Re: [PATCH bpf-next] xsk: build skb by page

[Magnus Karlsson <magnus.karlsson@gmail.com>]

On Wed, Dec 23, 2020 at 9:57 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>
> This patch is used to construct skb based on page to save memory copy
> overhead.
>
> Taking into account the problem of addr unaligned, and the
> possibility of frame size greater than page in the future.

Thanks Xuan for the patch set. Could you please share performance
numbers so we know how much this buys us? Would be good if you could
produce them for 64 bytes, 1500 bytes and something in the middle so
we can judge the benefits of this.

Please note that responses will be delayed this week and next due to
the Christmas and New Years holidays over here.

> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> ---
>  net/xdp/xsk.c | 68 ++++++++++++++++++++++++++++++++++++++++++++---------------
>  1 file changed, 51 insertions(+), 17 deletions(-)
>
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index ac4a317..7cab40f 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -430,6 +430,55 @@ static void xsk_destruct_skb(struct sk_buff *skb)
>         sock_wfree(skb);
>  }
>
> +static struct sk_buff *xsk_build_skb_bypage(struct xdp_sock *xs, struct xdp_desc *desc)
> +{
> +       char *buffer;
> +       u64 addr;
> +       u32 len, offset, copy, copied;
> +       int err, i;
> +       struct page *page;
> +       struct sk_buff *skb;
> +
> +       skb = sock_alloc_send_skb(&xs->sk, 0, 1, &err);
> +       if (unlikely(!skb))
> +               return NULL;
> +
> +       addr = desc->addr;
> +       len = desc->len;
> +
> +       buffer = xsk_buff_raw_get_data(xs->pool, addr);
> +       offset = offset_in_page(buffer);
> +       addr = buffer - (char *)xs->pool->addrs;
> +
> +       for (copied = 0, i = 0; copied < len; ++i) {
> +               page = xs->pool->umem->pgs[addr >> PAGE_SHIFT];
> +
> +               get_page(page);
> +
> +               copy = min((u32)(PAGE_SIZE - offset), len - copied);
> +
> +               skb_fill_page_desc(skb, i, page, offset, copy);
> +
> +               copied += copy;
> +               addr += copy;
> +               offset = 0;
> +       }
> +
> +       skb->len += len;
> +       skb->data_len += len;
> +       skb->truesize += len;
> +
> +       refcount_add(len, &xs->sk.sk_wmem_alloc);
> +
> +       skb->dev = xs->dev;
> +       skb->priority = xs->sk.sk_priority;
> +       skb->mark = xs->sk.sk_mark;
> +       skb_shinfo(skb)->destructor_arg = (void *)(long)addr;
> +       skb->destructor = xsk_destruct_skb;
> +
> +       return skb;
> +}
> +
>  static int xsk_generic_xmit(struct sock *sk)
>  {
>         struct xdp_sock *xs = xdp_sk(sk);
> @@ -445,40 +494,25 @@ static int xsk_generic_xmit(struct sock *sk)
>                 goto out;
>
>         while (xskq_cons_peek_desc(xs->tx, &desc, xs->pool)) {
> -               char *buffer;
> -               u64 addr;
> -               u32 len;
> -
>                 if (max_batch-- == 0) {
>                         err = -EAGAIN;
>                         goto out;
>                 }
>
> -               len = desc.len;
> -               skb = sock_alloc_send_skb(sk, len, 1, &err);
> +               skb = xsk_build_skb_bypage(xs, &desc);
>                 if (unlikely(!skb))
>                         goto out;
>
> -               skb_put(skb, len);
> -               addr = desc.addr;
> -               buffer = xsk_buff_raw_get_data(xs->pool, addr);
> -               err = skb_store_bits(skb, 0, buffer, len);
>                 /* This is the backpressure mechanism for the Tx path.
>                  * Reserve space in the completion queue and only proceed
>                  * if there is space in it. This avoids having to implement
>                  * any buffering in the Tx path.
>                  */
> -               if (unlikely(err) || xskq_prod_reserve(xs->pool->cq)) {
> +               if (xskq_prod_reserve(xs->pool->cq)) {
>                         kfree_skb(skb);
>                         goto out;
>                 }
>
> -               skb->dev = xs->dev;
> -               skb->priority = sk->sk_priority;
> -               skb->mark = sk->sk_mark;
> -               skb_shinfo(skb)->destructor_arg = (void *)(long)desc.addr;
> -               skb->destructor = xsk_destruct_skb;
> -
>                 err = __dev_direct_xmit(skb, xs->queue_id);
>                 if  (err == NETDEV_TX_BUSY) {
>                         /* Tell user-space to retry the send */
> --
> 1.8.3.1
>