SUNRPC: Make RTT measurement more precise (Send)

Some RPC transports have more overhead in their send_request
callouts than others. For example, for RPC-over-RDMA:

- Marshaling an RPC often has to DMA map the RPC arguments

- Registration methods perform memory registration as part of
  marshaling

To capture just server and network latencies more precisely: when
sending a Call, capture the rq_xtime timestamp _after_ the transport
header has been marshaled.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprt.c

@@ -1033,7 +1033,6 @@ void xprt_transmit(struct rpc_task *task)
 		return;
 
 	connect_cookie = xprt->connect_cookie;
-	req->rq_xtime = ktime_get();
 	status = xprt->ops->send_request(task);
 	trace_xprt_transmit(xprt, req->rq_xid, status);
 	if (status != 0) {

net/sunrpc/xprtrdma/transport.c

@@ -696,6 +696,7 @@ xprt_rdma_send_request(struct rpc_task *task)
 	/* Must suppress retransmit to maintain credits */
 	if (rqst->rq_connect_cookie == xprt->connect_cookie)
 		goto drop_connection;
+	rqst->rq_xtime = ktime_get();
 
 	__set_bit(RPCRDMA_REQ_F_PENDING, &req->rl_flags);
 	if (rpcrdma_ep_post(&r_xprt->rx_ia, &r_xprt->rx_ep, req))

net/sunrpc/xprtsock.c

@@ -527,6 +527,7 @@ static int xs_local_send_request(struct rpc_task *task)
 	xs_pktdump("packet data:",
 			req->rq_svec->iov_base, req->rq_svec->iov_len);
 
+	req->rq_xtime = ktime_get();
 	status = xs_sendpages(transport->sock, NULL, 0, xdr, req->rq_bytes_sent,
 			      true, &sent);
 	dprintk("RPC:       %s(%u) = %d\n",
@@ -589,6 +590,7 @@ static int xs_udp_send_request(struct rpc_task *task)
 
 	if (!xprt_bound(xprt))
 		return -ENOTCONN;
+	req->rq_xtime = ktime_get();
 	status = xs_sendpages(transport->sock, xs_addr(xprt), xprt->addrlen,
 			      xdr, req->rq_bytes_sent, true, &sent);
 
@@ -678,6 +680,7 @@ static int xs_tcp_send_request(struct rpc_task *task)
 	/* Continue transmitting the packet/record. We must be careful
 	 * to cope with writespace callbacks arriving _after_ we have
 	 * called sendmsg(). */
+	req->rq_xtime = ktime_get();
 	while (1) {
 		sent = 0;
 		status = xs_sendpages(transport->sock, NULL, 0, xdr,