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

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

On 10/16/2023 6:49 AM, Maciej Żenczykowski wrote:

>>>>
>>>> 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.
> 
> Eh, I really don't remember, but it wasn't anything earth shattering.
>  From what I recall it was just a matter of bumping mtu, and tweaking
> irq pinning to stronger cores.
> Indeed I'm not even certain that the mtu was required to be over 5gbps.
> Though I may be confusing some things, as at least some of the testing was done
> with the kernel's built in packet generator.
> 
>>>
>>>> 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.
> 
> Maybe just use an L3 mtu of 8000 then?  That's a nice round number...
> But I'm also fine with 8050 or 8100.. though 8100 seems 'rounder'.
> 
> I'm not sure what the actual overhead is... I guess we control the
> overhead in one direction, but not in the other, and there could be
> some slop, so we need to be a little generous?
> 
>>>
Hi Maciej,

   Sure. Let's go with 8000 to leave some space for headers. And would 
add the following paragraph as comment for readers to understand why 
this value was set:

"Although max mtu as dictated by u_ether is 15412 bytes, setting 
max_segment_size to 15426 would not be efficient. If user chooses 
segment size to be (> 8192), then we can't aggregate more than one 
buffer in each NTB (assuming each packet coming from network layer is > 
8192 bytes) as ep maxpacket limit is 16384. So let max_segment_size be 
limited to 8000 to allow atleast 2 packets to be aggregated reducing 
wastage of NTB buffer space"

Hope that would be fine.

Regards,
Krishna,