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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.
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