Re: [PATCH net-next v5 3/5] veth: implement Byte Queue Limits (BQL) for latency reduction

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

On 5/8/26 10:01, Simon Schippers wrote:
> On 5/7/26 22:45, Jesper Dangaard Brouer wrote:
>>
>>
>> On 07/05/2026 22.12, Simon Schippers wrote:
>>> On 5/7/26 21:09, Jesper Dangaard Brouer wrote:
>>>>
>>>>
>>>> On 07/05/2026 16.46, Simon Schippers wrote:
>>>>>
>>>>>
>>>>> On 5/7/26 16:34, Paolo Abeni wrote:
>>>>>> On 5/7/26 8:54 AM, Simon Schippers wrote:
>>>>>>> On 5/5/26 15:21, hawk@kernel.org wrote:
>>>>>>>> @@ -928,9 +968,13 @@ static int veth_xdp_rcv(struct veth_rq *rq, int budget,
>>>>>>>>                }
>>>>>>>>            } else {
>>>>>>>>                /* ndo_start_xmit */
>>>>>>>> -            struct sk_buff *skb = ptr;
>>>>>>>> +            bool bql_charged = veth_ptr_is_bql(ptr);
>>>>>>>> +            struct sk_buff *skb = veth_ptr_to_skb(ptr);
>>>>>>>>                  stats->xdp_bytes += skb->len;
>>>>>>>> +            if (peer_txq && bql_charged)
>>>>>>>> +                netdev_tx_completed_queue(peer_txq, 1, VETH_BQL_UNIT);
>>>>>>>
>>>>>>> In the discussion with Jonas [1], I left a comment explaining why I think
>>>>>>> this doesn’t work.
>>>>>>>
>>>>
>>>> I've experimented with doing the "completion" at NAPI-end in
>>>> veth_poll(), but that resulted in BQL limit being 128 packets, which
>>>> leads to bad latency results (not acceptable).
>>>> (See detailed report later)
>>>>
>>>>
>>>>>>> I still think first that adding an option to modify the hard-coded
>>>>>>> VETH_RING_SIZE is the way to go.
>>>>>>>
>>>>
>>>> Not against being able to modify VETH_RING_SIZE, but I don't think it is
>>>> the solution here.
>>>>
>>>> The simply solution is the configure BQL limit_min:
>>>>   `/sys/class/net/<dev>/queues/tx-N/byte_queue_limits/limit_min`
>>>>
>>>> My experiments (below) find that limit_min=8 is gives good performance.
>>>> We can simply set default to 8 as this still allows userspace to change
>>>> this later if lower latency is preferred.
>>>>
>>>>>>> Thanks!
>>>>>>>
>>>>>>> [1] Link: https://lore.kernel.org/netdev/e8cdba04-aa9a-45c6-9807-8274b62920df@tu-dortmund.de/
>>>>>>
>>>>>> In the above discussion a 20% regression is reported, which IMHO can't
>>>>>> be ignored. Still the tput figures in the data are extremely low,
>>>>>> something is possibly off?!? I would expect a few Mpps with pktgen on
>>>>>> top of veth, while the reported data is ~20-30Kpps.
>>>>>>
>>>>>> /P
>>>>>>
>>>>>
>>>>> The ~20-30Kpps occur when thousands of iptables rules are applied and
>>>>> an UDP userspace application is sending.
>>>>>
>>>>> And there is a 20% pktgen regression (no iptables rules applied).
>>>>>
>>>>
>>>> The pktgen test is a little dubious/weird and Jonas had to modify pktgen
>>>> to test this.   John Fastabend added a config to pktgen that allows us
>>>> to benchmarking egress qdisc path, this might be better to use this.
>>>> The samples/pktgen/pktgen_bench_xmit_mode_queue_xmit.sh is a demo usage.
>>>>
>>>> If redoing the tests, can you adjust limit_min to see the effect?
>>>>   /sys/class/net/<dev>/queues/tx-N/byte_queue_limits/limit_min
>>>>
>>>> 20% throughput performance regression is of-cause too much, but I will
>>>> remind us, that adding a qdisc will "cost" some overhead, that is a
>>>> configuration choice.  Our purpose here is to reduce bufferbloat and
>>>> latency, not optimize for throughput.
>>>>
>>>>
>>>>> I am pretty sure the reason is because the BQL limit is stuck at 2
>>>>> packets (because the completed queue is always called with 1 packet
>>>>> and not in a interrupt/timer with multiple packets...).
>>>>>
>>>>
>>>> I've run a lot of experiments, which I made AI write a report over, see attachment.  The TL;DR is that best performance vs latency tradeoff is defaulting BQL/DQL limit_min to be 8 packets.
>>>>
>>>> I fear this patchset will stall forever, if we keep searching for a perfect solution without any overhead.  The qdisc layer will be a baseline overhead.  The limit=2 packets is actually the optimal darkbuffer queue size, but I acknowledge that this causes too many qdisc requeue events (leading to overhead).  I suggest that I add another patch in V6, that defaults limit_min to 8 (separate patch to make it easier to revert/adjust later).
>>>>
>>>> I've talked with Jonas, and we want to experiment with different solutions to make BQL/DQL work better with virtual devices.
>>>>
>>>> This patchset helps our (production) use-case reduce mice-flow latency
>>>> from approx 22ms to 1.3ms for latency under-load.  Due to the consumer
>>>> namespace being the bottleneck the requeue overhead is negligible in
>>>> comparison.
>>>>
>>>> -Jesper
>>>
>>> First of all thanks for you work and I really see the advantages of
>>> avoiding bufferbloat :)
>>>
>>> But the key of the BQL algorithm, which is the *dynamic* adaption of the
>>> limit, is not working. Always calling netdev_completed_queue() with
>>> 1 packet results in a static limit of 2 packets (as seen by Jonas
>>> measurements), which you force up to 8 packets.
>>>
>>> So in the end this patchset has the same effect as just setting
>>> VETH_RING_SIZE to 8 (and giving an option to change this value).
>>>
>>
>> I've code up a time based BQL implementation, see attachment.
>> WDYT?
>>
>> --Jesper
>>
> 
> A step in the right direction, but I dislike that you call
> netdev_sent_queue() with at least 1 packet (never 0 packets).
> I am not sure if it works, and I am not sure about the parameter.
> 

Rethinking of it this could be fine, but really needs testing because:

The weird thing is that is that BQL's inflight != number of packets
in the ring and BQL's limit != "current ring size". Instead the BQL
limit describes the number of maximal allowed packets between
calls of netdev_sent_queue().



I messed up in my approach below. Forget it :P

> 
> I would propose doing it like other BQL implementations do
> (for example usbnet for which I adapted BQL [1] :) ):
> 
> Call netdev_sent_queue() with n_bql in a periodic work. n_bql would
> still be counted in veth_xdp_rcv() like you currently do (synchronized
> with the work via ring.consumer_lock?).
> 
> The only weird thing that remains is that BQL's inflight != number of
> packets in the ring and BQL's limit != "current ring size". Instead
> the BQL limit describes the number of maximal allowed packets between
> calls of netdev_sent_queue(), which occur periodically in a somewhat
> fixed time interval.
> I guess that could be fine, but it surely needs testing.
> 
> [1] Link: https://lore.kernel.org/netdev/20251106175615.26948-1-simon.schippers@tu-dortmund.de/
>