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

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Message-ID: <66d22955-bb20-44cf-8ad3-743ae272fec7@tu-dortmund.de>
Date: Wed, 5 Nov 2025 11:35:49 +0100
From: Simon Schippers <simon.schippers@...dortmund.de>
To: Eric Dumazet <edumazet@...gle.com>
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: [PATCH net-next v1 1/1] usbnet: Add support for Byte Queue Limits
 (BQL)

On 11/4/25 18:00, Eric Dumazet wrote:
> 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.

Yeah, I missed that.

I guess synchronizing with the lock of the sk_buff_head dev->done makes
sense? The same locking is also done right before in skb_dequeue.