netdev - Re: [PATCH v2 net-next 1/2] net: veth: add page

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Date:   Mon, 24 Apr 2023 11:17:16 +0200
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com, hawk@...nel.org,
        john.fastabend@...il.com, ast@...nel.org, daniel@...earbox.net
Subject: Re: [PATCH v2 net-next 1/2] net: veth: add page_pool for page
 recycling

> 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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