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Date: Wed, 16 Dec 2009 23:15:38 +1030
From: Rusty Russell <rusty@...tcorp.com.au>
To: Herbert Xu <herbert@...dor.apana.org.au>
Cc: Sridhar Samudrala <sri@...ibm.com>, mst@...hat.com,
netdev@...r.kernel.org
Subject: Re: [RFC PATCH] Regression in linux 2.6.32 virtio_net seen with vhost-net
On Wed, 16 Dec 2009 01:23:31 pm Herbert Xu wrote:
> On Wed, Dec 16, 2009 at 01:11:40PM +1030, Rusty Russell wrote:
> >
> > Thanks for the hint. They seem to use NAPI for xmit cleanup, so that's
> > what we should do? I'll try, but such a rewrite doesn't belong in 2.6.32.
>
> Well it depends. Real drivers can't touch the hardware so they're
> stuck with whatever the hardware does. For virtio we do have the
> flexibility of modifying the backend.
>
> Having said that, for existing backends that will signal when there
> is just a single free entry on the queue something like NAPI could
> reduce the overhead associated with the IRQs.
OK, this is unfortunately untested, but wanted to send it out tonight:
virtio_net: use NAPI for xmit (UNTESTED)
This is closer to the way tg3 and ixgbe do it: use the NAPI framework to
free transmitted packets. It neatens things a little as well.
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
@@ -47,6 +47,9 @@ struct virtnet_info
struct napi_struct napi;
unsigned int status;
+ /* We free packets and decide whether to restart xmit here. */
+ struct napi_struct xmit_napi;
+
/* Number of input buffers, and max we've ever had. */
unsigned int num, max;
@@ -60,6 +63,9 @@ struct virtnet_info
struct sk_buff_head recv;
struct sk_buff_head send;
+ /* Capacity left in xmit queue. */
+ unsigned int capacity;
+
/* Work struct for refilling if we run low on memory. */
struct delayed_work refill;
@@ -111,11 +117,8 @@ static void skb_xmit_done(struct virtque
{
struct virtnet_info *vi = svq->vdev->priv;
- /* Suppress further interrupts. */
- svq->vq_ops->disable_cb(svq);
-
/* We were probably waiting for more output buffers. */
- netif_wake_queue(vi->dev);
+ napi_schedule(&vi->xmit_napi);
}
static void receive_skb(struct net_device *dev, struct sk_buff *skb,
@@ -446,7 +449,7 @@ static unsigned int free_old_xmit_skbs(s
while ((skb = vi->svq->vq_ops->get_buf(vi->svq, &len)) != NULL) {
pr_debug("Sent skb %p\n", skb);
- __skb_unlink(skb, &vi->send);
+ skb_unlink(skb, &vi->send);
vi->dev->stats.tx_bytes += skb->len;
vi->dev->stats.tx_packets++;
tot_sgs += skb_vnet_hdr(skb)->num_sg;
@@ -455,6 +458,23 @@ static unsigned int free_old_xmit_skbs(s
return tot_sgs;
}
+static int virtnet_xmit_poll(struct napi_struct *xmit_napi, int budget)
+{
+ struct virtnet_info *vi =
+ container_of(xmit_napi, struct virtnet_info, xmit_napi);
+
+ if (netif_queue_stopped(vi->dev)) {
+ vi->capacity += free_old_xmit_skbs(vi);
+ if (vi->capacity >= 2 + MAX_SKB_FRAGS) {
+ /* Suppress further xmit interrupts. */
+ vi->svq->vq_ops->disable_cb(vi->svq);
+ napi_complete(xmit_napi);
+ netif_wake_queue(vi->dev);
+ }
+ }
+ return 1;
+}
+
static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)
{
struct scatterlist sg[2+MAX_SKB_FRAGS];
@@ -509,34 +529,22 @@ static netdev_tx_t start_xmit(struct sk_
struct virtnet_info *vi = netdev_priv(dev);
int capacity;
-again:
- /* Free up any pending old buffers before queueing new ones. */
- free_old_xmit_skbs(vi);
-
/* Try to transmit */
+ skb_queue_head(&vi->send, skb);
capacity = xmit_skb(vi, skb);
/* This can happen with OOM and indirect buffers. */
if (unlikely(capacity < 0)) {
+ skb_unlink(skb, &vi->send);
netif_stop_queue(dev);
dev_warn(&dev->dev, "Unexpected full queue\n");
- if (unlikely(!vi->svq->vq_ops->enable_cb(vi->svq))) {
- vi->svq->vq_ops->disable_cb(vi->svq);
- netif_start_queue(dev);
- goto again;
- }
+ /* If we missed an interrupt, we let virtnet_xmit_poll deal. */
+ if (unlikely(!vi->svq->vq_ops->enable_cb(vi->svq)))
+ napi_schedule(&vi->xmit_napi);
return NETDEV_TX_BUSY;
}
vi->svq->vq_ops->kick(vi->svq);
-
- /*
- * Put new one in send queue. You'd expect we'd need this before
- * xmit_skb calls add_buf(), since the callback can be triggered
- * immediately after that. But since the callback just triggers
- * another call back here, normal network xmit locking prevents the
- * race.
- */
- __skb_queue_head(&vi->send, skb);
+ vi->capacity = capacity;
/* Don't wait up for transmitted skbs to be freed. */
skb_orphan(skb);
@@ -544,15 +552,16 @@ again:
/* 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) {
- netif_stop_queue(dev);
- if (unlikely(!vi->svq->vq_ops->enable_cb(vi->svq))) {
- /* More just got used, free them then recheck. */
- capacity += free_old_xmit_skbs(vi);
- if (capacity >= 2+MAX_SKB_FRAGS) {
- netif_start_queue(dev);
- vi->svq->vq_ops->disable_cb(vi->svq);
- }
+ if (unlikely(capacity < 2+MAX_SKB_FRAGS)) {
+ /* Free old skbs; might make more capacity. */
+ vi->capacity = capacity + free_old_xmit_skbs(vi);
+ if (unlikely(vi->capacity < 2+MAX_SKB_FRAGS)) {
+ /* Make sure virtnet_xmit_poll sees updated capacity */
+ wmb();
+ netif_stop_queue(dev);
+ /* Missed xmit irq? virtnet_xmit_poll will deal. */
+ if (unlikely(!vi->svq->vq_ops->enable_cb(vi->svq)))
+ napi_schedule(&vi->xmit_napi);
}
}
@@ -590,6 +599,7 @@ static int virtnet_open(struct net_devic
struct virtnet_info *vi = netdev_priv(dev);
napi_enable(&vi->napi);
+ napi_enable(&vi->xmit_napi);
/* If all buffers were filled by other side before we napi_enabled, we
* won't get another interrupt, so process any outstanding packets
@@ -652,6 +662,7 @@ static int virtnet_close(struct net_devi
struct virtnet_info *vi = netdev_priv(dev);
napi_disable(&vi->napi);
+ napi_disable(&vi->xmit_napi);
return 0;
}
@@ -883,6 +894,7 @@ static int virtnet_probe(struct virtio_d
/* Set up our device-specific information */
vi = netdev_priv(dev);
netif_napi_add(dev, &vi->napi, virtnet_poll, napi_weight);
+ netif_napi_add(dev, &vi->xmit_napi, virtnet_xmit_poll, 64);
vi->dev = dev;
vi->vdev = vdev;
vdev->priv = vi;
--
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