[PATCH] RPC over UDP congestion control updates [3/8]

Improve the response to timeouts. As requests time out, we delay
timing out the remaining requests (in fact we follow exponential
backoff). This is done because we assume either that the round trip
time has been underestimated, or that the network/server is congested,
and we need to back off the resending of new requests.

include/linux/sunrpc/xprt.h

@@ -99,6 +99,7 @@ struct rpc_rqst {
 	u32			rq_bytes_sent;	/* Bytes we have sent */
 
 	long			rq_xtime;	/* when transmitted */
+	int			rq_ntimeo;
 	int			rq_nresend;
 };
 #define rq_svec			rq_snd_buf.head

net/sunrpc/xprt.c

@@ -77,6 +77,8 @@
 # define RPCDBG_FACILITY	RPCDBG_XPRT
 #endif
 
+#define XPRT_MAX_BACKOFF	(8)
+
 /*
  * Local functions
  */
@@ -931,6 +933,21 @@ xprt_write_space(struct sock *sk)
 	}
 }
 
+/*
+ * Exponential backoff for UDP retries
+ */
+static inline int
+xprt_expbackoff(struct rpc_task *task, struct rpc_rqst *req)
+{
+	int backoff;
+
+	req->rq_ntimeo++;
+	backoff = min(rpc_ntimeo(&task->tk_client->cl_rtt), XPRT_MAX_BACKOFF);
+	if (req->rq_ntimeo < (1 << backoff))
+		return 1;
+	return 0;
+}
+
 /*
  * RPC receive timeout handler.
  */
@@ -943,9 +960,16 @@ xprt_timer(struct rpc_task *task)
 	spin_lock(&xprt->sock_lock);
 	if (req->rq_received)
 		goto out;
+
+	if (!xprt->nocong) {
+		if (xprt_expbackoff(task, req)) {
+			rpc_add_timer(task, xprt_timer);
+			goto out_unlock;
+		}
+		rpc_inc_timeo(&task->tk_client->cl_rtt);
+		xprt_adjust_cwnd(req->rq_xprt, -ETIMEDOUT);
+	}
 	req->rq_nresend++;
-	rpc_inc_timeo(&task->tk_client->cl_rtt);
-	xprt_adjust_cwnd(xprt, -ETIMEDOUT);
 
 	dprintk("RPC: %4d xprt_timer (%s request)\n",
 		task->tk_pid, req ? "pending" : "backlogged");
@@ -954,6 +978,7 @@ xprt_timer(struct rpc_task *task)
 out:
 	task->tk_timeout = 0;
 	rpc_wake_up_task(task);
+out_unlock:
 	spin_unlock(&xprt->sock_lock);
 }
 
@@ -1076,16 +1101,9 @@ do_xprt_transmit(struct rpc_task *task)
 	dprintk("RPC: %4d xmit complete\n", task->tk_pid);
 	/* Set the task's receive timeout value */
 	if (!xprt->nocong) {
-		int backoff;
 		task->tk_timeout = rpc_calc_rto(&clnt->cl_rtt,
 				rpcproc_timer(clnt, task->tk_msg.rpc_proc));
-		/* If we are retransmitting, increment the timeout counter */
-		backoff = req->rq_nresend;
-		if (backoff) {
-			if (backoff > 7)
-				backoff = 7;
-			task->tk_timeout <<= backoff;
-		}
+		req->rq_ntimeo = 0;
 		if (task->tk_timeout > req->rq_timeout.to_maxval)
 			task->tk_timeout = req->rq_timeout.to_maxval;
 	} else