On Sun, Feb 21, 2016 at 08:40:59PM +0200, Michael S. Tsirkin wrote:
> For small packets data copy was observed to
> take up about 15% CPU time. Switch to build_skb
> and avoid the copy when using mergeable rx buffers.
>
> As a bonus, medium-size skbs that fit in a page will be
> completely linear.
>
> Of course, we now need to lower the lower bound on packet size,
> to make sure a sane number of skbs fits in rx socket buffer.
> By how much? I don't know yet.
>
> It might also be useful to prefetch the packet buffer since
> net stack will likely use it soon.
>
> Lightly tested, in particular, I didn't yet test what this
> actually does to performance - sending this out for early
> feedback/flames.
>
> TODO: it appears that Linux won't handle correctly the case of first
> buffer being very small (or consisting exclusively of virtio header).
> This is already the case for current code, so don't bother
> testing yet.
>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Oops. This should have been "RFC PATCH".
Please don't merge it yet, it's just a preview.
> ---
> drivers/net/virtio_net.c | 44 +++++++++++++++++++++++++++++++++++---------
> 1 file changed, 35 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 767ab11..20f8dda 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -347,6 +347,26 @@ err:
> return NULL;
> }
>
> +#define VNET_SKB_PAD (NET_SKB_PAD + NET_IP_ALIGN)
> +#define VNET_SKB_BUG (VNET_SKB_PAD < sizeof(struct virtio_net_hdr_mrg_rxbuf))
> +#define VNET_SKB_LEN(len) ((len) - sizeof(struct virtio_net_hdr_mrg_rxbuf))
> +#define VNET_SKB_OFF VNET_SKB_LEN(VNET_SKB_PAD)
> +
> +static struct sk_buff *vnet_build_skb(struct virtnet_info *vi,
> + void *buf,
> + unsigned int len, unsigned int truesize)
> +{
> + struct sk_buff *skb = build_skb(buf, truesize);
> +
> + if (!skb)
> + return NULL;
> +
> + skb_reserve(skb, VNET_SKB_PAD);
> + skb_put(skb, VNET_SKB_LEN(len));
> +
> + return skb;
> +}
> +
> static struct sk_buff *receive_mergeable(struct net_device *dev,
> struct virtnet_info *vi,
> struct receive_queue *rq,
> @@ -354,14 +374,13 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> unsigned int len)
> {
> void *buf = mergeable_ctx_to_buf_address(ctx);
> - struct virtio_net_hdr_mrg_rxbuf *hdr = buf;
> + struct virtio_net_hdr_mrg_rxbuf *hdr = buf + VNET_SKB_OFF;
> u16 num_buf = virtio16_to_cpu(vi->vdev, hdr->num_buffers);
> struct page *page = virt_to_head_page(buf);
> - int offset = buf - page_address(page);
> unsigned int truesize = max(len, mergeable_ctx_to_buf_truesize(ctx));
> + int offset;
>
> - struct sk_buff *head_skb = page_to_skb(vi, rq, page, offset, len,
> - truesize);
> + struct sk_buff *head_skb = vnet_build_skb(vi, buf, len, truesize);
> struct sk_buff *curr_skb = head_skb;
>
> if (unlikely(!curr_skb))
> @@ -606,11 +625,14 @@ static int add_recvbuf_big(struct virtnet_info *vi, struct receive_queue *rq,
>
> static unsigned int get_mergeable_buf_len(struct ewma_pkt_len *avg_pkt_len)
> {
> - const size_t hdr_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
> + unsigned int hdr;
> unsigned int len;
>
> - len = hdr_len + clamp_t(unsigned int, ewma_pkt_len_read(avg_pkt_len),
> - GOOD_PACKET_LEN, PAGE_SIZE - hdr_len);
> + hdr = ALIGN(VNET_SKB_PAD + sizeof(struct skb_shared_info),
> + MERGEABLE_BUFFER_ALIGN);
> +
> + len = hdr + clamp_t(unsigned int, ewma_pkt_len_read(avg_pkt_len),
> + 500 /* TODO */, PAGE_SIZE - hdr);
> return ALIGN(len, MERGEABLE_BUFFER_ALIGN);
> }
>
> @@ -626,7 +648,10 @@ static int add_recvbuf_mergeable(struct receive_queue *rq, gfp_t gfp)
> if (unlikely(!skb_page_frag_refill(len, alloc_frag, gfp)))
> return -ENOMEM;
>
> - buf = (char *)page_address(alloc_frag->page) + alloc_frag->offset;
> + BUILD_BUG_ON(VNET_SKB_BUG);
> +
> + buf = (char *)page_address(alloc_frag->page) + alloc_frag->offset +
> + VNET_SKB_OFF;
> ctx = mergeable_buf_to_ctx(buf, len);
> get_page(alloc_frag->page);
> alloc_frag->offset += len;
> @@ -641,7 +666,8 @@ static int add_recvbuf_mergeable(struct receive_queue *rq, gfp_t gfp)
> alloc_frag->offset += hole;
> }
>
> - sg_init_one(rq->sg, buf, len);
> + sg_init_one(rq->sg, buf,
> + len - VNET_SKB_OFF - sizeof(struct skb_shared_info));
> err = virtqueue_add_inbuf(rq->vq, rq->sg, 1, (void *)ctx, gfp);
> if (err < 0)
> put_page(virt_to_head_page(buf));
> --
> MST
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