Re: [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 1/9/26 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.

Okay :)
> 
>>
>>>
>>>> +               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.
> 
> The problem is that, at least for TUN, the socket is loosely coupled
> with the netdev. It means the netdev can go away while the socket
> might still exist. That's why vhost only talks to the socket, not the
> netdev. If we really want to go this way, here, we should at least
> check the existence of tun->dev first.

You are right, I missed that.

> 
>>
>>>
>>>> +
>>>> +       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.
> 
> Have you run a benchmark per patch? Or it might just be because the
> regression is not obvious. But at least this patch would introduce
> more atomic operations or it might just because the TUN doesn't
> support burst so pktgen can't have the best PPS.

No, I haven't. I see your point that this patch adds an additional
atomic test_and_clear_bit() (which will always return false without a
queue stop), and I should test that.

> 
> Thanks
> 
> 
>>
>>>
>>>>         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
>>>
>>
>