Re: [PATCH 2/2] usb: gadget: ncm: Add support to update wMaxSegmentSize via configfs

[Krishna Kurapati PSSNV]

On 10/14/2023 12:32 PM, Krishna Kurapati PSSNV wrote:
> On 10/14/2023 4:05 AM, Maciej Żenczykowski wrote:
> > > > > The intent of posting the diff was two fold:
> > > > >
> > > > > 1. The question Greg asked regarding why the max segment size was
> > > > > limited to 15014 was valid. When I thought about it, I actually wanted
> > > > > to limit the max MTU to 15000, so the max segment size automatically
> > > > > needs to be limited to 15014.
> > > >
> > > > Note that this is a *very* abstract value.
> > > > I get you want L3 MTU of 10 * 1500, but this value is not actually 
> > > > meaningful.
> > > >
> > > > IPv4/IPv6 fragmentation and IPv4/IPv6 TCP segmentation
> > > > do not result in a trivial multiplication of the standard 1500 byte
> > > > ethernet L3 MTU.
> > > > Indeed aggregating 2 1500 L3 mtu frames results in *different* sized
> > > > frames depending on which type of aggregation you do.
> > > > (and for tcp it even depends on the number and size of tcp options,
> > > > though it is often assumed that those take up 12 bytes, since that's 
> > > > the
> > > > normal for Linux-to-Linux tcp connections)
> > > >
> > > > For example if you aggregate N standard Linux ipv6/tcp L3 1500 mtu 
> > > > frames,
> > > > this means you have
> > > > N frames: ethernet (14) + ipv6 (40) + tcp (20) + tcp options (12) +
> > > > payload (1500-12-20-40=1500-72=1428)
> > > > post aggregation:
> > > > 1 frame: ethernet (14) + ipv6 (40) + tcp (20) + tcp options (12) +
> > > > payload (N*1428)
> > > >
> > > > so N * 1500 == N * (72 + 1428) --> 1 * (72 + N * 1428)
> > > >
> > > > That value of 72 is instead 52 for 'standard Linux ipv4/tcp),
> > > > it's 40/60 if there's no tcp options (which I think happens when
> > > > talking to windows)
> > > > it's different still with ipv4 fragmentation... and again different
> > > > with ipv6 fragmentation...
> > > > etc.
> > > >
> > > > ie. 15000 L3 mtu is exactly as meaningless as 14000 L3 mtu.
> > > > Either way you don't get full frames.
> > > >
> > > > As such I'd recommend going with whatever is the largest mtu that can
> > > > be meaningfully made to fit in 16K with all the NCM header overhead.
> > > > That's likely closer to 15500-16000 (though I have *not* checked).
> > > >
> > > > > But my commit text didn't mention this
> > > > > properly which was a mistake on my behalf. But when I looked at the
> > > > > code, limiting the max segment size 15014 would force the practical
> > > > > max_mtu to not cross 15000 although theoretical max_mtu was set to:
> > > > > (GETHER_MAX_MTU_SIZE - 15412) during registration of net device.
> > > > >
> > > > > So my assumption of limiting it to 15000 was wrong. It must be limited
> > > > > to 15412 as mentioned in u_ether.c  This inturn means we must limit
> > > > > max_segment_size to:
> > > > > GETHER_MAX_ETH_FRAME_LEN (GETHER_MAX_MTU_SIZE + ETH_HLEN)
> > > > > as mentioned in u_ether.c.
> > > > >
> > > > > I wanted to confirm that setting MAX_DATAGRAM_SIZE to
> > > > > GETHER_MAX_ETH_FRAME_LEN was correct.
> > > > >
> > > > > 2. I am not actually able to test with MTU beyond 15000. When my host
> > > > > device is a linux machine, the cdc_ncm.c limits max_segment_size to:
> > > > > CDC_NCM_MAX_DATAGRAM_SIZE               8192    /* bytes */
> > > >
> > > > In practice you get 50% of the benefits of infinitely large mtu by
> > > > going from 1500 to ~2980.
> > > > you get 75% of the benefits by going to ~6K
> > > > you get 87.5% of the benefits by going to ~12K
> > > > the benefits of going even higher are smaller and smaller...
> > > >   > If the host side is limited to 8192, maybe we should match that 
> > > > here too?
> > >
> > > Hi Maciej,
> > >
> > >    Thanks for the detailed explanation. I agree with you on setting
> > > device side also to 8192 instead of what max_mtu is present in u_ether
> > > or practical max segment size possible.
> > >
> > > > But the host side limitation of 8192 doesn't seem particularly sane 
> > > > either...
> > > > Maybe we should relax that instead?
> > > I really didn't understand why it was set to 8192 in first place.
> > >
> > > > (especially since for things like tcp zero copy you want an mtu which
> > > > is slighly more then N * 4096,
> > > > ie. around 4.5KB, 8.5KB, 12.5KB or something like that)
> > >
> > > I am not sure about host mode completely. If we want to increase though,
> > > just increasing the MAX_DATAGRAM_SIZE to some bigger value help ? (I
> > > don't know the entire code of cdc_ncm, so I might be wrong).
> > >
> > > Regards,
> > > Krishna,
> >
> > Hmm, I'm not sure.  I know I've experimented with high mtu ncm in the 
> > past
> > (around 2.5 years ago).  I got it working between my Linux desktop (host)
> > and a Pixel 6 (device/gadget) with absolutely no problems.
> >
> > I'm pretty sure I didn't change my desktop kernel, so I was probably
> > limited to 8192 there
> > (and I do more or less remember that).
> >  From what I vaguely remember, it wasn't difficult (at all) to hit
> > upwards of 7gbps for iperf tests.
> > I don't remember how close to the theoretical USB 10gbps maximum of
> > 9.7gbps I could get...
> > [this was never the real bottleneck / issue, so I didn't ever dig
> > particularly deep]
> >
> > I'm pretty sure my gadget side changes were non-configurable...
> > Probably just bumped one or two constants...
> Could you share what parameters you changed to get this high value of 
> iperf throughput.
>
> > I do *very* *vaguely* recall there being some funkiness though, where 
> > 8192 was
> > *less* efficient than some slightly smaller value.
> >
> > If I recall correctly the issue is that 8192 + ethernet overhead + NCM
> > overhead only fits *once* into 16384, which leaves a lot of space
> > wasted.
> > While ~7.5 kb + overhead fits twice and is thus a fair bit better.
> Right, same goes for using 5K vs 5.5K MTU. If MTU is 5K, 3 packets can 
> conveniently fit into an NTB but if its 5.5, at max only two (5.5k) 
> packets can fit in (essentially filling ~11k of the 16384 bytes and 
> wasting the rest)

Formatting gone wrong. So pasting the first paragraph again here:

"Right, same goes for using 5K vs 5.5K MTU. If MTU is 5K, 3 packets can
conveniently fit into an NTB but if its 5.5, at max only two (5.5k)
packets can fit in (essentially filling ~11k of the 16384 bytes and
wasting the rest)"

> And whether its Ipv4/Ipv6 like you mentioned on [1], the MTU is what NCM 
> layer receives and we append the Ethernet header and add NCM headers and 
> send it out after aggregation. Why can't we set the MAX_DATAGRAM_SIZE to 
> ~8050 or ~8100 ? The reason I say this value is, obviously setting it to 
> 8192 would not efficiently use the NTB buffer. We need to fill as much 
> space in buffer as possible and assuming that each packet received on 
> ncm layer is of MTU size set (not less that that), we can assume that 
> even if only 2 packets are aggregated (minimum aggregation possible), we 
> would be filling (2 * (8050 + ETH_HLEN) + (room for NCM headers)) would 
> almost be close to 16384 ep max packet size. I already check 8050 MTU 
> and it works. We can add a comment in code detailing the above 
> explanation and why we chose to use 8050 or 8100 as MAX_DATAGRAM_SIZE.
>
> Hope my reasoning of why we can chose 8.1K or 8.05K makes sense. Let me 
> know your thoughts on this.

[1]: 
https://lore.kernel.org/all/CANP3RGd4G4dkMOyg6wSX29NYP2mp=LhMhmZpoG=rgoCz=bh1=w@xxxxxxxxxxxxxx/

> Regards,
> Krishna,