From: Rusty Russell <rusty@rustcorp.com.au>
Subject: Hack: histogram of buffer sizes for virtio devices.

Currently triggered by a stats query (eg ifconfig) on a net device.
---
 drivers/net/virtio_net.c     |    2 ++
 drivers/virtio/virtio.c      |   10 ++++++++++
 drivers/virtio/virtio_ring.c |   34 ++++++++++++++++++++++++++++++++++
 include/linux/virtio.h       |    3 +++
 4 files changed, 49 insertions(+)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -727,6 +727,8 @@ static struct rtnl_link_stats64 *virtnet
 	tot->rx_length_errors = dev->stats.rx_length_errors;
 	tot->rx_frame_errors = dev->stats.rx_frame_errors;
 
+	virtio_dev_dump_histogram(vi->vdev);
+
 	return tot;
 }
 
diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c
@@ -108,6 +108,16 @@ void virtio_check_driver_offered_feature
 }
 EXPORT_SYMBOL_GPL(virtio_check_driver_offered_feature);
 
+void virtio_dev_dump_histogram(const struct virtio_device *vdev)
+{
+	const struct virtqueue *vq;
+
+	printk("Queue histogram for %s:\n", dev_name(&vdev->dev));
+	list_for_each_entry(vq, &vdev->vqs, list)
+		virtqueue_dump_histogram(vq);
+}
+EXPORT_SYMBOL_GPL(virtio_dev_dump_histogram);
+
 static int virtio_dev_probe(struct device *_d)
 {
 	int err, i;
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -120,6 +120,8 @@ struct vring_virtqueue
 	ktime_t last_add_time;
 #endif
 
+	unsigned int *histo;
+
 	/* Tokens for callbacks. */
 	void *data[];
 };
@@ -259,6 +261,8 @@ int virtqueue_add_buf(struct virtqueue *
 	BUG_ON(out + in > vq->vring.num);
 	BUG_ON(out + in == 0);
 
+	vq->histo[out+in]++;
+
 	/* If the host supports indirect descriptor tables, consider it. */
 	if (vq->indirect) {
 		bool try_indirect;
@@ -726,6 +730,7 @@ struct virtqueue *vring_new_virtqueue(un
 	}
 	vq->data[i] = NULL;
 
+	vq->histo = kzalloc(num * sizeof(vq->histo[0]), GFP_KERNEL);
 	return &vq->vq;
 }
 EXPORT_SYMBOL_GPL(vring_new_virtqueue);
@@ -772,4 +777,33 @@ unsigned int virtqueue_get_vring_size(st
 }
 EXPORT_SYMBOL_GPL(virtqueue_get_vring_size);
 
+void virtqueue_dump_histogram(const struct virtqueue *_vq)
+{
+	const struct vring_virtqueue *vq = to_vvq(_vq);
+	int i, j, start = 0, end = 0, max = 1;
+	char line[120];
+
+	for (i = 0; i < vq->vring.num; i++) {
+		if (!vq->histo[i])
+			continue;
+
+		end = i;
+		if (!vq->histo[start])
+			start = i;
+
+		if (vq->histo[i] > max)
+			max = vq->histo[i];
+	}
+
+	printk("Size distribution for %s (max=%u):\n", _vq->name, max);
+	for (i = start; i <= end; i++) {
+		unsigned int off;
+		off = sprintf(line, "%3u: %-7u ", i, vq->histo[i]);
+		for (j = 0; j < vq->histo[i] * 64 / max; j++)
+			line[off++] = '#';
+		line[off] = '\0';
+		printk("%s\n", line);
+	}
+}
+
 MODULE_LICENSE("GPL");
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -52,6 +52,9 @@ unsigned int virtqueue_get_vring_size(st
 
 int virtqueue_get_queue_index(struct virtqueue *vq);
 
+void virtio_dev_dump_histogram(const struct virtio_device *vdev);
+void virtqueue_dump_histogram(const struct virtqueue *vq);
+
 /**
  * virtio_device - representation of a device using virtio
  * @index: unique position on the virtio bus