rxrpc: Adjust retransmission backoff

Improve retransmission backoff by only backing off when we retransmit data
packets rather than when we set the lost ack timer.

To this end:

 (1) In rxrpc_resend(), use rxrpc_get_rto_backoff() when setting the
     retransmission timer and only tell it that we are retransmitting if we
     actually have things to retransmit.

     Note that it's possible for the retransmission algorithm to race with
     the processing of a received ACK, so we may see no packets needing
     retransmission.

 (2) In rxrpc_send_data_packet(), don't bump the backoff when setting the
     ack_lost_at timer, as it may then get bumped twice.

With this, when looking at one particular packet, the retransmission
intervals were seen to be 1.5ms, 2ms, 3ms, 5ms, 9ms, 17ms, 33ms, 71ms,
136ms, 264ms, 544ms, 1.088s, 2.1s, 4.2s and 8.3s.

Fixes: c410bf01 ("rxrpc: Fix the excessive initial retransmission timeout")
Suggested-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Marc Dionne <marc.dionne@auristor.com>
Tested-by: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Link: https://lore.kernel.org/r/164138117069.2023386.17446904856843997127.stgit@warthog.procyon.org.uk/Signed-off-by: David S. Miller <davem@davemloft.net>

net/rxrpc/call_event.c

@@ -157,7 +157,7 @@ static void rxrpc_congestion_timeout(struct rxrpc_call *call)
 static void rxrpc_resend(struct rxrpc_call *call, unsigned long now_j)
 {
 	struct sk_buff *skb;
-	unsigned long resend_at, rto_j;
+	unsigned long resend_at;
 	rxrpc_seq_t cursor, seq, top;
 	ktime_t now, max_age, oldest, ack_ts;
 	int ix;
@@ -165,10 +165,8 @@ static void rxrpc_resend(struct rxrpc_call *call, unsigned long now_j)
 
 	_enter("{%d,%d}", call->tx_hard_ack, call->tx_top);
 
-	rto_j = call->peer->rto_j;
-
 	now = ktime_get_real();
-	max_age = ktime_sub(now, jiffies_to_usecs(rto_j));
+	max_age = ktime_sub(now, jiffies_to_usecs(call->peer->rto_j));
 
 	spin_lock_bh(&call->lock);
 
@@ -213,7 +211,7 @@ static void rxrpc_resend(struct rxrpc_call *call, unsigned long now_j)
 	}
 
 	resend_at = nsecs_to_jiffies(ktime_to_ns(ktime_sub(now, oldest)));
-	resend_at += jiffies + rto_j;
+	resend_at += jiffies + rxrpc_get_rto_backoff(call->peer, retrans);
 	WRITE_ONCE(call->resend_at, resend_at);
 
 	if (unacked)

net/rxrpc/output.c

@@ -468,7 +468,7 @@ int rxrpc_send_data_packet(struct rxrpc_call *call, struct sk_buff *skb,
 			if (call->peer->rtt_count > 1) {
 				unsigned long nowj = jiffies, ack_lost_at;
 
-				ack_lost_at = rxrpc_get_rto_backoff(call->peer, retrans);
+				ack_lost_at = rxrpc_get_rto_backoff(call->peer, false);
 				ack_lost_at += nowj;
 				WRITE_ONCE(call->ack_lost_at, ack_lost_at);
 				rxrpc_reduce_call_timer(call, ack_lost_at, nowj,