[PATCH net-next] virtio-net: page_to_skb() use build_skb when there's sufficient tailroom

[Xuan Zhuo]

In page_to_skb(), if we have enough tailroom to save skb_shared_info, we
can use build_skb to create skb directly. No need to alloc for
additional space. And it can save a 'frags slot', which is very friendly
to GRO.

Here, if the payload of the received package is too small (less than
GOOD_COPY_LEN), we still choose to copy it directly to the space got by
napi_alloc_skb. So we can reuse these pages.

Testing Machine:
    The four queues of the network card are bound to the cpu1.

Test command:
    for ((i=0;i<5;++i)); do sockperf tp --ip 192.168.122.64 -m 1000 -t 150& done

The size of the udp package is 1000, so in the case of this patch, there
will always be enough tailroom to use build_skb. The sent udp packet
will be discarded because there is no port to receive it. The irqsoftd
of the machine is 100%, we observe the received quantity displayed by
sar -n DEV 1:

no build_skb:  956864.00 rxpck/s
build_skb:    1158465.00 rxpck/s

Signed-off-by: Xuan Zhuo <xuanz...@linux.alibaba.com>
Suggested-by: Jason Wang <jasow...@redhat.com>
---
 drivers/net/virtio_net.c | 51 ++++++++++++++++++++++++++--------------
 1 file changed, 33 insertions(+), 18 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index bb4ea9dbc16b..5071a8a8f57a 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -383,17 +383,11 @@ static struct sk_buff *page_to_skb(struct virtnet_info 
*vi,
 {
        struct sk_buff *skb;
        struct virtio_net_hdr_mrg_rxbuf *hdr;
-       unsigned int copy, hdr_len, hdr_padded_len;
-       char *p;
+       unsigned int copy, hdr_len, hdr_padded_len, tailroom, shinfo_size;
+       char *p, *hdr_p;
 
        p = page_address(page) + offset;
-
-       /* copy small packet so we can reuse these pages for small data */
-       skb = napi_alloc_skb(&rq->napi, GOOD_COPY_LEN);
-       if (unlikely(!skb))
-               return NULL;
-
-       hdr = skb_vnet_hdr(skb);
+       hdr_p = p;
 
        hdr_len = vi->hdr_len;
        if (vi->mergeable_rx_bufs)
@@ -401,24 +395,33 @@ static struct sk_buff *page_to_skb(struct virtnet_info 
*vi,
        else
                hdr_padded_len = sizeof(struct padded_vnet_hdr);
 
-       /* hdr_valid means no XDP, so we can copy the vnet header */
-       if (hdr_valid)
-               memcpy(hdr, p, hdr_len);
+       tailroom = truesize - len;
 
        len -= hdr_len;
        offset += hdr_padded_len;
        p += hdr_padded_len;
 
+       shinfo_size = SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
+
+       if (len > GOOD_COPY_LEN && tailroom >= shinfo_size) {
+               skb = build_skb(p, truesize);
+               if (unlikely(!skb))
+                       return NULL;
+
+               skb_put(skb, len);
+               goto ok;
+       }
+
+       /* copy small packet so we can reuse these pages for small data */
+       skb = napi_alloc_skb(&rq->napi, GOOD_COPY_LEN);
+       if (unlikely(!skb))
+               return NULL;
+
        copy = len;
        if (copy > skb_tailroom(skb))
                copy = skb_tailroom(skb);
        skb_put_data(skb, p, copy);
 
-       if (metasize) {
-               __skb_pull(skb, metasize);
-               skb_metadata_set(skb, metasize);
-       }
-
        len -= copy;
        offset += copy;
 
@@ -427,7 +430,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
                        skb_add_rx_frag(skb, 0, page, offset, len, truesize);
                else
                        put_page(page);
-               return skb;
+               goto ok;
        }
 
        /*
@@ -454,6 +457,18 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
        if (page)
                give_pages(rq, page);
 
+ok:
+       /* hdr_valid means no XDP, so we can copy the vnet header */
+       if (hdr_valid) {
+               hdr = skb_vnet_hdr(skb);
+               memcpy(hdr, hdr_p, hdr_len);
+       }
+
+       if (metasize) {
+               __skb_pull(skb, metasize);
+               skb_metadata_set(skb, metasize);
+       }
+
        return skb;
 }
 
-- 
2.31.0