Re: [PATCH net v3] tun/tap & vhost-net: make qdisc backpressure opt-in via IFF_BACKPRESSURE

[Simon Schippers]

On 7/14/26 18:50, Simon Schippers wrote:
> On 7/14/26 15:40, Michael S. Tsirkin wrote:
>> On Thu, Jul 09, 2026 at 11:55:11AM +0200, Simon Schippers wrote:
>>> Commit 1d6e569b7d0c ("tun/tap & vhost-net: avoid ptr_ring tail-drop
>>> when a qdisc is present") did not show a relevant performance regression
>>> in my testing but on Brett Sheffield's librecast testbed it shows a
>>> significant performance drop in a IPv6 multicast testcase. The regression
>>> can be pinpointed when multiple iperf3 TCP threads are sending. For 8
>>> threads the performance dropped from 13.5 Gbit/s to 9.13 Gbit/s. This is
>>> the reason why this patch makes the qdisc backpressure behavior opt-in.
>>>
>>> One option to accomplish the opt-in would be to set the default qdisc to
>>> noqueue at init. However this may also break userspace as users might
>>> have chosen a custom qdisc even though most of the qdiscs did nothing
>>> for tun/tap in the past due to missing backpressure...
>>>
>>> This is the reason why in this patch, the flag IFF_BACKPRESSURE is
>>> introduced instead which is required to enable the backpressure logic.
>>> This means the stopping logic in tun_net_xmit() and the waking logic in
>>> __tun_wake_queue() are skipped if the flag is disabled. Setting
>>> IFF_BACKPRESSURE makes an attached qdisc effective by stopping the queue
>>> instead of tail-dropping when the internal ring is full.
>>>
>>> To avoid a possible stall due to disabling IFF_BACKPRESSURE, the new
>>> helper tun_force_wake_queue() is implemented. The helper safely wakes the
>>> respective netdev queue and resets cons_cnt while the consumer_lock and
>>> the producer_lock of the ring are held. The helper is run in tun_attach()
>>> when a queue (re)attaches, in tun_set_iff() for attached tfiles, and
>>> in tun_queue_resize().
>>>
>>> The documentation in tuntap.rst is updated accordingly.
>>>
>>> Fixes: 1d6e569b7d0c ("tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present")
>>> Reported-by: Brett Sheffield <brett@xxxxxxxxxxxxx>
>>> Closes: https://lore.kernel.org/netdev/akVnoOYQOrt8k-Gu@xxxxxxxxxxxxxxxxxxxx/T/#u
>>> Signed-off-by: Simon Schippers <simon.schippers@xxxxxxxxxxxxxx>
>>
>>
>> The patch itself is fine:
>>
>> Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
>>
>> What bothers me is the timing: we
>> are past freeze and this new interface hasn't been tested
>> much.
>>
>> We can either apply this, or revert the original patch for now
>> and reapply with this for next.
> 
> I prefer applying it, but you maintainers must decide that.
> 

Hi,
just a gentle ping.
Has there been any decision on this patch?

Thanks!