Patchwork [RFC,3/3] virtio_net: limit xmit polling

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Submitter Krishna Kumar
Date June 2, 2011, 2:17 p.m.
Message ID <OF990F08C5.2ECE35B1-ON652578A3.004DC0FA-652578A3.004E3FE4@in.ibm.com>
Download mbox | patch
Permalink /patch/98390/
State Not Applicable
Delegated to: David Miller
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Comments

Krishna Kumar - June 2, 2011, 2:17 p.m.
> OK, I have something very similar, but I still dislike the screw the
> latency part: this path is exactly what the IBM guys seem to hit.  So I
> created two functions: one tries to free a constant number and another
> one up to capacity. I'll post that now.

Please review this patch to see if it looks reasonable (inline and
attachment):

1. Picked comments/code from Michael's code and Rusty's review.
2. virtqueue_min_capacity() needs to be called only if it returned
   empty the last time it was called.
3. Fix return value bug in free_old_xmit_skbs (hangs guest).
4. Stop queue only if capacity is not enough for next xmit.
5. Fix/clean some likely/unlikely checks (hopefully).
6. I think xmit_skb cannot return error since
    virtqueue_enable_cb_delayed() can return false only if
   3/4th space became available, which is what we check.
6. The comments for free_old_xmit_skbs needs to be more
    clear (not done).

I have done some minimal netperf tests with this.

With this patch, add_buf returning capacity seems to be useful - it
allows using fewer virtio API calls.

(See attached file: patch)

Signed-off-by: Krishna Kumar <krkumar2@in.ibm.com>
---
 drivers/net/virtio_net.c |  105 ++++++++++++++++++++++---------------
 1 file changed, 64 insertions(+), 41 deletions(-)

 }

 static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)
@@ -582,46 +598,53 @@ static int xmit_skb(struct virtnet_info
 static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct virtnet_info *vi = netdev_priv(dev);
-	int ret, n;
+	int capacity;

-	/* Free up space in the ring in case this is the first time we get
-	 * woken up after ring full condition.  Note: this might try to free
-	 * more than strictly necessary if the skb has a small
-	 * number of fragments, but keep it simple. */
-	free_old_xmit_skbs(vi, 0);
+	/* Try to free 2 buffers for every 1 xmit, to stay ahead. */
+	free_old_xmit_skbs(vi, 2);

 	/* Try to transmit */
-	ret = xmit_skb(vi, skb);
+	capacity = xmit_skb(vi, skb);

-	/* Failure to queue is unlikely. It's not a bug though: it might happen
-	 * if we get an interrupt while the queue is still mostly full.
-	 * We could stop the queue and re-enable callbacks (and possibly
return
-	 * TX_BUSY), but as this should be rare, we don't bother. */
-	if (unlikely(ret < 0)) {
+	if (unlikely(capacity < 0)) {
+		/*
+		 * Failure to queue should be impossible. The only way to
+		 * reach here is if we got a cb before 3/4th of space was
+		 * available. We could stop the queue and re-enable
+		 * callbacks (and possibly return TX_BUSY), but we don't
+		 * bother since this is impossible.
+		 */
 		if (net_ratelimit())
-			dev_info(&dev->dev, "TX queue failure: %d\n", ret);
+			dev_info(&dev->dev, "TX queue failure: %d\n", capacity);
 		dev->stats.tx_dropped++;
 		kfree_skb(skb);
 		return NETDEV_TX_OK;
 	}
+
 	virtqueue_kick(vi->svq);

 	/* Don't wait up for transmitted skbs to be freed. */
 	skb_orphan(skb);
 	nf_reset(skb);

-	/* Apparently nice girls don't return TX_BUSY; check capacity and stop
-	 * the queue before it gets out of hand.
-	 * Naturally, this wastes entries. */
-	/* We transmit one skb, so try to free at least two pending skbs.
-	 * This is so that we don't hog the skb memory unnecessarily. */
-	if (!likely(free_old_xmit_skbs(vi, 2))) {
-		netif_stop_queue(dev);
-		if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) {
-			/* More just got used, free them and recheck. */
-			if (!likely(free_old_xmit_skbs(vi, 0))) {
-				netif_start_queue(dev);
-				virtqueue_disable_cb(vi->svq);
+	/*
+	 * Apparently nice girls don't return TX_BUSY; check capacity and
+	 * stop the queue before it gets out of hand. Naturally, this wastes
+	 * entries.
+	 */
+	if (capacity < 2+MAX_SKB_FRAGS) {
+		/*
+		 * We don't have enough space for the next packet. Try
+		 * freeing more.
+		 */
+		if (likely(!free_old_xmit_skbs(vi, UINT_MAX))) {
+			netif_stop_queue(dev);
+			if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) {
+				/* More just got used, free them and recheck. */
+				if (likely(free_old_xmit_skbs(vi, UINT_MAX))) {
+					netif_start_queue(dev);
+					virtqueue_disable_cb(vi->svq);
+				}
 			}
 		}
 	}
Michael S. Tsirkin - June 2, 2011, 2:43 p.m.
On Thu, Jun 02, 2011 at 07:47:48PM +0530, Krishna Kumar2 wrote:
> > OK, I have something very similar, but I still dislike the screw the
> > latency part: this path is exactly what the IBM guys seem to hit.  So I
> > created two functions: one tries to free a constant number and another
> > one up to capacity. I'll post that now.
> 
> Please review this patch to see if it looks reasonable:

Hmm, since you decided to work on top of my patch,
I'd appreciate split-up fixes.

> 1. Picked comments/code from MST's code and Rusty's review.
> 2. virtqueue_min_capacity() needs to be called only if it returned
>    empty the last time it was called.
> 3. Fix return value bug in free_old_xmit_skbs (hangs guest).
> 4. Stop queue only if capacity is not enough for next xmit.

That's what we always did ...

> 5. Fix/clean some likely/unlikely checks (hopefully).
> 
> I have done some minimal netperf tests with this.
> 
> With this patch, add_buf returning capacity seems to be useful - it
> allows less virtio API calls.

Why bother? It's cheap ...

> 
> Signed-off-by: Krishna Kumar <krkumar2@in.ibm.com>
> ---
>  drivers/net/virtio_net.c |  105 ++++++++++++++++++++++---------------
>  1 file changed, 64 insertions(+), 41 deletions(-)
> 
> diff -ruNp org/drivers/net/virtio_net.c new/drivers/net/virtio_net.c
> --- org/drivers/net/virtio_net.c	2011-06-02 15:49:25.000000000 +0530
> +++ new/drivers/net/virtio_net.c	2011-06-02 19:13:02.000000000 +0530
> @@ -509,27 +509,43 @@ again:
>  	return received;
>  }
>  
> -/* Check capacity and try to free enough pending old buffers to enable queueing
> - * new ones.  If min_skbs > 0, try to free at least the specified number of skbs
> - * even if the ring already has sufficient capacity.  Return true if we can
> - * guarantee that a following virtqueue_add_buf will succeed. */
> -static bool free_old_xmit_skbs(struct virtnet_info *vi, int min_skbs)
> +/* Return true if freed a skb, else false */
> +static inline bool free_one_old_xmit_skb(struct virtnet_info *vi)
>  {
>  	struct sk_buff *skb;
>  	unsigned int len;
> -	bool r;
>  
> -	while ((r = virtqueue_min_capacity(vi->svq) < MAX_SKB_FRAGS + 2) ||
> -	       min_skbs-- > 0) {
> -		skb = virtqueue_get_buf(vi->svq, &len);
> -		if (unlikely(!skb))
> +	skb = virtqueue_get_buf(vi->svq, &len);
> +	if (unlikely(!skb))
> +		return 0;
> +
> +	pr_debug("Sent skb %p\n", skb);
> +	vi->dev->stats.tx_bytes += skb->len;
> +	vi->dev->stats.tx_packets++;
> +	dev_kfree_skb_any(skb);
> +	return 1;
> +}
> +
> +static bool free_old_xmit_skbs(struct virtnet_info *vi, int to_free)
> +{
> +	bool empty = virtqueue_min_capacity(vi->svq) < MAX_SKB_FRAGS + 2;
> +
> +	do {
> +		if (!free_one_old_xmit_skb(vi)) {
> +			/* No more skbs to free up */
>  			break;
> -		pr_debug("Sent skb %p\n", skb);
> -		vi->dev->stats.tx_bytes += skb->len;
> -		vi->dev->stats.tx_packets++;
> -		dev_kfree_skb_any(skb);
> -	}
> -	return r;
> +		}
> +
> +		if (empty) {
> +			/* Check again if there is enough space */
> +			empty = virtqueue_min_capacity(vi->svq) <
> +				MAX_SKB_FRAGS + 2;
> +		} else {
> +			--to_free;
> +		}
> +	} while (to_free > 0);
> +
> +	return !empty;
>  }

Why bother doing the capacity check in this function?

>  static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)
> @@ -582,46 +598,53 @@ static int xmit_skb(struct virtnet_info 
>  static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
>  {
>  	struct virtnet_info *vi = netdev_priv(dev);
> -	int ret, n;
> +	int capacity;
>  
> -	/* Free up space in the ring in case this is the first time we get
> -	 * woken up after ring full condition.  Note: this might try to free
> -	 * more than strictly necessary if the skb has a small
> -	 * number of fragments, but keep it simple. */
> -	free_old_xmit_skbs(vi, 0);
> +	/* Try to free 2 buffers for every 1 xmit, to stay ahead. */
> +	free_old_xmit_skbs(vi, 2);
>  
>  	/* Try to transmit */
> -	ret = xmit_skb(vi, skb);
> +	capacity = xmit_skb(vi, skb);
>  
> -	/* Failure to queue is unlikely. It's not a bug though: it might happen
> -	 * if we get an interrupt while the queue is still mostly full.
> -	 * We could stop the queue and re-enable callbacks (and possibly return
> -	 * TX_BUSY), but as this should be rare, we don't bother. */
> -	if (unlikely(ret < 0)) {
> +	if (unlikely(capacity < 0)) {
> +		/*
> +		 * Failure to queue should be impossible. The only way to
> +		 * reach here is if we got a cb before 3/4th of space was
> +		 * available. We could stop the queue and re-enable
> +		 * callbacks (and possibly return TX_BUSY), but we don't
> +		 * bother since this is impossible.

It's far from impossible.  The 3/4 thing is only a hint, and old devices
don't support it anyway.

> +		 */
>  		if (net_ratelimit())
> -			dev_info(&dev->dev, "TX queue failure: %d\n", ret);
> +			dev_info(&dev->dev, "TX queue failure: %d\n", capacity);
>  		dev->stats.tx_dropped++;
>  		kfree_skb(skb);
>  		return NETDEV_TX_OK;
>  	}
> +
>  	virtqueue_kick(vi->svq);
>  
>  	/* Don't wait up for transmitted skbs to be freed. */
>  	skb_orphan(skb);
>  	nf_reset(skb);
>  
> -	/* Apparently nice girls don't return TX_BUSY; check capacity and stop
> -	 * the queue before it gets out of hand.
> -	 * Naturally, this wastes entries. */
> -	/* We transmit one skb, so try to free at least two pending skbs.
> -	 * This is so that we don't hog the skb memory unnecessarily. */
> -	if (!likely(free_old_xmit_skbs(vi, 2))) {
> -		netif_stop_queue(dev);
> -		if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) {
> -			/* More just got used, free them and recheck. */
> -			if (!likely(free_old_xmit_skbs(vi, 0))) {
> -				netif_start_queue(dev);
> -				virtqueue_disable_cb(vi->svq);
> +	/*
> +	 * Apparently nice girls don't return TX_BUSY; check capacity and
> +	 * stop the queue before it gets out of hand. Naturally, this wastes
> +	 * entries. 
> +	 */
> +	if (capacity < 2+MAX_SKB_FRAGS) {
> +		/*
> +		 * We don't have enough space for the next packet. Try
> +		 * freeing more.
> +		 */
> +		if (likely(!free_old_xmit_skbs(vi, UINT_MAX))) {
> +			netif_stop_queue(dev);
> +			if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) {
> +				/* More just got used, free them and recheck. */
> +				if (likely(free_old_xmit_skbs(vi, UINT_MAX))) {

Is this where the bug was?

> +					netif_start_queue(dev);
> +					virtqueue_disable_cb(vi->svq);
> +				}
>  			}
>  		}
>  	}
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Krishna Kumar - June 2, 2011, 3:26 p.m.
"Michael S. Tsirkin" <mst@redhat.com> wrote on 06/02/2011 08:13:46 PM:

> > Please review this patch to see if it looks reasonable:
>
> Hmm, since you decided to work on top of my patch,
> I'd appreciate split-up fixes.

OK (that also explains your next comment).

> > 1. Picked comments/code from MST's code and Rusty's review.
> > 2. virtqueue_min_capacity() needs to be called only if it returned
> >    empty the last time it was called.
> > 3. Fix return value bug in free_old_xmit_skbs (hangs guest).
> > 4. Stop queue only if capacity is not enough for next xmit.
>
> That's what we always did ...

I had made the patch against your patch, hence this change (sorry for
the confusion!).

> > 5. Fix/clean some likely/unlikely checks (hopefully).
> >
> > I have done some minimal netperf tests with this.
> >
> > With this patch, add_buf returning capacity seems to be useful - it
> > allows less virtio API calls.
>
> Why bother? It's cheap ...

If add_buf retains it's functionality to return the capacity (it
is going to need a change to return 0 otherwise anyway), is it
useful to call another function at each xmit?

> > +static bool free_old_xmit_skbs(struct virtnet_info *vi, int to_free)
> > +{
> > +   bool empty = virtqueue_min_capacity(vi->svq) < MAX_SKB_FRAGS + 2;
> > +
> > +   do {
> > +      if (!free_one_old_xmit_skb(vi)) {
> > +         /* No more skbs to free up */
> >           break;
> > -      pr_debug("Sent skb %p\n", skb);
> > -      vi->dev->stats.tx_bytes += skb->len;
> > -      vi->dev->stats.tx_packets++;
> > -      dev_kfree_skb_any(skb);
> > -   }
> > -   return r;
> > +      }
> > +
> > +      if (empty) {
> > +         /* Check again if there is enough space */
> > +         empty = virtqueue_min_capacity(vi->svq) <
> > +            MAX_SKB_FRAGS + 2;
> > +      } else {
> > +         --to_free;
> > +      }
> > +   } while (to_free > 0);
> > +
> > +   return !empty;
> >  }
>
> Why bother doing the capacity check in this function?

To return whether we have enough space for next xmit. It should call
it only once unless space is running out. Does it sound OK?

> > -   if (unlikely(ret < 0)) {
> > +   if (unlikely(capacity < 0)) {
> > +      /*
> > +       * Failure to queue should be impossible. The only way to
> > +       * reach here is if we got a cb before 3/4th of space was
> > +       * available. We could stop the queue and re-enable
> > +       * callbacks (and possibly return TX_BUSY), but we don't
> > +       * bother since this is impossible.
>
> It's far from impossible.  The 3/4 thing is only a hint, and old devices
> don't support it anyway.

OK, I will re-put back your comment.

> > -   if (!likely(free_old_xmit_skbs(vi, 2))) {
> > -      netif_stop_queue(dev);
> > -      if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) {
> > -         /* More just got used, free them and recheck. */
> > -         if (!likely(free_old_xmit_skbs(vi, 0))) {
> > -            netif_start_queue(dev);
> > -            virtqueue_disable_cb(vi->svq);
> > +   /*
> > +    * Apparently nice girls don't return TX_BUSY; check capacity and
> > +    * stop the queue before it gets out of hand. Naturally, this
wastes
> > +    * entries.
> > +    */
> > +   if (capacity < 2+MAX_SKB_FRAGS) {
> > +      /*
> > +       * We don't have enough space for the next packet. Try
> > +       * freeing more.
> > +       */
> > +      if (likely(!free_old_xmit_skbs(vi, UINT_MAX))) {
> > +         netif_stop_queue(dev);
> > +         if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) {
> > +            /* More just got used, free them and recheck. */
> > +            if (likely(free_old_xmit_skbs(vi, UINT_MAX))) {
>
> Is this where the bug was?

Return value in free_old_xmit() was wrong. I will re-do against the
mainline kernel.

Thanks,

- KK

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Michael S. Tsirkin - June 2, 2011, 3:34 p.m.
On Thu, Jun 02, 2011 at 08:56:42PM +0530, Krishna Kumar2 wrote:
> "Michael S. Tsirkin" <mst@redhat.com> wrote on 06/02/2011 08:13:46 PM:
> 
> > > Please review this patch to see if it looks reasonable:
> >
> > Hmm, since you decided to work on top of my patch,
> > I'd appreciate split-up fixes.
> 
> OK (that also explains your next comment).
> 
> > > 1. Picked comments/code from MST's code and Rusty's review.
> > > 2. virtqueue_min_capacity() needs to be called only if it returned
> > >    empty the last time it was called.
> > > 3. Fix return value bug in free_old_xmit_skbs (hangs guest).
> > > 4. Stop queue only if capacity is not enough for next xmit.
> >
> > That's what we always did ...
> 
> I had made the patch against your patch, hence this change (sorry for
> the confusion!).
> 
> > > 5. Fix/clean some likely/unlikely checks (hopefully).
> > >
> > > I have done some minimal netperf tests with this.
> > >
> > > With this patch, add_buf returning capacity seems to be useful - it
> > > allows less virtio API calls.
> >
> > Why bother? It's cheap ...
> 
> If add_buf retains it's functionality to return the capacity (it
> is going to need a change to return 0 otherwise anyway), is it
> useful to call another function at each xmit?
> 
> > > +static bool free_old_xmit_skbs(struct virtnet_info *vi, int to_free)
> > > +{
> > > +   bool empty = virtqueue_min_capacity(vi->svq) < MAX_SKB_FRAGS + 2;
> > > +
> > > +   do {
> > > +      if (!free_one_old_xmit_skb(vi)) {
> > > +         /* No more skbs to free up */
> > >           break;
> > > -      pr_debug("Sent skb %p\n", skb);
> > > -      vi->dev->stats.tx_bytes += skb->len;
> > > -      vi->dev->stats.tx_packets++;
> > > -      dev_kfree_skb_any(skb);
> > > -   }
> > > -   return r;
> > > +      }
> > > +
> > > +      if (empty) {
> > > +         /* Check again if there is enough space */
> > > +         empty = virtqueue_min_capacity(vi->svq) <
> > > +            MAX_SKB_FRAGS + 2;
> > > +      } else {
> > > +         --to_free;
> > > +      }
> > > +   } while (to_free > 0);
> > > +
> > > +   return !empty;
> > >  }
> >
> > Why bother doing the capacity check in this function?
> 
> To return whether we have enough space for next xmit. It should call
> it only once unless space is running out. Does it sound OK?
> 
> > > -   if (unlikely(ret < 0)) {
> > > +   if (unlikely(capacity < 0)) {
> > > +      /*
> > > +       * Failure to queue should be impossible. The only way to
> > > +       * reach here is if we got a cb before 3/4th of space was
> > > +       * available. We could stop the queue and re-enable
> > > +       * callbacks (and possibly return TX_BUSY), but we don't
> > > +       * bother since this is impossible.
> >
> > It's far from impossible.  The 3/4 thing is only a hint, and old devices
> > don't support it anyway.
> 
> OK, I will re-put back your comment.
> 
> > > -   if (!likely(free_old_xmit_skbs(vi, 2))) {
> > > -      netif_stop_queue(dev);
> > > -      if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) {
> > > -         /* More just got used, free them and recheck. */
> > > -         if (!likely(free_old_xmit_skbs(vi, 0))) {
> > > -            netif_start_queue(dev);
> > > -            virtqueue_disable_cb(vi->svq);
> > > +   /*
> > > +    * Apparently nice girls don't return TX_BUSY; check capacity and
> > > +    * stop the queue before it gets out of hand. Naturally, this
> wastes
> > > +    * entries.
> > > +    */
> > > +   if (capacity < 2+MAX_SKB_FRAGS) {
> > > +      /*
> > > +       * We don't have enough space for the next packet. Try
> > > +       * freeing more.
> > > +       */
> > > +      if (likely(!free_old_xmit_skbs(vi, UINT_MAX))) {
> > > +         netif_stop_queue(dev);
> > > +         if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) {
> > > +            /* More just got used, free them and recheck. */
> > > +            if (likely(free_old_xmit_skbs(vi, UINT_MAX))) {
> >
> > Is this where the bug was?
> 
> Return value in free_old_xmit() was wrong. I will re-do against the
> mainline kernel.
> 
> Thanks,
> 
> - KK

Just noting that I'm working on that patch as well, it might
be more efficient if we don't both of us do this in parallel :)
Michael S. Tsirkin - June 2, 2011, 3:44 p.m.
On Thu, Jun 02, 2011 at 08:56:42PM +0530, Krishna Kumar2 wrote:
> Return value in free_old_xmit() was wrong.

Could you check my latest RFC pls?
Krishna Kumar - June 3, 2011, 4:08 a.m.
"Michael S. Tsirkin" <mst@redhat.com> wrote on 06/02/2011 09:04:23 PM:

> > > Is this where the bug was?
> >
> > Return value in free_old_xmit() was wrong. I will re-do against the
> > mainline kernel.
> >
> > Thanks,
> >
> > - KK
>
> Just noting that I'm working on that patch as well, it might
> be more efficient if we don't both of us do this in parallel :)

OK, but my intention was to work on a alternate approach, which
was the reason to base it against your patch.

I will check your latest patch.

thanks,

- KK

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Patch

diff -ruNp org/drivers/net/virtio_net.c new/drivers/net/virtio_net.c
--- org/drivers/net/virtio_net.c	2011-06-02 15:49:25.000000000 +0530
+++ new/drivers/net/virtio_net.c	2011-06-02 19:13:02.000000000 +0530
@@ -509,27 +509,43 @@  again:
 	return received;
 }

-/* Check capacity and try to free enough pending old buffers to enable
queueing
- * new ones.  If min_skbs > 0, try to free at least the specified number
of skbs
- * even if the ring already has sufficient capacity.  Return true if we
can
- * guarantee that a following virtqueue_add_buf will succeed. */
-static bool free_old_xmit_skbs(struct virtnet_info *vi, int min_skbs)
+/* Return true if freed a skb, else false */
+static inline bool free_one_old_xmit_skb(struct virtnet_info *vi)
 {
 	struct sk_buff *skb;
 	unsigned int len;
-	bool r;

-	while ((r = virtqueue_min_capacity(vi->svq) < MAX_SKB_FRAGS + 2) ||
-	       min_skbs-- > 0) {
-		skb = virtqueue_get_buf(vi->svq, &len);
-		if (unlikely(!skb))
+	skb = virtqueue_get_buf(vi->svq, &len);
+	if (unlikely(!skb))
+		return 0;
+
+	pr_debug("Sent skb %p\n", skb);
+	vi->dev->stats.tx_bytes += skb->len;
+	vi->dev->stats.tx_packets++;
+	dev_kfree_skb_any(skb);
+	return 1;
+}
+
+static bool free_old_xmit_skbs(struct virtnet_info *vi, int to_free)
+{
+	bool empty = virtqueue_min_capacity(vi->svq) < MAX_SKB_FRAGS + 2;
+
+	do {
+		if (!free_one_old_xmit_skb(vi)) {
+			/* No more skbs to free up */
 			break;
-		pr_debug("Sent skb %p\n", skb);
-		vi->dev->stats.tx_bytes += skb->len;
-		vi->dev->stats.tx_packets++;
-		dev_kfree_skb_any(skb);
-	}
-	return r;
+		}
+
+		if (empty) {
+			/* Check again if there is enough space */
+			empty = virtqueue_min_capacity(vi->svq) <
+				MAX_SKB_FRAGS + 2;
+		} else {
+			--to_free;
+		}
+	} while (to_free > 0);
+
+	return !empty;