Subject: Re: [PATCH net-next v8 2/4] vhost-net: wake queue of tun/tap after ptr_ring consume

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 12 Mar 2026 09:54:49 -0400

On Thu, Mar 12, 2026 at 02:06:37PM +0100, Simon Schippers wrote:
> Add tun_wake_queue() to tun.c and export it for use by vhost-net. The
> function validates that the file belongs to a tun/tap device,
> dereferences the tun_struct under RCU, and delegates to
> __tun_wake_queue().
> 
> vhost_net_buf_produce() now calls tun_wake_queue() after a successful
> batched consume of the ring to allow the netdev subqueue to be woken up.

A sentence missing here:
the point is to allow 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"

> Without the corresponding queue stopping (introduced in a subsequent
> commit), this patch alone causes a slight throughput regression for a
> tap+vhost-net setup sending to a qemu VM:
> 3.948 Mpps to 3.888 Mpps (-1.5%).
> 
> Details: AMD Ryzen 5 5600X at 4.3 GHz, 3200 MHz RAM, isolated QEMU
> threads, XDP drop program active in VM, pktgen sender; Avg over
> 20 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      | 21 +++++++++++++++++++++
>  drivers/vhost/net.c    | 15 +++++++++++----
>  include/linux/if_tun.h |  3 +++
>  3 files changed, 35 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index a82d665dab5f..b86582cc6cb6 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -3760,6 +3760,27 @@ struct ptr_ring *tun_get_tx_ring(struct file *file)
>  }
>  EXPORT_SYMBOL_GPL(tun_get_tx_ring);
>  
> +void tun_wake_queue(struct file *file)
> +{
> +	struct tun_file *tfile;
> +	struct tun_struct *tun;
> +
> +	if (file->f_op != &tun_fops)
> +		return;
> +	tfile = file->private_data;
> +	if (!tfile)
> +		return;
> +
> +	rcu_read_lock();
> +
> +	tun = rcu_dereference(tfile->tun);
> +	if (tun)
> +		__tun_wake_queue(tun, tfile);
> +
> +	rcu_read_unlock();
> +}
> +EXPORT_SYMBOL_GPL(tun_wake_queue);
> +
>  module_init(tun_init);
>  module_exit(tun_cleanup);
>  MODULE_DESCRIPTION(DRV_DESCRIPTION);
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 80965181920c..c8ef804ef28c 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -176,13 +176,19 @@ static void *vhost_net_buf_consume(struct vhost_net_buf *rxq)
>  	return ret;
>  }
>  
> -static int vhost_net_buf_produce(struct vhost_net_virtqueue *nvq)
> +static int vhost_net_buf_produce(struct sock *sk,
> +				 struct vhost_net_virtqueue *nvq)
>  {
> +	struct file *file = sk->sk_socket->file;
>  	struct vhost_net_buf *rxq = &nvq->rxq;
>  
>  	rxq->head = 0;
>  	rxq->tail = ptr_ring_consume_batched(nvq->rx_ring, rxq->queue,
>  					      VHOST_NET_BATCH);
> +
> +	if (rxq->tail)
> +		tun_wake_queue(file);
> +
>  	return rxq->tail;
>  }
>  
> @@ -209,14 +215,15 @@ static int vhost_net_buf_peek_len(void *ptr)
>  	return __skb_array_len_with_tag(ptr);
>  }
>  
> -static int vhost_net_buf_peek(struct vhost_net_virtqueue *nvq)
> +static int vhost_net_buf_peek(struct sock *sk,
> +			      struct vhost_net_virtqueue *nvq)
>  {
>  	struct vhost_net_buf *rxq = &nvq->rxq;
>  
>  	if (!vhost_net_buf_is_empty(rxq))
>  		goto out;
>  
> -	if (!vhost_net_buf_produce(nvq))
> +	if (!vhost_net_buf_produce(sk, nvq))
>  		return 0;
>  
>  out:
> @@ -995,7 +1002,7 @@ static int peek_head_len(struct vhost_net_virtqueue *rvq, struct sock *sk)
>  	unsigned long flags;
>  
>  	if (rvq->rx_ring)
> -		return vhost_net_buf_peek(rvq);
> +		return vhost_net_buf_peek(sk, rvq);
>  
>  	spin_lock_irqsave(&sk->sk_receive_queue.lock, flags);
>  	head = skb_peek(&sk->sk_receive_queue);
> diff --git a/include/linux/if_tun.h b/include/linux/if_tun.h
> index 80166eb62f41..ab3b4ebca059 100644
> --- a/include/linux/if_tun.h
> +++ b/include/linux/if_tun.h
> @@ -22,6 +22,7 @@ struct tun_msg_ctl {
>  #if defined(CONFIG_TUN) || defined(CONFIG_TUN_MODULE)
>  struct socket *tun_get_socket(struct file *);
>  struct ptr_ring *tun_get_tx_ring(struct file *file);
> +void tun_wake_queue(struct file *file);
>  
>  static inline bool tun_is_xdp_frame(void *ptr)
>  {
> @@ -55,6 +56,8 @@ static inline struct ptr_ring *tun_get_tx_ring(struct file *f)
>  	return ERR_PTR(-EINVAL);
>  }
>  
> +static inline void tun_wake_queue(struct file *f) {}
> +
>  static inline bool tun_is_xdp_frame(void *ptr)
>  {
>  	return false;
> -- 
> 2.43.0