lguest: Adaptive timeout

Since the correct timeout value varies, use a heuristic which adjusts
the timeout depending on how many packets we've seen.  This gives
slightly worse results, but doesn't need tweaking when GSO is
introduced.

500 usec	19.1887		xmit 561141 recv 1 timeout 559657
Dynamic (278)	20.1974		xmit 214510 recv 5 timeout 214491 usec 278
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Documentation/lguest/lguest.c

@@ -84,6 +84,7 @@ static void *guest_base;
 static unsigned long guest_limit, guest_max;
 /* The pipe for signal hander to write to. */
 static int timeoutpipe[2];
+static unsigned int timeout_usec = 500;
 
 /* a per-cpu variable indicating whose vcpu is currently running */
 static unsigned int __thread cpu_id;
@@ -905,7 +906,7 @@ static void block_vq(struct virtqueue *vq)
 	itm.it_interval.tv_sec = 0;
 	itm.it_interval.tv_usec = 0;
 	itm.it_value.tv_sec = 0;
-	itm.it_value.tv_usec = 500;
+	itm.it_value.tv_usec = timeout_usec;
 
 	setitimer(ITIMER_REAL, &itm, NULL);
 }
@@ -922,6 +923,7 @@ static void handle_net_output(int fd, struct virtqueue *vq, bool timeout)
 	unsigned int head, out, in, num = 0;
 	int len;
 	struct iovec iov[vq->vring.num];
+	static int last_timeout_num;
 
 	/* Keep getting output buffers from the Guest until we run out. */
 	while ((head = get_vq_desc(vq, iov, &out, &in)) != vq->vring.num) {
@@ -939,6 +941,14 @@ static void handle_net_output(int fd, struct virtqueue *vq, bool timeout)
 	/* Block further kicks and set up a timer if we saw anything. */
 	if (!timeout && num)
 		block_vq(vq);
+
+	if (timeout) {
+		if (num < last_timeout_num)
+			timeout_usec += 10;
+		else if (timeout_usec > 1)
+			timeout_usec--;
+		last_timeout_num = num;
+	}
 }
 
 /* This is where we handle a packet coming in from the tun device to our