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

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Message-ID: <CANn89iL6MjvOc8qEQpeQJPLX0Y3X0HmqNcmgHL4RzfcijPim5w@mail.gmail.com>
Date: Tue, 4 Nov 2025 09:00:14 -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 1/1] usbnet: Add support for Byte Queue Limits (BQL)

On Tue, Nov 4, 2025 at 8:14 AM Simon Schippers
<simon.schippers@...dortmund.de> wrote:
>
> The usbnet driver currently relies on fixed transmit queue lengths, which
> can lead to bufferbloat and large latency spikes under load -
> particularly with cellular modems.
> This patch adds support for Byte Queue Limits (BQL) to dynamically manage
> the transmit queue size and reduce latency without sacrificing
> throughput.
>
> Testing was performed on various devices using the usbnet driver for
> packet transmission:
>
> - DELOCK 66045: USB3 to 2.5 GbE adapter (ax88179_178a)
> - DELOCK 61969: USB2 to 1 GbE adapter (asix)
> - Quectel RM520: 5G modem (qmi_wwan)
> - USB2 Android tethering (cdc_ncm)
>
> No performance degradation was observed for iperf3 TCP or UDP traffic,
> while latency for a prioritized ping application was significantly
> reduced. For example, using the USB3 to 2.5 GbE adapter, which was fully
> utilized by iperf3 UDP traffic, the prioritized ping was improved from
> 1.6 ms to 0.6 ms. With the same setup but with a 100 Mbit/s Ethernet
> connection, the prioritized ping was improved from 35 ms to 5 ms.
>
> Signed-off-by: Simon Schippers <simon.schippers@...dortmund.de>
> ---
>  drivers/net/usb/usbnet.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
> index 62a85dbad31a..1994f03a78ad 100644
> --- a/drivers/net/usb/usbnet.c
> +++ b/drivers/net/usb/usbnet.c
> @@ -831,6 +831,7 @@ int usbnet_stop(struct net_device *net)
>
>         clear_bit(EVENT_DEV_OPEN, &dev->flags);
>         netif_stop_queue (net);
> +       netdev_reset_queue(net);
>
>         netif_info(dev, ifdown, dev->net,
>                    "stop stats: rx/tx %lu/%lu, errs %lu/%lu\n",
> @@ -939,6 +940,7 @@ int usbnet_open(struct net_device *net)
>         }
>
>         set_bit(EVENT_DEV_OPEN, &dev->flags);
> +       netdev_reset_queue(net);
>         netif_start_queue (net);
>         netif_info(dev, ifup, dev->net,
>                    "open: enable queueing (rx %d, tx %d) mtu %d %s framing\n",
> @@ -1500,6 +1502,7 @@ netdev_tx_t usbnet_start_xmit(struct sk_buff *skb, struct net_device *net)
>         case 0:
>                 netif_trans_update(net);
>                 __usbnet_queue_skb(&dev->txq, skb, tx_start);
> +               netdev_sent_queue(net, skb->len);
>                 if (dev->txq.qlen >= TX_QLEN (dev))
>                         netif_stop_queue (net);
>         }
> @@ -1563,6 +1566,7 @@ static inline void usb_free_skb(struct sk_buff *skb)
>  static void usbnet_bh(struct timer_list *t)
>  {
>         struct usbnet           *dev = timer_container_of(dev, t, delay);
> +       unsigned int bytes_compl = 0, pkts_compl = 0;
>         struct sk_buff          *skb;
>         struct skb_data         *entry;
>
> @@ -1574,6 +1578,8 @@ static void usbnet_bh(struct timer_list *t)
>                                 usb_free_skb(skb);
>                         continue;
>                 case tx_done:
> +                       bytes_compl += skb->len;
> +                       pkts_compl++;
>                         kfree(entry->urb->sg);
>                         fallthrough;
>                 case rx_cleanup:
> @@ -1584,6 +1590,8 @@ static void usbnet_bh(struct timer_list *t)
>                 }
>         }
>
> +       netdev_completed_queue(dev->net, pkts_compl, bytes_compl);
> +
>         /* restart RX again after disabling due to high error rate */
>         clear_bit(EVENT_RX_KILL, &dev->flags);
>

I think this is racy. usbnet_bh() can run from two different contexts,
at the same time (from two cpus)

1) From process context :
usbnet_bh_work()

2) From a timer. (dev->delay)


To use BQL, you will need to add mutual exclusion.