[PATCH net-next v1 0/1] usbnet: Add support for Byte Queue Limits (BQL)

[Simon Schippers <simon.schippers@tu-dortmund.de>]

On 11/5/25 11:35, Daniele Palmas wrote:
> Hello Simon,
> 
> Il giorno mer 5 nov 2025 alle ore 11:40 Simon Schippers
> <simon.schippers@tu-dortmund.de> ha scritto:
>>
>> On 11/4/25 18:02, Eric Dumazet wrote:
>>> On Tue, Nov 4, 2025 at 8:14 AM Simon Schippers
>>> <simon.schippers@tu-dortmund.de> wrote:
>>>>
>>>> During recent testing, I observed significant latency spikes when using
>>>> Quectel 5G modems under load. Investigation revealed that the issue was
>>>> caused by bufferbloat in the usbnet driver.
>>>>
>>>> In the current implementation, usbnet uses a fixed tx_qlen of:
>>>>
>>>> USB2: 60 * 1518 bytes = 91.08 KB
>>>> USB3: 60 * 5 * 1518 bytes = 454.80 KB
>>>>
>>>> Such large transmit queues can be problematic, especially for cellular
>>>> modems. For example, with a typical celluar link speed of 10 Mbit/s, a
>>>> fully occupied USB3 transmit queue results in:
>>>>
>>>> 454.80 KB / (10 Mbit/s / 8 bit/byte) = 363.84 ms
>>>>
>>>> of additional latency.
>>>
>>> Doesn't 5G need to push more packets to the driver to get good aggregation ?
>>>
>>
>> Yes, but not 455 KB for low speeds. 5G requires a queue of a few ms to
>> aggregate enough packets for a frame but not of several hundred ms as
>> calculated in my example. And yes, there are situations where 5G,
>> especially FR2 mmWave, reaches Gbit/s speeds where a big queue is
>> required. But the dynamic queue limit approach of BQL should be well
>> suited for these varying speeds.
>>
> 
> out of curiosity, related to the test with 5G Quectel, did you test
> enabling aggregation through QMAP (kernel module rmnet) or simply
> qmi_wwan raw_ip ?
> 
> Regards,
> Daniele
> 

Hi Daniele,

I simply used qmi_wwan. I actually never touched rmnet before.
Is the aggregation through QMAP what you and Eric mean with aggregation?
Because then I misunderstood it, because I was thinking about aggregating
enough (and not too many) packets in the usbnet queue.

Thanks

>>>>
>>>> To address this issue, this patch introduces support for
>>>> Byte Queue Limits (BQL) [1][2] in the usbnet driver. BQL dynamically
>>>> limits the amount of data queued in the driver, effectively reducing
>>>> latency without impacting throughput.
>>>> This implementation was successfully tested on several devices as
>>>> described in the commit.
>>>>
>>>>
>>>>
>>>> Future work
>>>>
>>>> Due to offloading, TCP often produces SKBs up to 64 KB in size.
>>>
>>> Only for rates > 500 Mbit. After BQL, we had many more improvements in
>>> the stack.
>>> https://lwn.net/Articles/564978/
>>>
>>>
>>
>> I also saw these large SKBs, for example, for my USB2 Android tethering,
>> which advertises a network speed of < 500 Mbit/s.
>> I saw these large SKBs by looking at the file:
>>
>> cat /sys/class/net/INTERFACE/queues/tx-0/byte_queue_limits/inflight
>>
>> For UDP-only traffic, inflight always maxed out at MTU size.
>>
>> Thank you for your replies!
>>
>>>> To
>>>> further decrease buffer bloat, I tried to disable TSO, GSO and LRO but it
>>>> did not have the intended effect in my tests. The only dirty workaround I
>>>> found so far was to call netif_stop_queue() whenever BQL sets
>>>> __QUEUE_STATE_STACK_XOFF. However, a proper solution to this issue would
>>>> be desirable.
>>>>
>>>> I also plan to publish a scientific paper on this topic in the near
>>>> future.
>>>>
>>>> Thanks,
>>>> Simon
>>>>
>>>> [1] https://medium.com/@tom_84912/byte-queue-limits-the-unauthorized-biography-61adc5730b83
>>>> [2] https://lwn.net/Articles/469652/
>>>>
>>>> Simon Schippers (1):
>>>>   usbnet: Add support for Byte Queue Limits (BQL)
>>>>
>>>>  drivers/net/usb/usbnet.c | 8 ++++++++
>>>>  1 file changed, 8 insertions(+)
>>>>
>>>> --
>>>> 2.43.0
>>>>
>>