Subject: Re: [PATCH v2 2/2] virtio_net: Defer skb allocation in receive path

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 13 Jan 2010 22:57:50 +0200

On Wed, Jan 13, 2010 at 12:53:38PM -0800, Shirley Ma wrote:
> virtio_net receives packets from its pre-allocated vring buffers, then it 
> delivers these packets to upper layer protocols as skb buffs. So it's not
> necessary to pre-allocate skb for each mergable buffer, then frees extra 
> skbs when buffers are merged into a large packet. This patch has deferred 
> skb allocation in receiving packets for both big packets and mergeable buffers
> to reduce skb pre-allocations and skb frees. It frees unused buffers by calling 
> detach_unused_buf in vring, so recv skb queue is not needed.
> 
> Signed-off-by: Shirley Ma <[email-masked]>
> ---
>  drivers/net/virtio_net.c |  427 +++++++++++++++++++++++++++-------------------
>  1 files changed, 248 insertions(+), 179 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index c708ecc..72b3f21 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -56,8 +56,7 @@ struct virtnet_info
>  	/* Host will merge rx buffers for big packets (shake it! shake it!) */
>  	bool mergeable_rx_bufs;
>  
> -	/* Receive & send queues. */
> -	struct sk_buff_head recv;
> +	/* Send queue. */
>  	struct sk_buff_head send;
>

How hard is it to get rid of send queue, btw?
  
>  	/* Work struct for refilling if we run low on memory. */
> @@ -75,34 +74,44 @@ struct skb_vnet_hdr {
>  	unsigned int num_sg;
>  };
>  
> +struct padded_vnet_hdr {
> +	struct virtio_net_hdr hdr;
> +	/*
> +	 * virtio_net_hdr should be in a separated sg buffer because of a
> +	 * QEMU bug, and data sg buffer shares same page with this header sg.
> +	 * This padding makes next sg 16 byte aligned after virtio_net_hdr.
> +	 */
> +	char padding[6];
> +};
> +
>  static inline struct skb_vnet_hdr *skb_vnet_hdr(struct sk_buff *skb)
>  {
>  	return (struct skb_vnet_hdr *)skb->cb;
>  }
>  
> -static void give_a_page(struct virtnet_info *vi, struct page *page)
> -{
> -	page->private = (unsigned long)vi->pages;
> -	vi->pages = page;
> -}
> -
> -static void trim_pages(struct virtnet_info *vi, struct sk_buff *skb)
> +/*
> + * private is used to chain pages for big packets, put the whole
> + * most recent used list in the beginning for reuse
> + */
> +static void give_pages(struct virtnet_info *vi, struct page *page)
>  {
> -	unsigned int i;
> +	struct page *end;
>  
> -	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
> -		give_a_page(vi, skb_shinfo(skb)->frags[i].page);
> -	skb_shinfo(skb)->nr_frags = 0;
> -	skb->data_len = 0;
> +	/* Find end of list, sew whole thing into vi->pages. */
> +	for (end = page; end->private; end = (struct page *)end->private);
> +	end->private = (unsigned long)vi->pages;
> +	vi->pages = page;
>  }
>  
>  static struct page *get_a_page(struct virtnet_info *vi, gfp_t gfp_mask)
>  {
>  	struct page *p = vi->pages;
>  
> -	if (p)
> +	if (p) {
>  		vi->pages = (struct page *)p->private;
> -	else
> +		/* clear private here, it is used to chain pages */
> +		p->private = 0;
> +	} else
>  		p = alloc_page(gfp_mask);
>  	return p;
>  }
> @@ -118,99 +127,142 @@ static void skb_xmit_done(struct virtqueue *svq)
>  	netif_wake_queue(vi->dev);
>  }
>  
> -static void receive_skb(struct net_device *dev, struct sk_buff *skb,
> -			unsigned len)
> +static void set_skb_frag(struct sk_buff *skb, struct page *page,
> +			 unsigned int offset, unsigned int *len)
>  {
> -	struct virtnet_info *vi = netdev_priv(dev);
> -	struct skb_vnet_hdr *hdr = skb_vnet_hdr(skb);
> -	int err;
> -	int i;
> -
> -	if (unlikely(len < sizeof(struct virtio_net_hdr) + ETH_HLEN)) {
> -		pr_debug("%s: short packet %i\n", dev->name, len);
> -		dev->stats.rx_length_errors++;
> -		goto drop;
> -	}
> +	int i = skb_shinfo(skb)->nr_frags;
> +	skb_frag_t *f;
> +
> +	f = &skb_shinfo(skb)->frags[i];
> +	f->size = min((unsigned)PAGE_SIZE - offset, *len);
> +	f->page_offset = offset;
> +	f->page = page;
> +
> +	skb->data_len += f->size;
> +	skb->len += f->size;
> +	skb_shinfo(skb)->nr_frags++;
> +	*len -= f->size;
> +}
>  
> -	if (vi->mergeable_rx_bufs) {
> -		unsigned int copy;
> -		char *p = page_address(skb_shinfo(skb)->frags[0].page);
> +static struct sk_buff *page_to_skb(struct virtnet_info *vi,
> +				   struct page *page, unsigned int len)
> +{
> +	struct sk_buff *skb;
> +	struct skb_vnet_hdr *hdr;
> +	unsigned int copy, hdr_len, offset;
> +	char *p;
>  
> -		if (len > PAGE_SIZE)
> -			len = PAGE_SIZE;
> -		len -= sizeof(struct virtio_net_hdr_mrg_rxbuf);
> +	p = page_address(page);
>  
> -		memcpy(&hdr->mhdr, p, sizeof(hdr->mhdr));
> -		p += sizeof(hdr->mhdr);
> +	/* copy small packet so we can reuse these pages for small data */
> +	skb = netdev_alloc_skb_ip_align(vi->dev, GOOD_COPY_LEN);
> +	if (unlikely(!skb))
> +		return NULL;
>  
> -		copy = len;
> -		if (copy > skb_tailroom(skb))
> -			copy = skb_tailroom(skb);
> +	hdr = skb_vnet_hdr(skb);
>  
> -		memcpy(skb_put(skb, copy), p, copy);
> +	if (vi->mergeable_rx_bufs) {
> +		hdr_len = sizeof hdr->mhdr;
> +		offset = hdr_len;
> +	} else {
> +		hdr_len = sizeof hdr->hdr;
> +		offset = sizeof(struct padded_vnet_hdr);
> +	}
>  
> -		len -= copy;
> +	memcpy(hdr, p, hdr_len);
>  
> -		if (!len) {
> -			give_a_page(vi, skb_shinfo(skb)->frags[0].page);
> -			skb_shinfo(skb)->nr_frags--;
> -		} else {
> -			skb_shinfo(skb)->frags[0].page_offset +=
> -				sizeof(hdr->mhdr) + copy;
> -			skb_shinfo(skb)->frags[0].size = len;
> -			skb->data_len += len;
> -			skb->len += len;
> -		}
> +	len -= hdr_len;
> +	p += offset;
>  
> -		while (--hdr->mhdr.num_buffers) {
> -			struct sk_buff *nskb;
> +	copy = len;
> +	if (copy > skb_tailroom(skb))
> +		copy = skb_tailroom(skb);
> +	memcpy(skb_put(skb, copy), p, copy);
>  
> -			i = skb_shinfo(skb)->nr_frags;
> -			if (i >= MAX_SKB_FRAGS) {
> -				pr_debug("%s: packet too long %d\n", dev->name,
> -					 len);
> -				dev->stats.rx_length_errors++;
> -				goto drop;
> -			}
> +	len -= copy;
> +	offset += copy;
>  
> -			nskb = vi->rvq->vq_ops->get_buf(vi->rvq, &len);
> -			if (!nskb) {
> -				pr_debug("%s: rx error: %d buffers missing\n",
> -					 dev->name, hdr->mhdr.num_buffers);
> -				dev->stats.rx_length_errors++;
> -				goto drop;
> -			}
> +	while (len) {
> +		set_skb_frag(skb, page, offset, &len);
> +		page = (struct page *)page->private;
> +		offset = 0;
> +	}
>  
> -			__skb_unlink(nskb, &vi->recv);
> -			vi->num--;
> +	if (page)
> +		give_pages(vi, page);
>  
> -			skb_shinfo(skb)->frags[i] = skb_shinfo(nskb)->frags[0];
> -			skb_shinfo(nskb)->nr_frags = 0;
> -			kfree_skb(nskb);
> +	return skb;
> +}
>  
> -			if (len > PAGE_SIZE)
> -				len = PAGE_SIZE;
> +static int receive_mergeable(struct virtnet_info *vi, struct sk_buff *skb)
> +{
> +	struct skb_vnet_hdr *hdr = skb_vnet_hdr(skb);
> +	struct page *page;
> +	int num_buf, i, len;
> +
> +	num_buf = hdr->mhdr.num_buffers;
> +	while (--num_buf) {
> +		i = skb_shinfo(skb)->nr_frags;
> +		if (i >= MAX_SKB_FRAGS) {
> +			pr_debug("%s: packet too long\n", skb->dev->name);
> +			skb->dev->stats.rx_length_errors++;
> +			return -EINVAL;
> +		}
>  
> -			skb_shinfo(skb)->frags[i].size = len;
> -			skb_shinfo(skb)->nr_frags++;
> -			skb->data_len += len;
> -			skb->len += len;
> +		page = vi->rvq->vq_ops->get_buf(vi->rvq, &len);
> +		if (!page) {
> +			pr_debug("%s: rx error: %d buffers missing\n",
> +				 skb->dev->name, hdr->mhdr.num_buffers);
> +			skb->dev->stats.rx_length_errors++;
> +			return -EINVAL;
>  		}
> -	} else {
> -		len -= sizeof(hdr->hdr);
> +		if (len > PAGE_SIZE)
> +			len = PAGE_SIZE;
> +
> +		set_skb_frag(skb, page, 0, &len);
> +
> +		--vi->num;
> +	}
> +	return 0;
> +}
> +
> +static void receive_buf(struct net_device *dev, void *buf, unsigned int len)
> +{
> +	struct virtnet_info *vi = netdev_priv(dev);
> +	struct sk_buff *skb;
> +	struct page *page;
> +	struct skb_vnet_hdr *hdr;
>  
> -		if (len <= MAX_PACKET_LEN)
> -			trim_pages(vi, skb);
> +	if (unlikely(len < sizeof(struct virtio_net_hdr) + ETH_HLEN)) {
> +		pr_debug("%s: short packet %i\n", dev->name, len);
> +		dev->stats.rx_length_errors++;
> +		if (vi->mergeable_rx_bufs || vi->big_packets)
> +			give_pages(vi, buf);
> +		else
> +			dev_kfree_skb(buf);
> +		return;
> +	}
>  
> -		err = pskb_trim(skb, len);
> -		if (err) {
> -			pr_debug("%s: pskb_trim failed %i %d\n", dev->name,
> -				 len, err);
> +	if (!vi->mergeable_rx_bufs && !vi->big_packets) {
> +		skb = buf;
> +		len -= sizeof(struct virtio_net_hdr);
> +		skb_trim(skb, len);
> +	} else {
> +		page = buf;
> +		skb = page_to_skb(vi, page, len);
> +		if (unlikely(!skb)) {
>  			dev->stats.rx_dropped++;
> -			goto drop;
> +			give_pages(vi, page);
> +			return;
>  		}
> +		if (vi->mergeable_rx_bufs)
> +			if (receive_mergeable(vi, skb)) {
> +				dev_kfree_skb(skb);
> +				return;
> +			}
>  	}
>  
> +	hdr = skb_vnet_hdr(skb);
>  	skb->truesize += skb->data_len;
>  	dev->stats.rx_bytes += skb->len;
>  	dev->stats.rx_packets++;
> @@ -267,110 +319,119 @@ static void receive_skb(struct net_device *dev, struct sk_buff *skb,
>  
>  frame_err:
>  	dev->stats.rx_frame_errors++;
> -drop:
>  	dev_kfree_skb(skb);
>  }
>  
> -static bool try_fill_recv_maxbufs(struct virtnet_info *vi, gfp_t gfp)
> +static int add_recvbuf_small(struct virtnet_info *vi, gfp_t gfp)
>  {
>  	struct sk_buff *skb;
> -	struct scatterlist sg[2+MAX_SKB_FRAGS];
> -	int num, err, i;
> -	bool oom = false;
> -
> -	sg_init_table(sg, 2+MAX_SKB_FRAGS);
> -	do {
> -		struct skb_vnet_hdr *hdr;
> +	struct skb_vnet_hdr *hdr;
> +	struct scatterlist sg[2];
> +	int err;
>  
> -		skb = netdev_alloc_skb_ip_align(vi->dev, MAX_PACKET_LEN);
> -		if (unlikely(!skb)) {
> -			oom = true;
> -			break;
> -		}
> +	skb = netdev_alloc_skb_ip_align(vi->dev, MAX_PACKET_LEN);
> +	if (unlikely(!skb))
> +		return -ENOMEM;
>  
> -		skb_put(skb, MAX_PACKET_LEN);
> +	skb_put(skb, MAX_PACKET_LEN);
>  
> -		hdr = skb_vnet_hdr(skb);
> -		sg_set_buf(sg, &hdr->hdr, sizeof(hdr->hdr));
> +	hdr = skb_vnet_hdr(skb);
> +	sg_set_buf(sg, &hdr->hdr, sizeof hdr->hdr);
>  
> -		if (vi->big_packets) {
> -			for (i = 0; i < MAX_SKB_FRAGS; i++) {
> -				skb_frag_t *f = &skb_shinfo(skb)->frags[i];
> -				f->page = get_a_page(vi, gfp);
> -				if (!f->page)
> -					break;
> +	skb_to_sgvec(skb, sg + 1, 0, skb->len);
>  
> -				f->page_offset = 0;
> -				f->size = PAGE_SIZE;
> +	err = vi->rvq->vq_ops->add_buf(vi->rvq, sg, 0, 2, skb);
> +	if (err < 0)
> +		dev_kfree_skb(skb);
>  
> -				skb->data_len += PAGE_SIZE;
> -				skb->len += PAGE_SIZE;
> +	return err;
> +}
>  
> -				skb_shinfo(skb)->nr_frags++;
> -			}
> +static int add_recvbuf_big(struct virtnet_info *vi, gfp_t gfp)
> +{
> +	struct scatterlist sg[MAX_SKB_FRAGS + 2];
> +	struct page *first, *list = NULL;
> +	char *p;
> +	int i, err, offset;
> +
> +	/* page in sg[MAX_SKB_FRAGS + 1] is list tail */
> +	for (i = MAX_SKB_FRAGS + 1; i > 1; --i) {
> +		first = get_a_page(vi, gfp);
> +		if (!first) {
> +			if (list)
> +				give_pages(vi, list);
> +			return -ENOMEM;
>  		}
> +		sg_set_buf(&sg[i], page_address(first), PAGE_SIZE);
>  
> -		num = skb_to_sgvec(skb, sg+1, 0, skb->len) + 1;
> -		skb_queue_head(&vi->recv, skb);
> +		/* chain new page in list head to match sg */
> +		first->private = (unsigned long)list;
> +		list = first;
> +	}
>  
> -		err = vi->rvq->vq_ops->add_buf(vi->rvq, sg, 0, num, skb);
> -		if (err < 0) {
> -			skb_unlink(skb, &vi->recv);
> -			trim_pages(vi, skb);
> -			kfree_skb(skb);
> -			break;
> -		}
> -		vi->num++;
> -	} while (err >= num);
> -	if (unlikely(vi->num > vi->max))
> -		vi->max = vi->num;
> -	vi->rvq->vq_ops->kick(vi->rvq);
> -	return !oom;
> +	first = get_a_page(vi, gfp);
> +	if (!first) {
> +		give_pages(vi, list);
> +		return -ENOMEM;
> +	}
> +	p = page_address(first);
> +
> +	/* sg[0], sg[1] share the same page */
> +	/* a separated sg[0] for  virtio_net_hdr only during to QEMU bug*/
> +	sg_set_buf(&sg[0], p, sizeof(struct virtio_net_hdr));
> +
> +	/* sg[1] for data packet, from offset */
> +	offset = sizeof(struct padded_vnet_hdr);
> +	sg_set_buf(&sg[1], p + offset, PAGE_SIZE - offset);
> +
> +	/* chain first in list head */
> +	first->private = (unsigned long)list;
> +	err = vi->rvq->vq_ops->add_buf(vi->rvq, sg, 0, MAX_SKB_FRAGS + 2,
> +				       first);
> +	if (err < 0)
> +		give_pages(vi, first);
> +
> +	return err;
>  }
>  
> -/* Returns false if we couldn't fill entirely (OOM). */
> -static bool try_fill_recv(struct virtnet_info *vi, gfp_t gfp)
> +static int add_recvbuf_mergeable(struct virtnet_info *vi, gfp_t gfp)
>  {
> -	struct sk_buff *skb;
> -	struct scatterlist sg[1];
> +	struct page *page;
> +	struct scatterlist sg;
>  	int err;
> -	bool oom = false;
> -
> -	if (!vi->mergeable_rx_bufs)
> -		return try_fill_recv_maxbufs(vi, gfp);
>  
> -	do {
> -		skb_frag_t *f;
> +	page = get_a_page(vi, gfp);
> +	if (!page)
> +		return -ENOMEM;
>  
> -		skb = netdev_alloc_skb_ip_align(vi->dev, GOOD_COPY_LEN);
> -		if (unlikely(!skb)) {
> -			oom = true;
> -			break;
> -		}
> +	sg_init_one(&sg, page_address(page), PAGE_SIZE);
>  
> -		f = &skb_shinfo(skb)->frags[0];
> -		f->page = get_a_page(vi, gfp);
> -		if (!f->page) {
> -			oom = true;
> -			kfree_skb(skb);
> -			break;
> -		}
> +	err = vi->rvq->vq_ops->add_buf(vi->rvq, &sg, 0, 1, page);
> +	if (err < 0)
> +		give_pages(vi, page);
>  
> -		f->page_offset = 0;
> -		f->size = PAGE_SIZE;
> +	return err;
> +}
>  
> -		skb_shinfo(skb)->nr_frags++;
> +/* Returns false if we couldn't fill entirely (OOM). */
> +static bool try_fill_recv(struct virtnet_info *vi, gfp_t gfp)
> +{
> +	int err;
> +	bool oom = false;
>  
> -		sg_init_one(sg, page_address(f->page), PAGE_SIZE);
> -		skb_queue_head(&vi->recv, skb);
> +	do {
> +		if (vi->mergeable_rx_bufs)
> +			err = add_recvbuf_mergeable(vi, gfp);
> +		else if (vi->big_packets)
> +			err = add_recvbuf_big(vi, gfp);
> +		else
> +			err = add_recvbuf_small(vi, gfp);
>  
> -		err = vi->rvq->vq_ops->add_buf(vi->rvq, sg, 0, 1, skb);
>  		if (err < 0) {
> -			skb_unlink(skb, &vi->recv);
> -			kfree_skb(skb);
> +			oom = true;
>  			break;
>  		}
> -		vi->num++;
> +		++vi->num;
>  	} while (err > 0);
>  	if (unlikely(vi->num > vi->max))
>  		vi->max = vi->num;
> @@ -408,15 +469,14 @@ static void refill_work(struct work_struct *work)
>  static int virtnet_poll(struct napi_struct *napi, int budget)
>  {
>  	struct virtnet_info *vi = container_of(napi, struct virtnet_info, napi);
> -	struct sk_buff *skb = NULL;
> +	void *buf;
>  	unsigned int len, received = 0;
>  
>  again:
>  	while (received < budget &&
> -	       (skb = vi->rvq->vq_ops->get_buf(vi->rvq, &len)) != NULL) {
> -		__skb_unlink(skb, &vi->recv);
> -		receive_skb(vi->dev, skb, len);
> -		vi->num--;
> +	       (buf = vi->rvq->vq_ops->get_buf(vi->rvq, &len)) != NULL) {
> +		receive_buf(vi->dev, buf, len);
> +		--vi->num;
>  		received++;
>  	}
>  
> @@ -496,9 +556,9 @@ static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)
>  
>  	/* Encode metadata header at front. */
>  	if (vi->mergeable_rx_bufs)
> -		sg_set_buf(sg, &hdr->mhdr, sizeof(hdr->mhdr));
> +		sg_set_buf(sg, &hdr->mhdr, sizeof hdr->mhdr);
>  	else
> -		sg_set_buf(sg, &hdr->hdr, sizeof(hdr->hdr));
> +		sg_set_buf(sg, &hdr->hdr, sizeof hdr->hdr);
>  
>  	hdr->num_sg = skb_to_sgvec(skb, sg+1, 0, skb->len) + 1;
>  	return vi->svq->vq_ops->add_buf(vi->svq, sg, hdr->num_sg, 0, skb);
> @@ -916,8 +976,7 @@ static int virtnet_probe(struct virtio_device *vdev)
>  			dev->features |= NETIF_F_HW_VLAN_FILTER;
>  	}
>  
> -	/* Initialize our empty receive and send queues. */
> -	skb_queue_head_init(&vi->recv);
> +	/* Initialize our empty send queue. */
>  	skb_queue_head_init(&vi->send);
>  
>  	err = register_netdev(dev);
> @@ -952,25 +1011,35 @@ free:
>  	return err;
>  }
>  
> +static void free_unused_bufs(struct virtnet_info *vi)
> +{
> +	void *buf;
> +	while (1) {
> +		buf = vi->rvq->vq_ops->detach_unused_buf(vi->rvq);
> +		if (!buf)
> +			break;
> +		if (vi->mergeable_rx_bufs || vi->big_packets)
> +			give_pages(vi, buf);
> +		else
> +			dev_kfree_skb(buf);
> +		--vi->num;
> +	}
> +	BUG_ON(vi->num != 0);
> +}
> +
>  static void __devexit virtnet_remove(struct virtio_device *vdev)
>  {
>  	struct virtnet_info *vi = vdev->priv;
> -	struct sk_buff *skb;
>  
>  	/* Stop all the virtqueues. */
>  	vdev->config->reset(vdev);
>  
> -	/* Free our skbs in send and recv queues, if any. */
> -	while ((skb = __skb_dequeue(&vi->recv)) != NULL) {
> -		kfree_skb(skb);
> -		vi->num--;
> -	}
> +	/* Free our skbs in send queue, if any. */
>  	__skb_queue_purge(&vi->send);
>  
> -	BUG_ON(vi->num != 0);
> -
>  	unregister_netdev(vi->dev);
>  	cancel_delayed_work_sync(&vi->refill);
> +	free_unused_bufs(vi);
>  
>  	vdev->config->del_vqs(vi->vdev);
>  
>