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

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Date:   Tue, 25 Apr 2023 16:13:07 +0200
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        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 2023/4/24 21:10, Maciej Fijalkowski wrote:
> >>> 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.
> > 
> > Intel drivers still work just fine at 192 headroom and split the page but
> > it makes it problematic for BIG TCP where MAX_SKB_FRAGS from shinfo needs
> 
> I am not sure why we are not enabling skb_shinfo(skb)->frag_list to support
> BIG TCP instead of increasing MAX_SKB_FRAGS, perhaps there was some disscution
> about this in the past I am not aware of?
> 
> > to be increased. So it's the tailroom that becomes the bottleneck, not the
> > headroom. I believe at some point we will convert our drivers to page_pool
> > with full 4k page dedicated for a single frame.
> 
> Can we use header splitting to ensure there is enough tailroom for
> napi_build_skb() or xdp_frame with shinfo?
> 

since veth_convert_skb_to_xdp_buff() runs in veth_poll() I think we can use
napi_build_skb(). I tested it and we get an improvement (9.65Gbps vs 9.2Gbps
for 1500B frames).

Regards,
Lorenzo

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