Message-ID: <20250924034534-mutt-send-email-mst@kernel.org>
Date: Wed, 24 Sep 2025 03:49:51 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Simon Schippers <simon.schippers@...dortmund.de>
Cc: willemdebruijn.kernel@...il.com, jasowang@...hat.com,
	eperezma@...hat.com, stephen@...workplumber.org, leiyang@...hat.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux.dev, kvm@...r.kernel.org,
	Tim Gebauer <tim.gebauer@...dortmund.de>
Subject: Re: [PATCH net-next v5 4/8] TUN & TAP: Wake netdev queue after
 consuming an entry

On Wed, Sep 24, 2025 at 09:42:45AM +0200, Simon Schippers wrote:
> On 24.09.25 08:55, Michael S. Tsirkin wrote:
> > On Wed, Sep 24, 2025 at 07:56:33AM +0200, Simon Schippers wrote:
> >> On 23.09.25 18:36, Michael S. Tsirkin wrote:
> >>> On Tue, Sep 23, 2025 at 12:15:49AM +0200, Simon Schippers wrote:
> >>>> The new wrappers tun_ring_consume/tap_ring_consume deal with consuming an
> >>>> entry of the ptr_ring and then waking the netdev queue when entries got
> >>>> invalidated to be used again by the producer.
> >>>> To avoid waking the netdev queue when the ptr_ring is full, it is checked
> >>>> if the netdev queue is stopped before invalidating entries. Like that the
> >>>> netdev queue can be safely woken after invalidating entries.
> >>>>
> >>>> The READ_ONCE in __ptr_ring_peek, paired with the smp_wmb() in
> >>>> __ptr_ring_produce within tun_net_xmit guarantees that the information
> >>>> about the netdev queue being stopped is visible after __ptr_ring_peek is
> >>>> called.
> >>>>
> >>>> The netdev queue is also woken after resizing the ptr_ring.
> >>>>
> >>>> Co-developed-by: Tim Gebauer <tim.gebauer@...dortmund.de>
> >>>> Signed-off-by: Tim Gebauer <tim.gebauer@...dortmund.de>
> >>>> Signed-off-by: Simon Schippers <simon.schippers@...dortmund.de>
> >>>> ---
> >>>>  drivers/net/tap.c | 44 +++++++++++++++++++++++++++++++++++++++++++-
> >>>>  drivers/net/tun.c | 47 +++++++++++++++++++++++++++++++++++++++++++++--
> >>>>  2 files changed, 88 insertions(+), 3 deletions(-)
> >>>>
> >>>> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
> >>>> index 1197f245e873..f8292721a9d6 100644
> >>>> --- a/drivers/net/tap.c
> >>>> +++ b/drivers/net/tap.c
> >>>> @@ -753,6 +753,46 @@ static ssize_t tap_put_user(struct tap_queue *q,
> >>>>  	return ret ? ret : total;
> >>>>  }
> >>>>  
> >>>> +static struct sk_buff *tap_ring_consume(struct tap_queue *q)
> >>>> +{
> >>>> +	struct netdev_queue *txq;
> >>>> +	struct net_device *dev;
> >>>> +	bool will_invalidate;
> >>>> +	bool stopped;
> >>>> +	void *ptr;
> >>>> +
> >>>> +	spin_lock(&q->ring.consumer_lock);
> >>>> +	ptr = __ptr_ring_peek(&q->ring);
> >>>> +	if (!ptr) {
> >>>> +		spin_unlock(&q->ring.consumer_lock);
> >>>> +		return ptr;
> >>>> +	}
> >>>> +
> >>>> +	/* Check if the queue stopped before zeroing out, so no ptr get
> >>>> +	 * produced in the meantime, because this could result in waking
> >>>> +	 * even though the ptr_ring is full.
> >>>
> >>> So what? Maybe it would be a bit suboptimal? But with your design, I do
> >>> not get what prevents this:
> >>>
> >>>
> >>> 	stopped? -> No
> >>> 		ring is stopped
> >>> 	discard
> >>>
> >>> and queue stays stopped forever
> >>>
> >>>
> >>
> >> I totally missed this (but I am not sure why it did not happen in my 
> >> testing with different ptr_ring sizes..).
> >>
> >> I guess you are right, there must be some type of locking.
> >> It probably makes sense to lock the netdev txq->_xmit_lock whenever the 
> >> consumer invalidates old ptr_ring entries (so when r->consumer_head >= 
> >> r->consumer_tail). The producer holds this lock with dev->lltx=false. Then 
> >> the consumer is able to wake the queue safely.
> >>
> >> So I would now just change the implementation to:
> >> tun_net_xmit:
> >> ...
> >> if ptr_ring_produce
> >>     // Could happen because of unproduce in vhost_net..
> >>     netif_tx_stop_queue
> >>     ...
> >>     goto drop
> >>
> >> if ptr_ring_full
> >>     netif_tx_stop_queue
> >> ...
> >>
> >> tun_ring_recv/tap_do_read (the implementation for the batched methods 
> >> would be done in the similar way):
> >> ...
> >> ptr_ring_consume
> >> if r->consumer_head >= r->consumer_tail
> >>     __netif_tx_lock_bh
> >>     netif_tx_wake_queue
> >>     __netif_tx_unlock_bh
> >>
> >> This implementation does not need any new ptr_ring helpers and no fancy 
> >> ordering tricks.
> >> Would this implementation be sufficient in your opinion?
> > 
> > 
> > Maybe you mean == ? Pls don't poke at ptr ring internals though.
> > What are we testing for here?
> > I think the point is that a batch of entries was consumed?
> > Maybe __ptr_ring_consumed_batch ? and a comment explaining
> > this returns true when last successful call to consume
> > freed up a batch of space in the ring for producer to make
> > progress.
> >
> 
> Yes, I mean ==.
> 
> Having a dedicated helper for this purpose makes sense. I just find
> the name __ptr_ring_consumed_batch a bit confusing next to
> __ptr_ring_consume_batched, since they both refer to different kinds of
> batches.

__ptr_ring_consume_created_space ?

/* Previous call to ptr_ring_consume created some space.
 *
 * NB: only refers to the last call to __ptr_ring_consume,
 * if you are calling ptr_ring_consume multiple times, you
 * have to check this multiple times.
 * Accordingly, do not use this after __ptr_ring_consume_batched.
 */

> > 
> > consumer_head == consumer_tail also happens rather a lot,
> > though thankfully not on every entry.
> > So taking tx lock each time this happens, even if queue
> > is not stopped, seems heavyweight.
> > 
> > 
> 
> Yes, I agree — but avoiding locking probably requires some fancy
> ordering tricks again..
> 
> 
> > 
> > 
> > 
> >>>> The order of the operations
> >>>> +	 * is ensured by barrier().
> >>>> +	 */
> >>>> +	will_invalidate = __ptr_ring_will_invalidate(&q->ring);
> >>>> +	if (unlikely(will_invalidate)) {
> >>>> +		rcu_read_lock();
> >>>> +		dev = rcu_dereference(q->tap)->dev;
> >>>> +		txq = netdev_get_tx_queue(dev, q->queue_index);
> >>>> +		stopped = netif_tx_queue_stopped(txq);
> >>>> +	}
> >>>> +	barrier();
> >>>> +	__ptr_ring_discard_one(&q->ring, will_invalidate);
> >>>> +
> >>>> +	if (unlikely(will_invalidate)) {
> >>>> +		if (stopped)
> >>>> +			netif_tx_wake_queue(txq);
> >>>> +		rcu_read_unlock();
> >>>> +	}
> >>>
> >>>
> >>> After an entry is consumed, you can detect this by checking
> >>>
> >>> 	                r->consumer_head >= r->consumer_tail
> >>>
> >>>
> >>> so it seems you could keep calling regular ptr_ring_consume
> >>> and check afterwards?
> >>>
> >>>
> >>>
> >>>
> >>>> +	spin_unlock(&q->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 +814,7 @@ 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) {
> >>>> @@ -1207,6 +1247,8 @@ int tap_queue_resize(struct tap_dev *tap)
> >>>>  	ret = ptr_ring_resize_multiple_bh(rings, n,
> >>>>  					  dev->tx_queue_len, GFP_KERNEL,
> >>>>  					  __skb_array_destroy_skb);
> >>>> +	if (netif_running(dev))
> >>>> +		netif_tx_wake_all_queues(dev);
> >>>>  
> >>>>  	kfree(rings);
> >>>>  	return ret;
> >>>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> >>>> index c6b22af9bae8..682df8157b55 100644
> >>>> --- a/drivers/net/tun.c
> >>>> +++ b/drivers/net/tun.c
> >>>> @@ -2114,13 +2114,53 @@ static ssize_t tun_put_user(struct tun_struct *tun,
> >>>>  	return total;
> >>>>  }
> >>>>  
> >>>> +static void *tun_ring_consume(struct tun_file *tfile)
> >>>> +{
> >>>> +	struct netdev_queue *txq;
> >>>> +	struct net_device *dev;
> >>>> +	bool will_invalidate;
> >>>> +	bool stopped;
> >>>> +	void *ptr;
> >>>> +
> >>>> +	spin_lock(&tfile->tx_ring.consumer_lock);
> >>>> +	ptr = __ptr_ring_peek(&tfile->tx_ring);
> >>>> +	if (!ptr) {
> >>>> +		spin_unlock(&tfile->tx_ring.consumer_lock);
> >>>> +		return ptr;
> >>>> +	}
> >>>> +
> >>>> +	/* Check if the queue stopped before zeroing out, so no ptr get
> >>>> +	 * produced in the meantime, because this could result in waking
> >>>> +	 * even though the ptr_ring is full. The order of the operations
> >>>> +	 * is ensured by barrier().
> >>>> +	 */
> >>>> +	will_invalidate = __ptr_ring_will_invalidate(&tfile->tx_ring);
> >>>> +	if (unlikely(will_invalidate)) {
> >>>> +		rcu_read_lock();
> >>>> +		dev = rcu_dereference(tfile->tun)->dev;
> >>>> +		txq = netdev_get_tx_queue(dev, tfile->queue_index);
> >>>> +		stopped = netif_tx_queue_stopped(txq);
> >>>> +	}
> >>>> +	barrier();
> >>>> +	__ptr_ring_discard_one(&tfile->tx_ring, will_invalidate);
> >>>> +
> >>>> +	if (unlikely(will_invalidate)) {
> >>>> +		if (stopped)
> >>>> +			netif_tx_wake_queue(txq);
> >>>> +		rcu_read_unlock();
> >>>> +	}
> >>>> +	spin_unlock(&tfile->tx_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);
> >>>>  	if (ptr)
> >>>>  		goto out;
> >>>>  	if (noblock) {
> >>>> @@ -2132,7 +2172,7 @@ 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)) {
> >>>> @@ -3621,6 +3661,9 @@ static int tun_queue_resize(struct tun_struct *tun)
> >>>>  					  dev->tx_queue_len, GFP_KERNEL,
> >>>>  					  tun_ptr_free);
> >>>>  
> >>>> +	if (netif_running(dev))
> >>>> +		netif_tx_wake_all_queues(dev);
> >>>> +
> >>>>  	kfree(rings);
> >>>>  	return ret;
> >>>>  }
> >>>> -- 
> >>>> 2.43.0
> >>>
> > 

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