rxrpc: Reduce unnecessary ack transmission

rxrpc_recvmsg_data() schedules an ACK to be transmitted every time at least
two packets have been consumed and any time it runs out of data and would
return -EAGAIN to the caller.  Both events may occur within a single loop,
however, and if the I/O thread is quick enough it may send duplicate ACKs.

The ACKs are sent to inform the peer that more space has been made in the
local Rx window, but the I/O thread is going to send an ACK every couple of
DATA packets anyway, so we end up sending a lot more ACKs than we really
need to.

So reduce the rate at which recvmsg() schedules ACKs, such that if the I/O
thread sends ACKs at its normal faster rate, recvmsg() won't actually
schedule ACKs until the Rx flow stops (call->rx_consumed is cleared any
time we transmit an ACK for that call, resetting the counter used by
recvmsg).

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org

net/rxrpc/recvmsg.c

@@ -137,7 +137,7 @@ static void rxrpc_rotate_rx_window(struct rxrpc_call *call)
 	/* Check to see if there's an ACK that needs sending. */
 	acked = atomic_add_return(call->rx_consumed - old_consumed,
 				  &call->ackr_nr_consumed);
-	if (acked > 2 &&
+	if (acked > 8 &&
 	    !test_and_set_bit(RXRPC_CALL_RX_IS_IDLE, &call->flags))
 		rxrpc_poke_call(call, rxrpc_call_poke_idle);
 }