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

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Message-ID: <CANn89iLLwWvbnCKKRrV2c7eo+4UduLVgZUWR=ZoZ+SPHRGf=wg@mail.gmail.com>
Date: Tue, 4 Nov 2025 09:02:22 -0800
From: Eric Dumazet <edumazet@...gle.com>
To: Simon Schippers <simon.schippers@...dortmund.de>
Cc: oneukum@...e.com, andrew+netdev@...n.ch, davem@...emloft.net, 
	kuba@...nel.org, pabeni@...hat.com, netdev@...r.kernel.org, 
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v1 0/1] usbnet: Add support for Byte Queue Limits (BQL)

On Tue, Nov 4, 2025 at 8:14 AM Simon Schippers
<simon.schippers@...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 ?

>
> 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/


> 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
>