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Date: Fri, 20 Nov 2020 11:18:34 +0300 From: Igor Russkikh <irusskikh@...vell.com> To: Maciej Fijalkowski <maciej.fijalkowski@...el.com>, "Ramsay, Lincoln" <Lincoln.Ramsay@...i.com> CC: Florian Westphal <fw@...len.de>, "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, <netdev@...r.kernel.org>, "Dmitry Bogdanov [C]" <dbogdanov@...vell.com> Subject: Re: [EXT] Re: [PATCH v3] aquantia: Remove the build_skb path On 20/11/2020 1:49 am, Maciej Fijalkowski wrote: > External Email > > ---------------------------------------------------------------------- > On Thu, Nov 19, 2020 at 10:34:48PM +0000, Ramsay, Lincoln wrote: >> When performing IPv6 forwarding, there is an expectation that SKBs >> will have some headroom. When forwarding a packet from the aquantia >> driver, this does not always happen, triggering a kernel warning. >> >> The build_skb path fails to allow for an SKB header, but the hardware >> buffer it is built around won't allow for this anyway. Just always use > the >> slower codepath that copies memory into an allocated SKB. >> >> Signed-off-by: Lincoln Ramsay <lincoln.ramsay@...ngear.com> >> --- > > (Next time please include in the subject the tree that you're targetting > the patch) > > I feel like it's only a workaround, not a real solution. On previous > thread Igor says: > > "The limitation here is we can't tell HW on granularity less than 1K." > > Are you saying that the minimum headroom that we could provide is 1k? We can tell HW to place packets with 4 bytes granularity addresses, but the problem is the length granularity of this buffer - 1K. This means we can do as Ramsay initially suggested - just offset the packet placement. But then we have to guarantee that 1K after this offset is available to HW. Since normal layout is 1400 packets - we do use 2K (half page) for each packet. This way we reuse each allocated page for at least two packets (and putting skb_shared into the remaining 512b). Obviously we may allocate 4K page for a single packet, and tell HW that it can use 3K for data. This'll give 1K headroom. Quite an overload - assuming IMIX is of 0.5K - 1.4K.. Of course that depends on a usecase. If you know all your traffic is 16K jumbos - putting 1K headroom is very small overhead on memory usage. > Maybe put more pressure on memory side and pull in order-1 pages, provide > this big headroom and tailroom for skb_shared_info and use build_skb by > default? With standard 1500 byte MTU. I know many customers do consider AQC chips in near embedded environments (routers, etc). They really do care about memories. So that could be risky. > This issue would pop up again if this driver would like to support XDP > where 256 byte headroom will have to be provided. Actually it already popped. Thats one of the reasons I'm delaying with xdp patch series for this driver. I think the best tradeoff here would be allocating order 1 or 2 pages (i.e. 8K or 16K), and reuse the page for multiple placements of 2K XDP packets: (256+2048)*3 = 6912 (1K overhead for each 3 packets) (256+2048)*7 = 16128 (200b overhead over 7 packets) Regards, Igor
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