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

[Krishna Kurapati PSSNV <quic_kriskura@quicinc.com>]

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@mail.gmail.com/

> Regards,
> Krishna,
>