Re: [EXT] Re: [PATCH v3] aquantia: Remove the build_skb path

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Fri, Nov 20, 2020 at 11:18:34AM +0300, Igor Russkikh wrote:
> 
> 
> 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@opengear.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.

Ok, I see, thanks for clarifying.

> 
> Since normal layout is 1400 packets - we do use 2K (half page) for each packet.

What is 'normal layout is 1400 packets' ? Didn't you mean the 1500 byte
standard MTU? So this is what you've been trying to tell me - that for
1500 byte mtu and 1k HW granularity you need to provide to HW 2k of
contiguous space, correct?

> This way we reuse each allocated page for at least two packets (and putting
> skb_shared into the remaining 512b).

I don't think I follow that. I thought that 2k needs to be exclusive for
HW and now you're saying that for remaining 512 bytes you can do whatever
you want.

If that's true then I think you can have build_skb support and I don't see
that 1k granularity as a limitation.

> 
> 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.

We have a knob that is controlled by ethtool's priv flag so you can change
the memory model and pull the build_skb out of the picture. Just FYI.

> 
> > 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)

And for XDP_PASS you would use build_skb? Then tailroom needs to be
provided.

> 
> Regards,
>   Igor
> 
> 
>