Subject: Re: [RFC] defer skb allocation in virtio_net -- mergable buff part

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 25 Aug 2009 14:41:43 +0300

Wanted to try this awhile now, good to see this worked on!
Some comments inline.

On Wed, Aug 12, 2009 at 11:33:51PM -0700, Shirley Ma wrote:
> Guest 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 it when it's useless. 
> 
> This patch has deferred skb allocation to when receiving packets, 
> it reduces skb pre-allocations and skb_frees. And it induces two 
> page list: freed_pages and used_page list, used_pages is used to 
> track pages pre-allocated, it is only useful when removing virtio_net.
> 
> This patch has tested and measured against 2.6.31-rc4 git,
> I thought this patch will improve large packet performance, but I saw
> netperf TCP_STREAM performance improved for small packet for both 
> local guest to host and host to local guest cases. It also reduces 
> UDP packets drop rate from host to local guest. I am not fully understand 
> why.
> 
> The netperf results from my laptop are:
> 
> mtu=1500
> netperf -H xxx -l 120
> 
> 		w/o patch	w/i patch (two runs)	
> guest to host:  3336.84Mb/s   3730.14Mb/s ~ 3582.88Mb/s
> 
> host to guest:  3165.10Mb/s   3370.39Mb/s ~ 3407.96Mb/s
> 
> Here is the patch for your review. The same approach can apply to non-mergable
> buffs too, so we can use code in common.

Yes, we should do that. The way to test is to hack virtio to ignore
host support for mergeable buffers.

> If there is no objection, I will 
> submit the non-mergable buffs patch later.
> 

The whole page cache management in virtio net is too complex.
Since it seems you bypass it for some cases, this might be where
savings come from?

> Signed-off-by: Shirley Ma <[email-masked]>
> ---
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 2a6e81d..e31ebc9 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -17,6 +17,7 @@
>   * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
>   */
>  //#define DEBUG
> +#include <linux/list.h>
>  #include <linux/netdevice.h>
>  #include <linux/etherdevice.h>
>  #include <linux/ethtool.h>
> @@ -39,6 +40,12 @@ module_param(gso, bool, 0444);
>  
>  #define VIRTNET_SEND_COMMAND_SG_MAX    2
>  
> +struct page_list
> +{

Kernel style is "struct page_list {".
Also, prefix with virtnet_?

> +	struct page *page;
> +	struct list_head list;
> +};
> +
>  struct virtnet_info
>  {
>  	struct virtio_device *vdev;
> @@ -72,6 +79,8 @@ struct virtnet_info
>  
>  	/* Chain pages by the private ptr. */
>  	struct page *pages;

Do we need the pages list now? Can we do without?

Pls document fields below.

> +	struct list_head used_pages;

Seems a waste to have this list just for dev down.
Extend virtio to give us all buffers from vq
on shutdown?

> +	struct list_head freed_pages;
>  };
>  
>  static inline void *skb_vnet_hdr(struct sk_buff *skb)
> @@ -106,6 +115,26 @@ static struct page *get_a_page(struct virtnet_info *vi, gfp_t gfp_mask)
>  	return p;
>  }
>  
> +static struct page_list *get_a_free_page(struct virtnet_info *vi, gfp_t gfp_mask)

always called with gfp_mask GFP_ATOMIC?
Also - line too long here and elsewere. Run checkpatch?

> +{
> +	struct page_list *plist;
> +
> +	if (list_empty(&vi->freed_pages)) {

special handling for empty list looks a bit ugly
really necessary?

> +		plist = kmalloc(sizeof(struct page_list), gfp_mask);

sizeof *plist

> +		if (!plist)
> +			return NULL;
> +		list_add_tail(&plist->list, &vi->freed_pages);
> +		plist->page = alloc_page(gfp_mask);
> +	} else {
> +		plist = list_first_entry(&vi->freed_pages, struct page_list, list);

is the first entry special? pls document in struct definition

> +		if (!plist->page)
> +			plist->page = alloc_page(gfp_mask);

what does it mean plist->page == NULL? Please document this
in struct definition.

> +	}
> +	if (plist->page)
> +		list_move_tail(&plist->list, &vi->used_pages);
> +	return plist;
> +}
> +
>  static void skb_xmit_done(struct virtqueue *svq)
>  {
>  	struct virtnet_info *vi = svq->vdev->priv;
> @@ -121,14 +150,14 @@ static void skb_xmit_done(struct virtqueue *svq)
>  	tasklet_schedule(&vi->tasklet);
>  }
>  
> -static void receive_skb(struct net_device *dev, struct sk_buff *skb,
> +static void receive_skb(struct net_device *dev, void *buf,

what is buf, here? can it have proper type and not void*?

>  			unsigned len)
>  {
>  	struct virtnet_info *vi = netdev_priv(dev);
> -	struct virtio_net_hdr *hdr = skb_vnet_hdr(skb);
>  	int err;
>  	int i;
> -
> +	struct sk_buff *skb = NULL;
> +	struct virtio_net_hdr *hdr = NULL;

do we have to init these to NULL?

>  	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++;
> @@ -136,15 +165,30 @@ static void receive_skb(struct net_device *dev, struct sk_buff *skb,
>  	}
>  
>  	if (vi->mergeable_rx_bufs) {
> -		struct virtio_net_hdr_mrg_rxbuf *mhdr = skb_vnet_hdr(skb);
> +		struct virtio_net_hdr_mrg_rxbuf *mhdr;
>  		unsigned int copy;
> -		char *p = page_address(skb_shinfo(skb)->frags[0].page);
> +		skb_frag_t *f;
> +		struct page_list *plist = (struct page_list *)buf;

cast not necessary

> +		char *p = page_address(plist->page);
> +
> +		skb = netdev_alloc_skb(vi->dev, GOOD_COPY_LEN + NET_IP_ALIGN);
> +		if (unlikely(!skb)) {
> +			/* drop the packet */

obvious, but okay ...

> +			dev->stats.rx_dropped++;

but what does the below do? Maybe add a comment ...

> +			list_move_tail(&plist->list, &vi->freed_pages);
> +			return;
> +		}
> +
> +		skb_reserve(skb, NET_IP_ALIGN);
>  
>  		if (len > PAGE_SIZE)
>  			len = PAGE_SIZE;
>  		len -= sizeof(struct virtio_net_hdr_mrg_rxbuf);
>  
> -		memcpy(hdr, p, sizeof(*mhdr));
> +		mhdr = skb_vnet_hdr(skb);
> +		memcpy(mhdr, p, sizeof(*mhdr));
> +		hdr = (struct virtio_net_hdr *)mhdr;
> +
>  		p += sizeof(*mhdr);
>  
>  		copy = len;
> @@ -155,20 +199,20 @@ static void receive_skb(struct net_device *dev, struct sk_buff *skb,
>  
>  		len -= copy;
>  
> -		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 +=
> +		if (len) {
> +			f = &skb_shinfo(skb)->frags[0];
> +			f->page = plist->page;
> +			skb_shinfo(skb)->frags[0].page_offset =
>  				sizeof(*mhdr) + copy;
>  			skb_shinfo(skb)->frags[0].size = len;
>  			skb->data_len += len;
>  			skb->len += len;
> +			skb_shinfo(skb)->nr_frags++;
> +			plist->page = NULL;
>  		}
> +		list_move_tail(&plist->list, &vi->freed_pages);
>  
>  		while (--mhdr->num_buffers) {
> -			struct sk_buff *nskb;
> -
>  			i = skb_shinfo(skb)->nr_frags;
>  			if (i >= MAX_SKB_FRAGS) {
>  				pr_debug("%s: packet too long %d\n", dev->name,
> @@ -177,30 +221,30 @@ static void receive_skb(struct net_device *dev, struct sk_buff *skb,
>  				goto drop;
>  			}
>  
> -			nskb = vi->rvq->vq_ops->get_buf(vi->rvq, &len);
> -			if (!nskb) {
> +			plist = vi->rvq->vq_ops->get_buf(vi->rvq, &len);
> +			if (!plist) {
>  				pr_debug("%s: rx error: %d buffers missing\n",
>  					 dev->name, mhdr->num_buffers);
>  				dev->stats.rx_length_errors++;
>  				goto drop;
>  			}
> -
> -			__skb_unlink(nskb, &vi->recv);
>  			vi->num--;
> -
> -			skb_shinfo(skb)->frags[i] = skb_shinfo(nskb)->frags[0];
> -			skb_shinfo(nskb)->nr_frags = 0;
> -			kfree_skb(nskb);
> -
> +			f = &skb_shinfo(skb)->frags[i];
> +			f->page = plist->page;
> +			f->page_offset = 0;

I though skb is pre-zeroed anyway. No?

> +			
>  			if (len > PAGE_SIZE)
>  				len = PAGE_SIZE;
> -

please don't change empty lines arbitrarily

>  			skb_shinfo(skb)->frags[i].size = len;
>  			skb_shinfo(skb)->nr_frags++;
>  			skb->data_len += len;
>  			skb->len += len;
> +			plist->page = NULL;
> +			list_move_tail(&plist->list, &vi->freed_pages);

did we set all fields here? how about truesize?
Does skb_add_rx_frag do what we want by any chance?

>  		}
>  	} else {
> +		skb = (struct sk_buff *)buf;

don't cast away void

> +		hdr = skb_vnet_hdr(skb);
>  		len -= sizeof(struct virtio_net_hdr);
>  
>  		if (len <= MAX_PACKET_LEN)
> @@ -329,7 +373,6 @@ static void try_fill_recv_maxbufs(struct virtnet_info *vi)
>  
>  static void try_fill_recv(struct virtnet_info *vi)
>  {
> -	struct sk_buff *skb;
>  	struct scatterlist sg[1];
>  	int err;
>  
> @@ -339,39 +382,21 @@ static void try_fill_recv(struct virtnet_info *vi)
>  	}
>  
>  	for (;;) {
> -		skb_frag_t *f;
> -
> -		skb = netdev_alloc_skb(vi->dev, GOOD_COPY_LEN + NET_IP_ALIGN);
> -		if (unlikely(!skb))
> -			break;
> -
> -		skb_reserve(skb, NET_IP_ALIGN);
> -
> -		f = &skb_shinfo(skb)->frags[0];
> -		f->page = get_a_page(vi, GFP_ATOMIC);
> -		if (!f->page) {
> -			kfree_skb(skb);
> +		struct page_list *plist = get_a_free_page(vi, GFP_ATOMIC);
> +		if (!plist || !plist->page)
>  			break;
> -		}
> -
> -		f->page_offset = 0;
> -		f->size = PAGE_SIZE;
> -
> -		skb_shinfo(skb)->nr_frags++;
> -
> -		sg_init_one(sg, page_address(f->page), PAGE_SIZE);
> -		skb_queue_head(&vi->recv, skb);
> -
> -		err = vi->rvq->vq_ops->add_buf(vi->rvq, sg, 0, 1, skb);
> +		sg_init_one(sg, page_address(plist->page), PAGE_SIZE);
> +		err = vi->rvq->vq_ops->add_buf(vi->rvq, sg, 0, 1, plist);
>  		if (err) {
> -			skb_unlink(skb, &vi->recv);
> -			kfree_skb(skb);
> +			list_move_tail(&plist->list, &vi->freed_pages);
>  			break;
>  		}
>  		vi->num++;
>  	}
> +
>  	if (unlikely(vi->num > vi->max))
>  		vi->max = vi->num;
> +	

unnecessary empty lines - if you like them, make a
separate patch. and seems to have trailing whitespace.

>  	vi->rvq->vq_ops->kick(vi->rvq);
>  }
>  
> @@ -388,14 +413,15 @@ static void skb_recv_done(struct virtqueue *rvq)
>  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 = NULL;

Does it need to be initialized?

>  	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);
> +	       (buf = vi->rvq->vq_ops->get_buf(vi->rvq, &len)) != NULL) {
> +		if (!vi->mergeable_rx_bufs)
> +			__skb_unlink((struct sk_buff *)buf, &vi->recv);

cast not necessary

> +		receive_skb(vi->dev, buf, len);
>  		vi->num--;
>  		received++;
>  	}
> @@ -893,6 +919,8 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	vi->vdev = vdev;
>  	vdev->priv = vi;
>  	vi->pages = NULL;
> +	INIT_LIST_HEAD(&vi->used_pages);
> +	INIT_LIST_HEAD(&vi->freed_pages);
>  
>  	/* If they give us a callback when all buffers are done, we don't need
>  	 * the timer. */
> @@ -969,6 +997,7 @@ static void virtnet_remove(struct virtio_device *vdev)
>  {
>  	struct virtnet_info *vi = vdev->priv;
>  	struct sk_buff *skb;
> +	struct page_list *plist, *tp;
>  
>  	/* Stop all the virtqueues. */
>  	vdev->config->reset(vdev);
> @@ -977,14 +1006,23 @@ static void virtnet_remove(struct virtio_device *vdev)
>  		del_timer_sync(&vi->xmit_free_timer);
>  
>  	/* Free our skbs in send and recv queues, if any. */
> -	while ((skb = __skb_dequeue(&vi->recv)) != NULL) {
> -		kfree_skb(skb);
> -		vi->num--;
> +	if (!vi->mergeable_rx_bufs) {
> +		while ((skb = __skb_dequeue(&vi->recv)) != NULL) {
> +			kfree_skb(skb);
> +			vi->num--;
> +		}
> +		BUG_ON(vi->num != 0);
> +	} else {
> +		list_splice_init(&vi->used_pages, &vi->freed_pages);
> +		list_for_each_entry_safe(plist, tp, &vi->freed_pages, list) {
> +			vi->num--;
> +			if (plist->page)
> +				__free_pages(plist->page, 0);
> +			kfree(plist);
> +		}

BUG_ON here as well?

>  	}

So, you are fixing this to share code. Good.

>  	__skb_queue_purge(&vi->send);
>  
> -	BUG_ON(vi->num != 0);
> -
>  	unregister_netdev(vi->dev);
>  
>  	vdev->config->del_vqs(vi->vdev);
> 
> 
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