[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 1/8/26 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?

> 
>> +               rcu_read_unlock();
>> +       }
> 
> Btw, this function duplicates a lot of logic of tap_ring_consume() we
> should consider to merge the logic.

Yes, it is largely the same approach, but it would require accessing the
net_device each time.

> 
>> +
>> +       spin_unlock(&ring->consumer_lock);
>> +
>> +       return ptr;
>> +}
>> +
>>  static void *tun_ring_recv(struct tun_file *tfile, int noblock, int *err)
>>  {
>>         DECLARE_WAITQUEUE(wait, current);
>>         void *ptr = NULL;
>>         int error = 0;
>>
>> -       ptr = ptr_ring_consume(&tfile->tx_ring);
>> +       ptr = tun_ring_consume(tfile);
> 
> I'm not sure having a separate patch like this may help. For example,
> it will introduce performance regression.

I ran benchmarks for the whole patch set with noqueue (where the queue is
not stopped to preserve the old behavior), as described in the cover
letter, and observed no performance regression. This leads me to conclude
that there is no performance impact because of this patch when the queue
is not stopped.

> 
>>         if (ptr)
>>                 goto out;
>>         if (noblock) {
>> @@ -2131,7 +2152,7 @@ static void *tun_ring_recv(struct tun_file *tfile, int noblock, int *err)
>>
>>         while (1) {
>>                 set_current_state(TASK_INTERRUPTIBLE);
>> -               ptr = ptr_ring_consume(&tfile->tx_ring);
>> +               ptr = tun_ring_consume(tfile);
>>                 if (ptr)
>>                         break;
>>                 if (signal_pending(current)) {
>> --
>> 2.43.0
>>
> 
> Thanks
>