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

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Submitter Michael S. Tsirkin
Date June 1, 2011, 9:50 a.m.
Message ID <1ec8eec325839ecf2eac9930a230361e7956047c.1306921434.git.mst@redhat.com>
Download mbox | patch
Permalink /patch/98160/
State Not Applicable
Delegated to: David Miller
Headers show

Comments

Michael S. Tsirkin - June 1, 2011, 9:50 a.m.
Current code might introduce a lot of latency variation
if there are many pending bufs at the time we
attempt to transmit a new one. This is bad for
real-time applications and can't be good for TCP either.

Free up just enough to both clean up all buffers
eventually and to be able to xmit the next packet.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 drivers/net/virtio_net.c |   62 ++++++++++++++++++++++++++--------------------
 1 files changed, 35 insertions(+), 27 deletions(-)
Rusty Russell - June 2, 2011, 3:54 a.m.
On Wed, 1 Jun 2011 12:50:03 +0300, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> Current code might introduce a lot of latency variation
> if there are many pending bufs at the time we
> attempt to transmit a new one. This is bad for
> real-time applications and can't be good for TCP either.
> 
> Free up just enough to both clean up all buffers
> eventually and to be able to xmit the next packet.

OK, I found this quite confusing to read.

> -	while ((skb = virtqueue_get_buf(vi->svq, &len)) != NULL) {
> +	while ((r = virtqueue_min_capacity(vi->svq) < MAX_SKB_FRAGS + 2) ||
> +	       min_skbs-- > 0) {
> +		skb = virtqueue_get_buf(vi->svq, &len);
> +		if (unlikely(!skb))
> +			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;
>  }

Gah... what a horrible loop.

Basically, this patch makes hard-to-read code worse, and we should try
to make it better.

Currently, xmit *can* fail when an xmit interrupt wakes the queue, but
the packet(s) xmitted didn't free up enough space for the new packet.
With indirect buffers this only happens if we hit OOM (and thus go to
direct buffers).

We could solve this by only waking the queue in skb_xmit_done if the
capacity is >= 2 + MAX_SKB_FRAGS.  But can we do it without a race?

If not, then I'd really prefer to see this, because I think it's clearer:

        // Try to free 2 buffers for every 1 xmit, to stay ahead.
        free_old_buffers(2)

        if (!add_buf()) {
                // Screw latency, free them all.
                free_old_buffers(UINT_MAX)
                // OK, this can happen if we are using direct buffers,
                // and the xmit interrupt woke us but the packets
                // xmitted were smaller than this one.  Rare though.
                if (!add_buf())
                        Whinge and stop queue, maybe loop.
        }

        if (capacity < 2 + MAX_SKB_FRAGS) {
                // We don't have enough for the next packet?  Try
                // freeing more.
                free_old_buffers(UINT_MAX);
                if (capacity < 2 + MAX_SKB_FRAGS) {
                        Stop queue, maybe loop.
        }

The current code makes my head hurt :(

Thoughts?
Rusty.
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Michael S. Tsirkin - June 2, 2011, 1:34 p.m.
On Thu, Jun 02, 2011 at 01:24:57PM +0930, Rusty Russell wrote:
> On Wed, 1 Jun 2011 12:50:03 +0300, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > Current code might introduce a lot of latency variation
> > if there are many pending bufs at the time we
> > attempt to transmit a new one. This is bad for
> > real-time applications and can't be good for TCP either.
> > 
> > Free up just enough to both clean up all buffers
> > eventually and to be able to xmit the next packet.
> 
> OK, I found this quite confusing to read.
> 
> > -	while ((skb = virtqueue_get_buf(vi->svq, &len)) != NULL) {
> > +	while ((r = virtqueue_min_capacity(vi->svq) < MAX_SKB_FRAGS + 2) ||
> > +	       min_skbs-- > 0) {
> > +		skb = virtqueue_get_buf(vi->svq, &len);
> > +		if (unlikely(!skb))
> > +			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;
> >  }
> 
> Gah... what a horrible loop.
> 
> Basically, this patch makes hard-to-read code worse, and we should try
> to make it better.
> 
> Currently, xmit *can* fail when an xmit interrupt wakes the queue, but
> the packet(s) xmitted didn't free up enough space for the new packet.
> With indirect buffers this only happens if we hit OOM (and thus go to
> direct buffers).
> 
> We could solve this by only waking the queue in skb_xmit_done if the
> capacity is >= 2 + MAX_SKB_FRAGS.  But can we do it without a race?

I don't think so.

> If not, then I'd really prefer to see this, because I think it's clearer:
> 
>         // Try to free 2 buffers for every 1 xmit, to stay ahead.
>         free_old_buffers(2)
> 
>         if (!add_buf()) {
>                 // Screw latency, free them all.
>                 free_old_buffers(UINT_MAX)
>                 // OK, this can happen if we are using direct buffers,
>                 // and the xmit interrupt woke us but the packets
>                 // xmitted were smaller than this one.  Rare though.
>                 if (!add_buf())
>                         Whinge and stop queue, maybe loop.
>         }
> 
>         if (capacity < 2 + MAX_SKB_FRAGS) {
>                 // We don't have enough for the next packet?  Try
>                 // freeing more.
>                 free_old_buffers(UINT_MAX);
>                 if (capacity < 2 + MAX_SKB_FRAGS) {
>                         Stop queue, maybe loop.
>         }
> 
> The current code makes my head hurt :(
> 
> Thoughts?
> Rusty.

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.

Patch

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index a0ee78d..6045510 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -509,17 +509,27 @@  again:
 	return received;
 }
 
-static void free_old_xmit_skbs(struct virtnet_info *vi)
+/* 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)
 {
 	struct sk_buff *skb;
 	unsigned int len;
+	bool r;
 
-	while ((skb = virtqueue_get_buf(vi->svq, &len)) != NULL) {
+	while ((r = virtqueue_min_capacity(vi->svq) < MAX_SKB_FRAGS + 2) ||
+	       min_skbs-- > 0) {
+		skb = virtqueue_get_buf(vi->svq, &len);
+		if (unlikely(!skb))
+			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;
 }
 
 static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)
@@ -572,27 +582,24 @@  static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)
 static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct virtnet_info *vi = netdev_priv(dev);
-	int capacity;
+	int ret, n;
 
-	/* Free up any pending old buffers before queueing new ones. */
-	free_old_xmit_skbs(vi);
+	/* 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 transmit */
-	capacity = xmit_skb(vi, skb);
-
-	/* This can happen with OOM and indirect buffers. */
-	if (unlikely(capacity < 0)) {
-		if (net_ratelimit()) {
-			if (likely(capacity == -ENOMEM)) {
-				dev_warn(&dev->dev,
-					 "TX queue failure: out of memory\n");
-			} else {
-				dev->stats.tx_fifo_errors++;
-				dev_warn(&dev->dev,
-					 "Unexpected TX queue failure: %d\n",
-					 capacity);
-			}
-		}
+	ret = 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 (net_ratelimit())
+			dev_info(&dev->dev, "TX queue failure: %d\n", ret);
 		dev->stats.tx_dropped++;
 		kfree_skb(skb);
 		return NETDEV_TX_OK;
@@ -603,15 +610,16 @@  static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 	skb_orphan(skb);
 	nf_reset(skb);
 
-	/* Apparently nice girls don't return TX_BUSY; stop the queue
-	 * before it gets out of hand.  Naturally, this wastes entries. */
-	if (capacity < 2+MAX_SKB_FRAGS) {
+	/* 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 then recheck. */
-			free_old_xmit_skbs(vi);
-			capacity = virtqueue_min_capacity(vi->svq);
-			if (capacity >= 2+MAX_SKB_FRAGS) {
+			/* 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);
 			}