rxrpc: Implement slow-start

Implement RxRPC slow-start, which is similar to RFC 5681 for TCP.  A
tracepoint is added to log the state of the congestion management algorithm
and the decisions it makes.

Notes:

 (1) Since we send fixed-size DATA packets (apart from the final packet in
     each phase), counters and calculations are in terms of packets rather
     than bytes.

 (2) The ACK packet carries the equivalent of TCP SACK.

 (3) The FLIGHT_SIZE calculation in RFC 5681 doesn't seem particularly
     suited to SACK of a small number of packets.  It seems that, almost
     inevitably, by the time three 'duplicate' ACKs have been seen, we have
     narrowed the loss down to one or two missing packets, and the
     FLIGHT_SIZE calculation ends up as 2.

 (4) In rxrpc_resend(), if there was no data that apparently needed
     retransmission, we transmit a PING ACK to ask the peer to tell us what
     its Rx window state is.

Signed-off-by: David Howells <dhowells@redhat.com>

1 files changed, 34 insertions, 2 deletions

diff --git a/net/rxrpc/call_event.c b/net/rxrpc/call_event.c
index 05b94d1acf52..0e8478012212 100644
--- a/net/rxrpc/call_event.c
+++ b/net/rxrpc/call_event.c
@@ -147,6 +147,14 @@ void rxrpc_propose_ACK(struct rxrpc_call *call, u8 ack_reason,
 }
 
 /*
+ * Handle congestion being detected by the retransmit timeout.
+ */
+static void rxrpc_congestion_timeout(struct rxrpc_call *call)
+{
+	set_bit(RXRPC_CALL_RETRANS_TIMEOUT, &call->flags);
+}
+
+/*
  * Perform retransmission of NAK'd and unack'd packets.
  */
 static void rxrpc_resend(struct rxrpc_call *call)
@@ -154,9 +162,9 @@ static void rxrpc_resend(struct rxrpc_call *call)
 	struct rxrpc_skb_priv *sp;
 	struct sk_buff *skb;
 	rxrpc_seq_t cursor, seq, top;
-	ktime_t now = ktime_get_real(), max_age, oldest,  resend_at;
+	ktime_t now = ktime_get_real(), max_age, oldest, resend_at, ack_ts;
 	int ix;
-	u8 annotation, anno_type;
+	u8 annotation, anno_type, retrans = 0, unacked = 0;
 
 	_enter("{%d,%d}", call->tx_hard_ack, call->tx_top);
 
@@ -193,10 +201,13 @@ static void rxrpc_resend(struct rxrpc_call *call)
 					oldest = skb->tstamp;
 				continue;
 			}
+			if (!(annotation & RXRPC_TX_ANNO_RESENT))
+				unacked++;
 		}
 
 		/* Okay, we need to retransmit a packet. */
 		call->rxtx_annotations[ix] = RXRPC_TX_ANNO_RETRANS | annotation;
+		retrans++;
 		trace_rxrpc_retransmit(call, seq, annotation | anno_type,
 				       ktime_to_ns(ktime_sub(skb->tstamp, max_age)));
 	}
@@ -210,6 +221,25 @@ static void rxrpc_resend(struct rxrpc_call *call)
 		    * reached the nsec timeout yet.
 		    */
 
+	if (unacked)
+		rxrpc_congestion_timeout(call);
+
+	/* If there was nothing that needed retransmission then it's likely
+	 * that an ACK got lost somewhere.  Send a ping to find out instead of
+	 * retransmitting data.
+	 */
+	if (!retrans) {
+		rxrpc_set_timer(call, rxrpc_timer_set_for_resend);
+		spin_unlock_bh(&call->lock);
+		ack_ts = ktime_sub(now, call->acks_latest_ts);
+		if (ktime_to_ns(ack_ts) < call->peer->rtt)
+			goto out;
+		rxrpc_propose_ACK(call, RXRPC_ACK_PING, 0, 0, true, false,
+				  rxrpc_propose_ack_ping_for_lost_ack);
+		rxrpc_send_call_packet(call, RXRPC_PACKET_TYPE_ACK);
+		goto out;
+	}
+
 	/* Now go through the Tx window and perform the retransmissions.  We
 	 * have to drop the lock for each send.  If an ACK comes in whilst the
 	 * lock is dropped, it may clear some of the retransmission markers for
@@ -260,6 +290,7 @@ static void rxrpc_resend(struct rxrpc_call *call)
 
 out_unlock:
 	spin_unlock_bh(&call->lock);
+out:
 	_leave("");
 }
 
@@ -293,6 +324,7 @@ recheck_state:
 	if (time_after_eq(now, call->expire_at)) {
 		rxrpc_abort_call("EXP", call, 0, RX_CALL_TIMEOUT, ETIME);
 		set_bit(RXRPC_CALL_EV_ABORT, &call->events);
+		goto recheck_state;
 	}
 
 	if (test_and_clear_bit(RXRPC_CALL_EV_ACK, &call->events) ||