On Thu, 1 Mar 2018 11:19:05 +0800 Jason Wang <jasow...@redhat.com> wrote:
> We used to do data copy through xdp_linearize_page() for the buffer
> without sufficient headroom, it brings extra complexity without
> helping for the performance. So this patch remove it and switch to use
> generic XDP routine to handle this case.
I don't like where this is going. I don't like intermixing the native
XDP and generic XDP in this way, for several reasons:
1. XDP generic is not feature complete, e.g. cpumap will drop these
packets. It might not be possible to implement some features, think
of (AF_XDP) zero-copy.
2. This can easily cause out-of-order packets.
3. It makes it harder to troubleshoot, when diagnosing issues
around #1, we have a hard time determining what path an XDP packet
took (the xdp tracepoints doesn't know).
[...]
> @@ -590,25 +526,14 @@ static struct sk_buff *receive_small(struct net_device
> *dev,
> if (unlikely(hdr->hdr.gso_type))
> goto err_xdp;
>
> + /* This happnes when headroom is not enough because
> + * the buffer was refilled before XDP is set. This
> + * only happen for several packets, for simplicity,
> + * offload them to generic XDP routine.
In my practical tests, I also saw that sometime my ping packets were
traveling this code-path, even after a long time when XDP were attached.
This worries me a bit, for troubleshooting purposes... as this can give
a strange user experience given point #1.
> + */
> if (unlikely(xdp_headroom < virtnet_get_headroom(vi))) {
> - int offset = buf - page_address(page) + header_offset;
> - unsigned int tlen = len + vi->hdr_len;
> - u16 num_buf = 1;
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
