Subject: Re: Network performance with small packets

From: Rusty Russell <[email-masked]>
Date: Wed, 9 Feb 2011 11:07:20 +1030

On Wed, 2 Feb 2011 03:12:22 pm Michael S. Tsirkin wrote:
> On Wed, Feb 02, 2011 at 10:09:18AM +0530, Krishna Kumar2 wrote:
> > > "Michael S. Tsirkin" <[email-masked]> 02/02/2011 03:11 AM
> > >
> > > On Tue, Feb 01, 2011 at 01:28:45PM -0800, Shirley Ma wrote:
> > > > On Tue, 2011-02-01 at 23:21 +0200, Michael S. Tsirkin wrote:
> > > > > Confused. We compare capacity to skb frags, no?
> > > > > That's sg I think ...
> > > >
> > > > Current guest kernel use indirect buffers, num_free returns how many
> > > > available descriptors not skb frags. So it's wrong here.
> > > >
> > > > Shirley
> > >
> > > I see. Good point. In other words when we complete the buffer
> > > it was indirect, but when we add a new one we
> > > can not allocate indirect so we consume.
> > > And then we start the queue and add will fail.
> > > I guess we need some kind of API to figure out
> > > whether the buf we complete was indirect?

I've finally read this thread... I think we need to get more serious
with our stats gathering to diagnose these kind of performance issues.

This is a start; it should tell us what is actually happening to the
virtio ring(s) without significant performance impact...

Subject: virtio: CONFIG_VIRTIO_STATS

For performance problems we'd like to know exactly what the ring looks
like.  This patch adds stats indexed by how-full-ring-is; we could extend
it to also record them by how-used-ring-is if we need.

Signed-off-by: Rusty Russell <[email-masked]>

diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
--- a/drivers/virtio/Kconfig
+++ b/drivers/virtio/Kconfig
@@ -7,6 +7,14 @@ config VIRTIO_RING
 	tristate
 	depends on VIRTIO
 
+config VIRTIO_STATS
+	bool "Virtio debugging stats (EXPERIMENTAL)"
+	depends on VIRTIO_RING
+	select DEBUG_FS
+	---help---
+	  Virtio stats collected by how full the ring is at any time,
+	  presented under debugfs/virtio/<name>-<vq>/<num-used>/
+
 config VIRTIO_PCI
 	tristate "PCI driver for virtio devices (EXPERIMENTAL)"
 	depends on PCI && EXPERIMENTAL
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
@@ -21,6 +21,7 @@
 #include <linux/virtio_config.h>
 #include <linux/device.h>
 #include <linux/slab.h>
+#include <linux/debugfs.h>
 
 /* virtio guest is communicating with a virtual "device" that actually runs on
  * a host processor.  Memory barriers are used to control SMP effects. */
@@ -95,6 +96,11 @@ struct vring_virtqueue
 	/* How to notify other side. FIXME: commonalize hcalls! */
 	void (*notify)(struct virtqueue *vq);
 
+#ifdef CONFIG_VIRTIO_STATS
+	struct vring_stat *stats;
+	struct dentry *statdir;
+#endif
+
 #ifdef DEBUG
 	/* They're supposed to lock for us. */
 	unsigned int in_use;
@@ -106,6 +112,87 @@ struct vring_virtqueue
 
 #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq)
 
+#ifdef CONFIG_VIRTIO_STATS
+/* We have an array of these, indexed by how full the ring is. */
+struct vring_stat {
+	/* How many interrupts? */
+	size_t interrupt_nowork, interrupt_work;
+	/* How many non-notify kicks, how many notify kicks, how many add notify? */
+	size_t kick_no_notify, kick_notify, add_notify;
+	/* How many adds? */
+	size_t add_direct, add_indirect, add_fail;
+	/* How many gets? */
+	size_t get;
+	/* How many disable callbacks? */
+	size_t disable_cb;
+	/* How many enables? */
+	size_t enable_cb_retry, enable_cb_success;
+};
+
+static struct dentry *virtio_stats;
+
+static void create_stat_files(struct vring_virtqueue *vq)
+{
+	char name[80];
+	unsigned int i;
+
+	/* Racy in theory, but we don't care. */
+	if (!virtio_stats)
+		virtio_stats = debugfs_create_dir("virtio-stats", NULL);
+
+	sprintf(name, "%s-%s", dev_name(&vq->vq.vdev->dev), vq->vq.name);
+	vq->statdir = debugfs_create_dir(name, virtio_stats);
+
+	for (i = 0; i < vq->vring.num; i++) {
+		struct dentry *dir;
+
+		sprintf(name, "%i", i);
+		dir = debugfs_create_dir(name, vq->statdir);
+		debugfs_create_size_t("interrupt_nowork", 0400, dir,
+				      &vq->stats[i].interrupt_nowork);
+		debugfs_create_size_t("interrupt_work", 0400, dir,
+				      &vq->stats[i].interrupt_work);
+		debugfs_create_size_t("kick_no_notify", 0400, dir,
+				      &vq->stats[i].kick_no_notify);
+		debugfs_create_size_t("kick_notify", 0400, dir,
+				      &vq->stats[i].kick_notify);
+		debugfs_create_size_t("add_notify", 0400, dir,
+				      &vq->stats[i].add_notify);
+		debugfs_create_size_t("add_direct", 0400, dir,
+				      &vq->stats[i].add_direct);
+		debugfs_create_size_t("add_indirect", 0400, dir,
+				      &vq->stats[i].add_indirect);
+		debugfs_create_size_t("add_fail", 0400, dir,
+				      &vq->stats[i].add_fail);
+		debugfs_create_size_t("get", 0400, dir,
+				      &vq->stats[i].get);
+		debugfs_create_size_t("disable_cb", 0400, dir,
+				      &vq->stats[i].disable_cb);
+		debugfs_create_size_t("enable_cb_retry", 0400, dir,
+				      &vq->stats[i].enable_cb_retry);
+		debugfs_create_size_t("enable_cb_success", 0400, dir,
+				      &vq->stats[i].enable_cb_success);
+	}
+}
+
+static void delete_stat_files(struct vring_virtqueue *vq)
+{
+	debugfs_remove_recursive(vq->statdir);
+}
+
+#define add_stat(vq, name)						\
+	do {								\
+		struct vring_virtqueue *_vq = (vq);			\
+		_vq->stats[_vq->num_free - _vq->vring.num].name++;	\
+	} while (0)
+
+#else
+#define add_stat(vq, name)
+static void delete_stat_files(struct vring_virtqueue *vq)
+{
+}
+#endif
+
 /* Set up an indirect table of descriptors and add it to the queue. */
 static int vring_add_indirect(struct vring_virtqueue *vq,
 			      struct scatterlist sg[],
@@ -121,6 +208,8 @@ static int vring_add_indirect(struct vri
 	if (!desc)
 		return -ENOMEM;
 
+	add_stat(vq, add_indirect);
+
 	/* Transfer entries from the sg list into the indirect page */
 	for (i = 0; i < out; i++) {
 		desc[i].flags = VRING_DESC_F_NEXT;
@@ -183,17 +272,22 @@ int virtqueue_add_buf_gfp(struct virtque
 	BUG_ON(out + in == 0);
 
 	if (vq->num_free < out + in) {
+		add_stat(vq, add_fail);
 		pr_debug("Can't add buf len %i - avail = %i\n",
 			 out + in, vq->num_free);
 		/* FIXME: for historical reasons, we force a notify here if
 		 * there are outgoing parts to the buffer.  Presumably the
 		 * host should service the ring ASAP. */
-		if (out)
+		if (out) {
+			add_stat(vq, add_notify);
 			vq->notify(&vq->vq);
+		}
 		END_USE(vq);
 		return -ENOSPC;
 	}
 
+	add_stat(vq, add_direct);
+
 	/* We're about to use some buffers from the free list. */
 	vq->num_free -= out + in;
 
@@ -248,9 +342,12 @@ void virtqueue_kick(struct virtqueue *_v
 	/* Need to update avail index before checking if we should notify */
 	virtio_mb();
 
-	if (!(vq->vring.used->flags & VRING_USED_F_NO_NOTIFY))
+	if (!(vq->vring.used->flags & VRING_USED_F_NO_NOTIFY)) {
+		add_stat(vq, kick_notify);
 		/* Prod other side to tell it about changes. */
 		vq->notify(&vq->vq);
+	} else
+		add_stat(vq, kick_no_notify);
 
 	END_USE(vq);
 }
@@ -294,6 +391,8 @@ void *virtqueue_get_buf(struct virtqueue
 
 	START_USE(vq);
 
+	add_stat(vq, get);
+
 	if (unlikely(vq->broken)) {
 		END_USE(vq);
 		return NULL;
@@ -333,6 +432,7 @@ void virtqueue_disable_cb(struct virtque
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
 
+	add_stat(vq, disable_cb);
 	vq->vring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT;
 }
 EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
@@ -348,10 +448,12 @@ bool virtqueue_enable_cb(struct virtqueu
 	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
 	virtio_mb();
 	if (unlikely(more_used(vq))) {
+		add_stat(vq, enable_cb_retry);
 		END_USE(vq);
 		return false;
 	}
 
+	add_stat(vq, enable_cb_success);
 	END_USE(vq);
 	return true;
 }
@@ -387,10 +489,12 @@ irqreturn_t vring_interrupt(int irq, voi
 	struct vring_virtqueue *vq = to_vvq(_vq);
 
 	if (!more_used(vq)) {
+		add_stat(vq, interrupt_nowork);
 		pr_debug("virtqueue interrupt with no work for %p\n", vq);
 		return IRQ_NONE;
 	}
 
+	add_stat(vq, interrupt_work);
 	if (unlikely(vq->broken))
 		return IRQ_HANDLED;
 
@@ -451,6 +555,15 @@ struct virtqueue *vring_new_virtqueue(un
 	}
 	vq->data[i] = NULL;
 
+#ifdef CONFIG_VIRTIO_STATS
+	vq->stats = kzalloc(sizeof(*vq->stats) * num, GFP_KERNEL);
+	if (!vq->stats) {
+		kfree(vq);
+		return NULL;
+	}
+	create_stat_files(vq);
+#endif
+
 	return &vq->vq;
 }
 EXPORT_SYMBOL_GPL(vring_new_virtqueue);
@@ -458,6 +571,7 @@ EXPORT_SYMBOL_GPL(vring_new_virtqueue);
 void vring_del_virtqueue(struct virtqueue *vq)
 {
 	list_del(&vq->list);
+	delete_stat_files(to_vvq(vq));
 	kfree(to_vvq(vq));
 }
 EXPORT_SYMBOL_GPL(vring_del_virtqueue);