Re: [PATCH net-next 1/4] virtio-net: mergeable buffer size should include virtio-net header

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Nov 12, 2013 at 02:21:22PM -0800, Michael Dalton wrote:
> Commit 2613af0ed18a ("virtio_net: migrate mergeable rx buffers to page
> frag allocators") changed the mergeable receive buffer size from PAGE_SIZE
> to MTU-size. However, the merge buffer size does not take into account the
> size of the virtio-net header. Consequently, packets that are MTU-size
> will take two buffers intead of one (to store the virtio-net header),
> substantially decreasing the throughput of MTU-size traffic due to TCP
> window / SKB truesize effects.
> 
> This commit changes the mergeable buffer size to include the virtio-net
> header. The buffer size is cacheline-aligned because skb_page_frag_refill
> will not automatically align the requested size.
> 
> Benchmarks taken from an average of 5 netperf 30-second TCP_STREAM runs
> between two QEMU VMs on a single physical machine. Each VM has two VCPUs and
> vhost enabled. All VMs and vhost threads run in a single 4 CPU cgroup
> cpuset, using cgroups to ensure that other processes in the system will not
> be scheduled on the benchmark CPUs. Transmit offloads and mergeable receive
> buffers are enabled, but guest_tso4 / guest_csum are explicitly disabled to
> force MTU-sized packets on the receiver.
> 
> next-net trunk before 2613af0ed18a (PAGE_SIZE buf): 3861.08Gb/s
> net-next trunk (MTU 1500- packet uses two buf due to size bug): 4076.62Gb/s
> net-next trunk (MTU 1480- packet fits in one buf): 6301.34Gb/s
> net-next trunk w/ size fix (MTU 1500 - packet fits in one buf): 6445.44Gb/s
> 
> Suggested-by: Eric Northup <digitaleric@google.com>
> Signed-off-by: Michael Dalton <mwdalton@google.com>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  drivers/net/virtio_net.c | 30 ++++++++++++++++--------------
>  1 file changed, 16 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 01f4eb5..69fb225 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -36,7 +36,10 @@ module_param(csum, bool, 0444);
>  module_param(gso, bool, 0444);
>  
>  /* FIXME: MTU in config. */
> -#define MAX_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
> +#define GOOD_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
> +#define MERGE_BUFFER_LEN (ALIGN(GOOD_PACKET_LEN + \
> +                                sizeof(struct virtio_net_hdr_mrg_rxbuf), \
> +                                L1_CACHE_BYTES))
>  #define GOOD_COPY_LEN	128
>  
>  #define VIRTNET_DRIVER_VERSION "1.0.0"
> @@ -314,10 +317,10 @@ static int receive_mergeable(struct receive_queue *rq, struct sk_buff *head_skb)
>  			head_skb->dev->stats.rx_length_errors++;
>  			return -EINVAL;
>  		}
> -		if (unlikely(len > MAX_PACKET_LEN)) {
> +		if (unlikely(len > MERGE_BUFFER_LEN)) {
>  			pr_debug("%s: rx error: merge buffer too long\n",
>  				 head_skb->dev->name);
> -			len = MAX_PACKET_LEN;
> +			len = MERGE_BUFFER_LEN;
>  		}
>  		if (unlikely(num_skb_frags == MAX_SKB_FRAGS)) {
>  			struct sk_buff *nskb = alloc_skb(0, GFP_ATOMIC);
> @@ -336,18 +339,17 @@ static int receive_mergeable(struct receive_queue *rq, struct sk_buff *head_skb)
>  		if (curr_skb != head_skb) {
>  			head_skb->data_len += len;
>  			head_skb->len += len;
> -			head_skb->truesize += MAX_PACKET_LEN;
> +			head_skb->truesize += MERGE_BUFFER_LEN;
>  		}
>  		page = virt_to_head_page(buf);
>  		offset = buf - (char *)page_address(page);
>  		if (skb_can_coalesce(curr_skb, num_skb_frags, page, offset)) {
>  			put_page(page);
>  			skb_coalesce_rx_frag(curr_skb, num_skb_frags - 1,
> -					     len, MAX_PACKET_LEN);
> +					     len, MERGE_BUFFER_LEN);
>  		} else {
>  			skb_add_rx_frag(curr_skb, num_skb_frags, page,
> -					offset, len,
> -					MAX_PACKET_LEN);
> +					offset, len, MERGE_BUFFER_LEN);
>  		}
>  		--rq->num;
>  	}
> @@ -383,7 +385,7 @@ static void receive_buf(struct receive_queue *rq, void *buf, unsigned int len)
>  		struct page *page = virt_to_head_page(buf);
>  		skb = page_to_skb(rq, page,
>  				  (char *)buf - (char *)page_address(page),
> -				  len, MAX_PACKET_LEN);
> +				  len, MERGE_BUFFER_LEN);
>  		if (unlikely(!skb)) {
>  			dev->stats.rx_dropped++;
>  			put_page(page);
> @@ -471,11 +473,11 @@ static int add_recvbuf_small(struct receive_queue *rq, gfp_t gfp)
>  	struct skb_vnet_hdr *hdr;
>  	int err;
>  
> -	skb = __netdev_alloc_skb_ip_align(vi->dev, MAX_PACKET_LEN, gfp);
> +	skb = __netdev_alloc_skb_ip_align(vi->dev, GOOD_PACKET_LEN, gfp);
>  	if (unlikely(!skb))
>  		return -ENOMEM;
>  
> -	skb_put(skb, MAX_PACKET_LEN);
> +	skb_put(skb, GOOD_PACKET_LEN);
>  
>  	hdr = skb_vnet_hdr(skb);
>  	sg_set_buf(rq->sg, &hdr->hdr, sizeof hdr->hdr);
> @@ -542,20 +544,20 @@ static int add_recvbuf_mergeable(struct receive_queue *rq, gfp_t gfp)
>  	int err;
>  
>  	if (gfp & __GFP_WAIT) {
> -		if (skb_page_frag_refill(MAX_PACKET_LEN, &vi->alloc_frag,
> +		if (skb_page_frag_refill(MERGE_BUFFER_LEN, &vi->alloc_frag,
>  					 gfp)) {
>  			buf = (char *)page_address(vi->alloc_frag.page) +
>  			      vi->alloc_frag.offset;
>  			get_page(vi->alloc_frag.page);
> -			vi->alloc_frag.offset += MAX_PACKET_LEN;
> +			vi->alloc_frag.offset += MERGE_BUFFER_LEN;
>  		}
>  	} else {
> -		buf = netdev_alloc_frag(MAX_PACKET_LEN);
> +		buf = netdev_alloc_frag(MERGE_BUFFER_LEN);
>  	}
>  	if (!buf)
>  		return -ENOMEM;
>  
> -	sg_init_one(rq->sg, buf, MAX_PACKET_LEN);
> +	sg_init_one(rq->sg, buf, MERGE_BUFFER_LEN);
>  	err = virtqueue_add_inbuf(rq->vq, rq->sg, 1, buf, gfp);
>  	if (err < 0)
>  		put_page(virt_to_head_page(buf));
> -- 
> 1.8.4.1