netdev - Re: [bpf-next V2 PATCH 7/8] bpf/xdp: non-map redirect can avoid calling ndo_xdp

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Date:   Thu, 31 May 2018 09:16:46 -0700
From:   Song Liu <liu.song.a23@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Networking <netdev@...r.kernel.org>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [bpf-next V2 PATCH 7/8] bpf/xdp: non-map redirect can avoid
 calling ndo_xdp_flush

On Thu, May 31, 2018 at 2:00 AM, Jesper Dangaard Brouer
<brouer@...hat.com> wrote:
> This is the first real user of the XDP_XMIT_FLUSH flag.
>
> As pointed out many times, XDP_REDIRECT without using BPF maps is
> significant slower than the map variant.  This is primary due to the
> lack of bulking, as the ndo_xdp_flush operation is required after each
> frame (to avoid frames hanging on the egress device).
>
> It is still possible to optimize this case.  Instead of invoking two
> NDO indirect calls, which are very expensive with CONFIG_RETPOLINE,
> instead instruct ndo_xdp_xmit to flush via XDP_XMIT_FLUSH flag.
>
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>

Acked-by: Song Liu <songliubraving@...com>

> ---
>  net/core/filter.c |    3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 6a21dbcad350..6981b4608979 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3056,10 +3056,9 @@ static int __bpf_tx_xdp(struct net_device *dev,
>         if (unlikely(!xdpf))
>                 return -EOVERFLOW;
>
> -       sent = dev->netdev_ops->ndo_xdp_xmit(dev, 1, &xdpf, 0);
> +       sent = dev->netdev_ops->ndo_xdp_xmit(dev, 1, &xdpf, XDP_XMIT_FLUSH);
>         if (sent <= 0)
>                 return sent;
> -       dev->netdev_ops->ndo_xdp_flush(dev);
>         return 0;
>  }
>
>