> On 2023/4/23 22:20, Lorenzo Bianconi wrote:
> >> On 2023/4/23 2:54, Lorenzo Bianconi wrote:
> >>> struct veth_priv {
> >>> @@ -727,17 +729,20 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
> >>> goto drop;
> >>>
> >>> /* Allocate skb head */
> >>> - page = alloc_page(GFP_ATOMIC | __GFP_NOWARN);
> >>> + page = page_pool_dev_alloc_pages(rq->page_pool);
> >>> if (!page)
> >>> goto drop;
> >>>
> >>> nskb = build_skb(page_address(page), PAGE_SIZE);
> >>
> >> If page pool is used with PP_FLAG_PAGE_FRAG, maybe there is some additional
> >> improvement for the MTU 1500B case, it seem a 4K page is able to hold two skb.
> >> And we can reduce the memory usage too, which is a significant saving if page
> >> size is 64K.
> >
> > please correct if I am wrong but I think the 1500B MTU case does not fit in the
> > half-page buffer size since we need to take into account VETH_XDP_HEADROOM.
> > In particular:
> >
> > - VETH_BUF_SIZE = 2048
> > - VETH_XDP_HEADROOM = 256 + 2 = 258
>
> On some arch the NET_IP_ALIGN is zero.
>
> I suppose XDP_PACKET_HEADROOM are for xdp_frame and data_meta, it seems
> xdp_frame is only 40 bytes for 64 bit arch and max size of metalen is 32
> as xdp_metalen_invalid() suggest, is there any other reason why we need
> 256 bytes here?
XDP_PACKET_HEADROOM must be greater than (40 + 32)B because you may want push
new data at the beginning of the xdp_buffer/xdp_frame running
bpf_xdp_adjust_head() helper.
I think 256B has been selected for XDP_PACKET_HEADROOM since it is 4 cachelines
(but I can be wrong).
There was a discussion in the past to reduce XDP_PACKET_HEADROOM to 192B but
this is not merged yet and it is not related to this series. We can address
your comments in a follow-up patch when XDP_PACKET_HEADROOM series is merged.
Regards,
Lorenzo
>
> > - max_headsize = SKB_WITH_OVERHEAD(VETH_BUF_SIZE - VETH_XDP_HEADROOM) = 1470
> >
> > Even in this case we will need the consume a full page. In fact, performances
> > are a little bit worse:
> >
> > MTU 1500: tcp throughput ~ 8.3Gbps
> >
> > Do you agree or am I missing something?
> >
> > Regards,
> > Lorenzo
>
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