcp_fastretrans_alert(struct sock *sk, const u32 prior_snd_una,
 		break;
 	case TCP_CA_Loss:
 		tcp_process_loss(sk, flag, num_dupack, rexmit);
+		if (icsk->icsk_ca_state != TCP_CA_Loss)
+			tcp_update_rto_time(tp);
 		tcp_identify_packet_loss(sk, ack_flag);
 		if (!(icsk->icsk_ca_state == TCP_CA_Open ||
 		      (*ack_flag & FLAG_LOST_RETRANS)))
@@ -6454,6 +6468,7 @@ static void tcp_rcv_synrecv_state_fastopen(struct sock *sk)
 		tcp_try_undo_recovery(sk);
 
 	/* Reset rtx states to prevent spurious retransmits_timed_out() */
+	tcp_update_rto_time(tp);
 	tp->retrans_stamp = 0;
 	inet_csk(sk)->icsk_retransmits = 0;