rxrpc: Send pings to get RTT data

Send a PING ACK packet to the peer when we get a new incoming call from a
peer we don't have a record for.  The PING RESPONSE ACK packet will tell us
the following about the peer:

 (1) its receive window size

 (2) its MTU sizes

 (3) its support for jumbo DATA packets

 (4) if it supports slow start (similar to RFC 5681)

 (5) an estimate of the RTT

This is necessary because the peer won't normally send us an ACK until it
gets to the Rx phase and we send it a packet, but we would like to know
some of this information before we start sending packets.

A pair of tracepoints are added so that RTT determination can be observed.

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

1 files changed, 22 insertions, 0 deletions

diff --git a/net/rxrpc/output.c b/net/rxrpc/output.c
index 817fb0e82d6a..0d89cd3f2c01 100644
--- a/net/rxrpc/output.c
+++ b/net/rxrpc/output.c
@@ -57,6 +57,9 @@ static size_t rxrpc_fill_out_ack(struct rxrpc_call *call,
 	pkt->ack.reason		= call->ackr_reason;
 	pkt->ack.nAcks		= top - hard_ack;
 
+	if (pkt->ack.reason == RXRPC_ACK_PING)
+		pkt->whdr.flags |= RXRPC_REQUEST_ACK;
+
 	if (after(top, hard_ack)) {
 		seq = hard_ack + 1;
 		do {
@@ -97,6 +100,7 @@ int rxrpc_send_call_packet(struct rxrpc_call *call, u8 type)
 	struct kvec iov[2];
 	rxrpc_serial_t serial;
 	size_t len, n;
+	bool ping = false;
 	int ioc, ret;
 	u32 abort_code;
 
@@ -147,6 +151,7 @@ int rxrpc_send_call_packet(struct rxrpc_call *call, u8 type)
 			ret = 0;
 			goto out;
 		}
+		ping = (call->ackr_reason == RXRPC_ACK_PING);
 		n = rxrpc_fill_out_ack(call, pkt);
 		call->ackr_reason = 0;
 
@@ -183,12 +188,29 @@ int rxrpc_send_call_packet(struct rxrpc_call *call, u8 type)
 		goto out;
 	}
 
+	if (ping) {
+		call->ackr_ping = serial;
+		smp_wmb();
+		/* We need to stick a time in before we send the packet in case
+		 * the reply gets back before kernel_sendmsg() completes - but
+		 * asking UDP to send the packet can take a relatively long
+		 * time, so we update the time after, on the assumption that
+		 * the packet transmission is more likely to happen towards the
+		 * end of the kernel_sendmsg() call.
+		 */
+		call->ackr_ping_time = ktime_get_real();
+		set_bit(RXRPC_CALL_PINGING, &call->flags);
+		trace_rxrpc_rtt_tx(call, rxrpc_rtt_tx_ping, serial);
+	}
 	ret = kernel_sendmsg(conn->params.local->socket,
 			     &msg, iov, ioc, len);
+	if (ping)
+		call->ackr_ping_time = ktime_get_real();
 
 	if (ret < 0 && call->state < RXRPC_CALL_COMPLETE) {
 		switch (type) {
 		case RXRPC_PACKET_TYPE_ACK:
+			clear_bit(RXRPC_CALL_PINGING, &call->flags);
 			rxrpc_propose_ACK(call, pkt->ack.reason,
 					  ntohs(pkt->ack.maxSkew),
 					  ntohl(pkt->ack.serial),