Re: [PATCH net] tun/tap & vhost-net: make qdisc backpressure opt-in via IFF_BACKPRESSURE

["Michael S. Tsirkin" <mst@redhat.com>]

On Sat, Jul 04, 2026 at 01:20:58PM +0200, Simon Schippers wrote:
> Commit 1d6e569b7d0c ("tun/tap & vhost-net: avoid ptr_ring tail-drop
> when a qdisc is present") did not show a relevant performance regression
> in my testing but on Brett Sheffield's librecast testbed it shows a
> significant performance drop. The regression can be pinpointed when
> multiple iperf3 TCP threads are sending. For 8 threads the performance
> dropped from 13.5 Gbit/s to 9.13 Gbit/s. This is the reason why this
> patch makes the qdisc backpressure behavior opt-in.
> 
> One option to accomplish the opt-in would be to set the default qdisc to
> noqueue at init. However this may also break userspace as users might
> have chosen a custom qdisc even though most of the qdiscs did nothing
> for tun/tap in the past due to missing backpressure...
> 
> This is the reason why in this patch, the flag IFF_BACKPRESSURE is
> introduced instead which is required to enable the backpressure logic.
> This means the stopping logic in tun_net_xmit() and the waking logic in
> __tun_wake_queue() are skipped if the flag is disabled.
> 
> In tun_set_iff(), netif_tx_wake_all_queues() is replaced with looping
> over all tfiles in which the netdev queues are woken and cons_cnt is
> reset while the consumer_lock and producer_lock are held. This is to
> ensure that tun_net_xmit() can not stop the queue concurrently, avoiding
> a possible stall.
> 
> The documentation in tuntap.rst is updated accordingly.
> 
> Fixes: 1d6e569b7d0c ("tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present")
> Reported-by: Brett Sheffield <brett@librecast.net>
> Closes: https://lore.kernel.org/netdev/akVnoOYQOrt8k-Gu@karahi.librecast.net/T/#u
> Signed-off-by: Simon Schippers <simon.schippers@tu-dortmund.de>

I don't object to this approach. At the same time - a new UAPI outside
the merge window? Is this acceptable to net maintainers?


> ---
>  Documentation/networking/tuntap.rst | 17 +++++++++++++++++
>  drivers/net/tun.c                   | 29 +++++++++++++++++++++++------
>  include/uapi/linux/if_tun.h         |  1 +
>  tools/include/uapi/linux/if_tun.h   |  1 +
>  4 files changed, 42 insertions(+), 6 deletions(-)
> 
> diff --git a/Documentation/networking/tuntap.rst b/Documentation/networking/tuntap.rst
> index 4d7087f727be..599264825dd2 100644
> --- a/Documentation/networking/tuntap.rst
> +++ b/Documentation/networking/tuntap.rst
> @@ -206,6 +206,23 @@ enable is true we enable it, otherwise we disable it::
>        return ioctl(fd, TUNSETQUEUE, (void *)&ifr);
>    }
>  
> +3.4 qdisc backpressure
> +----------------------
> +
> +Starting with Linux 7.2, IFF_BACKPRESSURE can be set to enable qdisc
> +backpressure. Without it, TX drops occur when the internal ring buffer is
> +full. With it, the kernel stops the TX queue instead, letting the qdisc
> +hold packets. Drops only occur as a rare race. This can benefit protocols
> +like TCP that react to drops. Backpressure requires a qdisc to be
> +attached and has no effect with noqueue.
> +
> +The TUN/TAP ring buffer size can be reduced alongside this flag to
> +further shift buffering into the qdisc and reduce bufferbloat, but comes
> +at possible performance cost.
> +
> +When running multiple network streams in parallel, the flag may reduce
> +performance due to the extra overhead of the backpressure mechanism.
> +
>  Universal TUN/TAP device driver Frequently Asked Question
>  =========================================================
>  
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index ffbe6f13fb1f..3bf8a73a0816 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -98,7 +98,8 @@ static void tun_default_link_ksettings(struct net_device *dev,
>  #define TUN_FASYNC	IFF_ATTACH_QUEUE
>  
>  #define TUN_FEATURES (IFF_NO_PI | IFF_ONE_QUEUE | IFF_VNET_HDR | \
> -		      IFF_MULTI_QUEUE | IFF_NAPI | IFF_NAPI_FRAGS)
> +		      IFF_MULTI_QUEUE | IFF_NAPI | IFF_NAPI_FRAGS | \
> +		      IFF_BACKPRESSURE)
>  
>  #define GOODCOPY_LEN 128
>  
> @@ -1077,7 +1078,8 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  
>  	spin_lock(&tfile->tx_ring.producer_lock);
>  	ret = __ptr_ring_produce(&tfile->tx_ring, skb);
> -	if (!qdisc_txq_has_no_queue(queue) &&
> +	if ((tun->flags & IFF_BACKPRESSURE) &&
> +	    !qdisc_txq_has_no_queue(queue) &&
>  	    __ptr_ring_check_produce(&tfile->tx_ring) == -ENOSPC) {
>  		netif_tx_stop_queue(queue);
>  		/* Paired with smp_mb() in __tun_wake_queue() */
> @@ -2151,8 +2153,12 @@ static ssize_t tun_put_user(struct tun_struct *tun,
>  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);
> +	struct netdev_queue *txq;
> +
> +	if (!(tun->flags & IFF_BACKPRESSURE))
> +		return;
> +
> +	txq = netdev_get_tx_queue(tun->dev, tfile->queue_index);
>  
>  	/* Paired with smp_mb__after_atomic() in tun_net_xmit() */
>  	smp_mb();
> @@ -2893,8 +2899,19 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
>  	/* Make sure persistent devices do not get stuck in
>  	 * xoff state.
>  	 */
> -	if (netif_running(tun->dev))
> -		netif_tx_wake_all_queues(tun->dev);
> +	if (netif_running(tun->dev)) {
> +		for (int i = 0; i < tun->numqueues; i++) {
> +			struct tun_file *i_tfile;
> +
> +			i_tfile = rtnl_dereference(tun->tfiles[i]);
> +			spin_lock_bh(&i_tfile->tx_ring.consumer_lock);
> +			spin_lock(&i_tfile->tx_ring.producer_lock);
> +			netif_wake_subqueue(tun->dev, i_tfile->queue_index);
> +			i_tfile->cons_cnt = 0;
> +			spin_unlock(&i_tfile->tx_ring.producer_lock);
> +			spin_unlock_bh(&i_tfile->tx_ring.consumer_lock);
> +		}
> +	}
>  
>  	strscpy(ifr->ifr_name, tun->dev->name);
>  	return 0;
> diff --git a/include/uapi/linux/if_tun.h b/include/uapi/linux/if_tun.h
> index 79d53c7a1ebd..73a77141315c 100644
> --- a/include/uapi/linux/if_tun.h
> +++ b/include/uapi/linux/if_tun.h
> @@ -69,6 +69,7 @@
>  #define IFF_NAPI_FRAGS	0x0020
>  /* Used in TUNSETIFF to bring up tun/tap without carrier */
>  #define IFF_NO_CARRIER	0x0040
> +#define IFF_BACKPRESSURE	0x0080
>  #define IFF_NO_PI	0x1000
>  /* This flag has no real effect */
>  #define IFF_ONE_QUEUE	0x2000
> diff --git a/tools/include/uapi/linux/if_tun.h b/tools/include/uapi/linux/if_tun.h
> index 2ec07de1d73b..97b670f5bc0a 100644
> --- a/tools/include/uapi/linux/if_tun.h
> +++ b/tools/include/uapi/linux/if_tun.h
> @@ -67,6 +67,7 @@
>  #define IFF_TAP		0x0002
>  #define IFF_NAPI	0x0010
>  #define IFF_NAPI_FRAGS	0x0020
> +#define IFF_BACKPRESSURE	0x0080
>  #define IFF_NO_PI	0x1000
>  /* This flag has no real effect */
>  #define IFF_ONE_QUEUE	0x2000
> -- 
> 2.43.0