rxrpc: Express protocol timeouts in terms of RTT

Express protocol timeouts for data retransmission and deferred ack
generation in terms on RTT rather than specified timeouts once we have
sufficient RTT samples.

For the moment, this requires just one RTT sample to be able to use this
for ack deferral and two for data retransmission.

The data retransmission timeout is set at RTT*1.5 and the ACK deferral
timeout is set at RTT.

Note that the calculated timeout is limited to a minimum of 4ns to make
sure it doesn't happen too quickly.
Signed-off-by: David Howells <dhowells@redhat.com>

net/rxrpc/call_event.c

@@ -52,7 +52,7 @@ static void __rxrpc_propose_ACK(struct rxrpc_call *call, u8 ack_reason,
 				enum rxrpc_propose_ack_trace why)
 {
 	enum rxrpc_propose_ack_outcome outcome = rxrpc_propose_ack_use;
-	unsigned long now, ack_at, expiry = rxrpc_soft_ack_delay;
+	unsigned long expiry = rxrpc_soft_ack_delay;
 	s8 prior = rxrpc_ack_priority[ack_reason];
 
 	/* Pings are handled specially because we don't want to accidentally
@@ -116,7 +116,13 @@ static void __rxrpc_propose_ACK(struct rxrpc_call *call, u8 ack_reason,
 		    background)
 			rxrpc_queue_call(call);
 	} else {
-		now = jiffies;
+		unsigned long now = jiffies, ack_at;
+
+		if (call->peer->rtt_usage > 0)
+			ack_at = nsecs_to_jiffies(call->peer->rtt);
+		else
+			ack_at = expiry;
+
 		ack_at = jiffies + expiry;
 		if (time_before(ack_at, call->ack_at)) {
 			WRITE_ONCE(call->ack_at, ack_at);
@@ -160,14 +166,22 @@ static void rxrpc_resend(struct rxrpc_call *call, unsigned long now_j)
 	struct sk_buff *skb;
 	unsigned long resend_at;
 	rxrpc_seq_t cursor, seq, top;
-	ktime_t now, max_age, oldest, ack_ts;
+	ktime_t now, max_age, oldest, ack_ts, timeout, min_timeo;
 	int ix;
 	u8 annotation, anno_type, retrans = 0, unacked = 0;
 
 	_enter("{%d,%d}", call->tx_hard_ack, call->tx_top);
 
+	if (call->peer->rtt_usage > 1)
+		timeout = ns_to_ktime(call->peer->rtt * 3 / 2);
+	else
+		timeout = ms_to_ktime(rxrpc_resend_timeout);
+	min_timeo = ns_to_ktime((1000000000 / HZ) * 4);
+	if (ktime_before(timeout, min_timeo))
+		timeout = min_timeo;
+
 	now = ktime_get_real();
-	max_age = ktime_sub_ms(now, rxrpc_resend_timeout * 1000 / HZ);
+	max_age = ktime_sub(now, timeout);
 
 	spin_lock_bh(&call->lock);
 

net/rxrpc/sendmsg.c

@@ -226,6 +226,13 @@ static void rxrpc_queue_packet(struct rxrpc_sock *rx, struct rxrpc_call *call,
 	} else {
 		unsigned long now = jiffies, resend_at;
 
+		if (call->peer->rtt_usage > 1)
+			resend_at = nsecs_to_jiffies(call->peer->rtt * 3 / 2);
+		else
+			resend_at = rxrpc_resend_timeout;
+		if (resend_at < 1)
+			resend_at = 1;
+
 		resend_at = now + rxrpc_resend_timeout;
 		WRITE_ONCE(call->resend_at, resend_at);
 		rxrpc_reduce_call_timer(call, resend_at, now,