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

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Message-ID: <ZEZ/xXcOv9Co5Vif@boxer>
Date:   Mon, 24 Apr 2023 15:10:29 +0200
From:   Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>
CC:     Lorenzo Bianconi <lorenzo.bianconi@...hat.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 Mon, Apr 24, 2023 at 07:58:07PM +0800, Yunsheng Lin wrote:
> On 2023/4/24 17:17, Lorenzo Bianconi wrote:
> >> 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.

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
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.

> 
> It worth mentioning that the performance gain in this patch is at the cost of
> more memory usage, at most of VETH_RING_SIZE(256) + PP_ALLOC_CACHE_SIZE(128)
> pages is used.
> 
> IMHO, it seems better to limit the memory usage as much as possible, or provide a
> way to disable/enable page pool for user.

I think that this argument is valuable due to the purpose that veth can
serve, IMHO it wouldn't be an argument for a standard PF driver. It would
be interesting to see how veth would work with PP_FLAG_PAGE_FRAG.