Subject: Re: [PATCH net-next v10 1/4] tun/tap: add ptr_ring consume helper with netdev queue wakeup

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 6 May 2026 18:18:51 -0400

On Wed, May 06, 2026 at 04:10:30PM +0200, Simon Schippers wrote:
> Introduce tun_ring_consume() that wraps ptr_ring_consume() and calls
> __tun_wake_queue(). The latter wakes the stopped netdev subqueue once
> half of the ring capacity has been consumed, tracked via the new
> cons_cnt field in tun_file. cons_cnt is updated while holding the ring
> consumer lock, avoiding races. As a safety net, the queue is also woken
> when the ring becomes empty. The point is to allow the queue to be
> stopped when it gets full, which is required for traffic shaping -
> implemented by the following "avoid ptr_ring tail-drop when a qdisc
> is present". That patch also explains the pairing of the smp_mb()
> of __tun_wake_queue().
> 
> Without the corresponding queue stopping, this patch alone causes no
> regression for a tap setup sending to a qemu VM: 1.132 Mpps
> to 1.144 Mpps.
> 
> Details: AMD Ryzen 5 5600X at 4.3 GHz, 3200 MHz RAM, isolated QEMU
> threads, pktgen sender; Avg over 50 runs @ 100,000,000 packets;
> SRSO and spectre v2 mitigations disabled.
> 
> Co-developed-by: Tim Gebauer <[email-masked]>
> Signed-off-by: Tim Gebauer <[email-masked]>
> Signed-off-by: Simon Schippers <[email-masked]>
> ---
>  drivers/net/tun.c | 54 +++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 50 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index b183189f1853..00ecf128fe8e 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -145,6 +145,7 @@ struct tun_file {
>  	struct list_head next;
>  	struct tun_struct *detached;
>  	struct ptr_ring tx_ring;
> +	int cons_cnt;
>  	struct xdp_rxq_info xdp_rxq;
>  };
>  
> @@ -557,6 +558,13 @@ void tun_ptr_free(void *ptr)
>  }
>  EXPORT_SYMBOL_GPL(tun_ptr_free);
>  
> +static void tun_reset_cons_cnt(struct tun_file *tfile)
> +{
> +	spin_lock(&tfile->tx_ring.consumer_lock);
> +	tfile->cons_cnt = 0;
> +	spin_unlock(&tfile->tx_ring.consumer_lock);
> +}
> +
>  static void tun_queue_purge(struct tun_file *tfile)
>  {
>  	void *ptr;
> @@ -564,6 +572,7 @@ static void tun_queue_purge(struct tun_file *tfile)
>  	while ((ptr = ptr_ring_consume(&tfile->tx_ring)) != NULL)
>  		tun_ptr_free(ptr);
>  
> +	tun_reset_cons_cnt(tfile);
>  	skb_queue_purge(&tfile->sk.sk_write_queue);
>  	skb_queue_purge(&tfile->sk.sk_error_queue);
>  }
> @@ -730,6 +739,7 @@ static int tun_attach(struct tun_struct *tun, struct file *file,
>  		goto out;
>  	}
>  
> +	tun_reset_cons_cnt(tfile);
>  	tfile->queue_index = tun->numqueues;
>  	tfile->socket.sk->sk_shutdown &= ~RCV_SHUTDOWN;
>  
> @@ -2115,13 +2125,46 @@ static ssize_t tun_put_user(struct tun_struct *tun,
>  	return total;
>  }
>  
> -static void *tun_ring_recv(struct tun_file *tfile, int noblock, int *err)
> +/* Callers must hold ring.consumer_lock */
> +static void __tun_wake_queue(struct tun_struct *tun,
> +			     struct tun_file *tfile, int consumed)
> +{
> +	struct netdev_queue *txq = netdev_get_tx_queue(tun->dev,
> +						tfile->queue_index);
> +
> +	/* Paired with smp_mb__after_atomic() in tun_net_xmit() */
> +	smp_mb();
> +	if (netif_tx_queue_stopped(txq)) {
> +		tfile->cons_cnt += consumed;
> +		if (tfile->cons_cnt >= tfile->tx_ring.size / 2 ||
> +		    __ptr_ring_empty(&tfile->tx_ring)) {
> +			netif_tx_wake_queue(txq);
> +			tfile->cons_cnt = 0;
> +		}
> +	}
> +}
> +
> +static void *tun_ring_consume(struct tun_struct *tun, struct tun_file *tfile)
> +{
> +	void *ptr;
> +
> +	spin_lock(&tfile->tx_ring.consumer_lock);
> +	ptr = __ptr_ring_consume(&tfile->tx_ring);
> +	if (ptr)
> +		__tun_wake_queue(tun, tfile, 1);
> +
> +	spin_unlock(&tfile->tx_ring.consumer_lock);
> +	return ptr;
> +}
> +
> +static void *tun_ring_recv(struct tun_struct *tun, 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(tun, tfile);
>  	if (ptr)
>  		goto out;
>  	if (noblock) {
> @@ -2133,7 +2176,7 @@ 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(tun, tfile);
>  		if (ptr)
>  			break;
>  		if (signal_pending(current)) {

So based on commit log I expected all calls to ptr_ring_consume to
be replaced with tun_ring_consume, but it looks like tun_queue_purge
still calls ptr_ring_consume.
I suspect that together with patch 4 it can sometimes leave us stuck
with a stopped queue and an empty ring, forever.

> @@ -2170,7 +2213,7 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
>  
>  	if (!ptr) {
>  		/* Read frames from ring */
> -		ptr = tun_ring_recv(tfile, noblock, &err);
> +		ptr = tun_ring_recv(tun, tfile, noblock, &err);
>  		if (!ptr)
>  			return err;
>  	}
> @@ -3406,6 +3449,8 @@ static int tun_chr_open(struct inode *inode, struct file * file)
>  		return -ENOMEM;
>  	}
>  
> +	tun_reset_cons_cnt(tfile);
> +
>  	mutex_init(&tfile->napi_mutex);
>  	RCU_INIT_POINTER(tfile->tun, NULL);
>  	tfile->flags = 0;
> @@ -3614,6 +3659,7 @@ static int tun_queue_resize(struct tun_struct *tun)
>  	for (i = 0; i < tun->numqueues; i++) {
>  		tfile = rtnl_dereference(tun->tfiles[i]);
>  		rings[i] = &tfile->tx_ring;
> +		tun_reset_cons_cnt(tfile);
>  	}
>  	list_for_each_entry(tfile, &tun->disabled, next)
>  		rings[i++] = &tfile->tx_ring;
> -- 
> 2.43.0