[PATCH net-next v7 3/9] tun/tap: add ptr_ring consume helper with netdev queue wakeup

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

On 1/23/26 04:05, Jason Wang wrote:
> On Thu, Jan 22, 2026 at 1:35 PM Jason Wang <jasowang@redhat.com> wrote:
>>
>> On Wed, Jan 21, 2026 at 5:33 PM Simon Schippers
>> <simon.schippers@tu-dortmund.de> wrote:
>>>
>>> On 1/9/26 07:02, Jason Wang wrote:
>>>> On Thu, Jan 8, 2026 at 3:41 PM Simon Schippers
>>>> <simon.schippers@tu-dortmund.de> wrote:
>>>>>
>>>>> On 1/8/26 04:38, Jason Wang wrote:
>>>>>> On Thu, Jan 8, 2026 at 5:06 AM Simon Schippers
>>>>>> <simon.schippers@tu-dortmund.de> wrote:
>>>>>>>
>>>>>>> Introduce {tun,tap}_ring_consume() helpers that wrap __ptr_ring_consume()
>>>>>>> and wake the corresponding netdev subqueue when consuming an entry frees
>>>>>>> space in the underlying ptr_ring.
>>>>>>>
>>>>>>> Stopping of the netdev queue when the ptr_ring is full will be introduced
>>>>>>> in an upcoming commit.
>>>>>>>
>>>>>>> Co-developed-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
>>>>>>> Signed-off-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
>>>>>>> Signed-off-by: Simon Schippers <simon.schippers@tu-dortmund.de>
>>>>>>> ---
>>>>>>>  drivers/net/tap.c | 23 ++++++++++++++++++++++-
>>>>>>>  drivers/net/tun.c | 25 +++++++++++++++++++++++--
>>>>>>>  2 files changed, 45 insertions(+), 3 deletions(-)
>>>>>>>
>>>>>>> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
>>>>>>> index 1197f245e873..2442cf7ac385 100644
>>>>>>> --- a/drivers/net/tap.c
>>>>>>> +++ b/drivers/net/tap.c
>>>>>>> @@ -753,6 +753,27 @@ static ssize_t tap_put_user(struct tap_queue *q,
>>>>>>>         return ret ? ret : total;
>>>>>>>  }
>>>>>>>
>>>>>>> +static void *tap_ring_consume(struct tap_queue *q)
>>>>>>> +{
>>>>>>> +       struct ptr_ring *ring = &q->ring;
>>>>>>> +       struct net_device *dev;
>>>>>>> +       void *ptr;
>>>>>>> +
>>>>>>> +       spin_lock(&ring->consumer_lock);
>>>>>>> +
>>>>>>> +       ptr = __ptr_ring_consume(ring);
>>>>>>> +       if (unlikely(ptr && __ptr_ring_consume_created_space(ring, 1))) {
>>>>>>> +               rcu_read_lock();
>>>>>>> +               dev = rcu_dereference(q->tap)->dev;
>>>>>>> +               netif_wake_subqueue(dev, q->queue_index);
>>>>>>> +               rcu_read_unlock();
>>>>>>> +       }
>>>>>>> +
>>>>>>> +       spin_unlock(&ring->consumer_lock);
>>>>>>> +
>>>>>>> +       return ptr;
>>>>>>> +}
>>>>>>> +
>>>>>>>  static ssize_t tap_do_read(struct tap_queue *q,
>>>>>>>                            struct iov_iter *to,
>>>>>>>                            int noblock, struct sk_buff *skb)
>>>>>>> @@ -774,7 +795,7 @@ static ssize_t tap_do_read(struct tap_queue *q,
>>>>>>>                                         TASK_INTERRUPTIBLE);
>>>>>>>
>>>>>>>                 /* Read frames from the queue */
>>>>>>> -               skb = ptr_ring_consume(&q->ring);
>>>>>>> +               skb = tap_ring_consume(q);
>>>>>>>                 if (skb)
>>>>>>>                         break;
>>>>>>>                 if (noblock) {
>>>>>>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>>>>>>> index 8192740357a0..7148f9a844a4 100644
>>>>>>> --- a/drivers/net/tun.c
>>>>>>> +++ b/drivers/net/tun.c
>>>>>>> @@ -2113,13 +2113,34 @@ static ssize_t tun_put_user(struct tun_struct *tun,
>>>>>>>         return total;
>>>>>>>  }
>>>>>>>
>>>>>>> +static void *tun_ring_consume(struct tun_file *tfile)
>>>>>>> +{
>>>>>>> +       struct ptr_ring *ring = &tfile->tx_ring;
>>>>>>> +       struct net_device *dev;
>>>>>>> +       void *ptr;
>>>>>>> +
>>>>>>> +       spin_lock(&ring->consumer_lock);
>>>>>>> +
>>>>>>> +       ptr = __ptr_ring_consume(ring);
>>>>>>> +       if (unlikely(ptr && __ptr_ring_consume_created_space(ring, 1))) {
>>>>>>
>>>>>> I guess it's the "bug" I mentioned in the previous patch that leads to
>>>>>> the check of __ptr_ring_consume_created_space() here. If it's true,
>>>>>> another call to tweak the current API.
>>>>>>
>>>>>>> +               rcu_read_lock();
>>>>>>> +               dev = rcu_dereference(tfile->tun)->dev;
>>>>>>> +               netif_wake_subqueue(dev, tfile->queue_index);
>>>>>>
>>>>>> This would cause the producer TX_SOFTIRQ to run on the same cpu which
>>>>>> I'm not sure is what we want.
>>>>>
>>>>> What else would you suggest calling to wake the queue?
>>>>
>>>> I don't have a good method in my mind, just want to point out its implications.
>>>
>>> I have to admit I'm a bit stuck at this point, particularly with this
>>> aspect.
>>>
>>> What is the correct way to pass the producer CPU ID to the consumer?
>>> Would it make sense to store smp_processor_id() in the tfile inside
>>> tun_net_xmit(), or should it instead be stored in the skb (similar to the
>>> XDP bit)? In the latter case, my concern is that this information may
>>> already be significantly outdated by the time it is used.
>>>
>>> Based on that, my idea would be for the consumer to wake the producer by
>>> invoking a new function (e.g., tun_wake_queue()) on the producer CPU via
>>> smp_call_function_single().
>>> Is this a reasonable approach?
>>
>> I'm not sure but it would introduce costs like IPI.
>>
>>>
>>> More generally, would triggering TX_SOFTIRQ on the consumer CPU be
>>> considered a deal-breaker for the patch set?
>>
>> It depends on whether or not it has effects on the performance.
>> Especially when vhost is pinned.
> 
> I meant we can benchmark to see the impact. For example, pin vhost to
> a specific CPU and the try to see the impact of the TX_SOFTIRQ.
> 
> Thanks
> 

I ran benchmarks with vhost pinned to CPU 0 using taskset -p -c 0 ...
for both the stock and patched versions. The benchmarks were run with
the full patch series applied, since testing only patches 1-3 would not
be meaningful - the queue is never stopped in that case, so no
TX_SOFTIRQ is triggered.

Compared to the non-pinned CPU benchmarks in the cover letter,
performance is lower for pktgen with a single thread but higher with
four threads. The results show no regression for the patched version,
with even slight performance improvements observed:

+-------------------------+-----------+----------------+
| pktgen benchmarks to    | Stock     | Patched with   |
| Debian VM, i5 6300HQ,   |           | fq_codel qdisc |
| 100M packets            |           |                |
| vhost pinned to core 0  |           |                |
+-----------+-------------+-----------+----------------+
| TAP       | Transmitted | 452 Kpps  | 454 Kpps       |
|  +        +-------------+-----------+----------------+
| vhost-net | Lost        | 1154 Kpps | 0              |
+-----------+-------------+-----------+----------------+

+-------------------------+-----------+----------------+
| pktgen benchmarks to    | Stock     | Patched with   |
| Debian VM, i5 6300HQ,   |           | fq_codel qdisc |
| 100M packets            |           |                |
| vhost pinned to core 0  |           |                |
| *4 threads*             |           |                |
+-----------+-------------+-----------+----------------+
| TAP       | Transmitted | 71 Kpps   | 79 Kpps        |
|  +        +-------------+-----------+----------------+
| vhost-net | Lost        | 1527 Kpps | 0              |
+-----------+-------------+-----------+----------------+

+------------------------+-------------+----------------+
| iperf3 TCP benchmarks  | Stock       | Patched with   |
| to Debian VM 120s      |             | fq_codel qdisc |
| vhost pinned to core 0 |             |                |
+------------------------+-------------+----------------+
| TAP                    | 22.0 Gbit/s | 22.0 Gbit/s    |
|  +                     |             |                |
| vhost-net              |             |                |
+------------------------+-------------+----------------+

+---------------------------+-------------+----------------+
| iperf3 TCP benchmarks     | Stock       | Patched with   |
| to Debian VM 120s         |             | fq_codel qdisc |
| vhost pinned to core 0    |             |                |
| *4 iperf3 client threads* |             |                |
+---------------------------+-------------+----------------+
| TAP                       | 21.4 Gbit/s | 21.5 Gbit/s    |
|  +                        |             |                |
| vhost-net                 |             |                |
+---------------------------+-------------+----------------+