Subject: Re: [PATCH net-next v9 4/4] tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 28 Apr 2026 08:50:21 -0400

On Tue, Apr 28, 2026 at 02:38:59PM +0200, Simon Schippers wrote:
> This commit prevents tail-drop when a qdisc is present and the ptr_ring
> becomes full. Once an entry is successfully produced and the ptr_ring
> reaches capacity, the netdev queue is stopped instead of dropping
> subsequent packets.
> 
> If producing an entry fails anyways due to a race, tun_net_xmit returns
> NETDEV_TX_BUSY, again avoiding a drop. Such races are expected because
> LLTX is enabled and the transmit path operates without the usual locking.
> 
> If no qdisc is present, the previous tail-drop behavior is preserved.
> 
> The existing __tun_wake_queue() function of the consumer races with the
> producer for waking/stopping the netdev queue: the consumer may drain
> the ring just as the producer stops the queue, leading to a permanent
> stall. To avoid this, the producer re-checks the ring after stopping
> and wakes the queue itself if space was just made. An
> smp_mb__after_atomic() is required so the re-peek of the ring sees any
> drain that the consumer performed.
> smp_mb__after_atomic() pairs with the test_and_clear_bit() inside of
> netif_wake_subqueue():
> 
> Consumer CPU                  Producer CPU
> ========================      =========================
> __ptr_ring_consume()
> netif_wake_subqueue()         netif_tx_stop_queue()
>           /\                  smp_mb__after_atomic()
>           ||                  __ptr_ring_produce_peek()
> contains RMW operation
>  test_and_clear_bit()
>           /\
>           ||
>  "Fully ordered RMW:
> smp_mb() before + after"
>     - atomic_t.txt
> 
> Benchmarks:
> The benchmarks show a slight regression in raw transmission performance,
> though no packets are lost anymore.

Could you include the packets received as well?
To demonstrate the gains/lack of loss. 

> 
> The previously introduced threshold to only wake after the queue stopped
> and half of the ring was consumed showed to be a descent choice:
> Waking the queue whenever a consume made space in the ring strongly
> degrades performance for tap, while waking only when the ring is empty
> is too late and also hurts throughput for tap & tap+vhost-net.
> Other ratios (3/4, 7/8) showed similar results (not shown here), so
> 1/2 was chosen for the sake of simplicity for both tun/tap and
> tun/tap+vhost-net.
> 
> Test setup:
> AMD Ryzen 5 5600X at 4.3 GHz, 3200 MHz RAM, isolated QEMU threads;
> Average over 50 runs @ 100,000,000 packets. SRSO and spectre v2
> mitigations disabled.
> 
> Note for tap+vhost-net:
> XDP drop program active in VM -> ~2.5x faster, slower for tap due to
> more syscalls (high utilization of entry_SYSRETQ_unsafe_stack in perf)
> 
> +--------------------------+--------------+----------------+----------+
> | 1 thread                 | Stock        | Patched with   | diff     |
> | sending                  |              | fq_codel qdisc |          |
> +------------+-------------+--------------+----------------+----------+
> | TAP        | Transmitted | 1.136 Mpps   | 1.130 Mpps     | -0.6%    |
> |            +-------------+--------------+----------------+----------+
> |            | Lost/s      | 3.758 Mpps   | 0 pps          |          |
> +------------+-------------+--------------+----------------+----------+
> | TAP        | Transmitted | 3.858 Mpps   | 3.816 Mpps     | -1.1%    |
> |            +-------------+--------------+----------------+----------+
> | +vhost-net | Lost/s      | 789.8 Kpps   | 0 pps          |          |
> +------------+-------------+--------------+----------------+----------+
> 
> +--------------------------+--------------+----------------+----------+
> | 2 threads                | Stock        | Patched with   | diff     |
> | sending                  |              | fq_codel qdisc |          |
> +------------+-------------+--------------+----------------+----------+
> | TAP        | Transmitted | 1.117 Mpps   | 1.087 Mpps     | -2.7%    |
> |            +-------------+--------------+----------------+----------+
> |            | Lost/s      | 8.476 Mpps   | 0 pps          |          |
> +------------+-------------+--------------+----------------+----------+
> | TAP        | Transmitted | 3.679 Mpps   | 3.464 Mpps     | -5.8%    |
> |            +-------------+--------------+----------------+----------+
> | +vhost-net | Lost/s      | 5.306 Mpps   | 0 pps          |          |
> +------------+-------------+--------------+----------------+----------+
> 
> 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 | 30 ++++++++++++++++++++++++++++--
>  1 file changed, 28 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index efe809597622..c2a1618cc9db 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1011,6 +1011,8 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  	struct netdev_queue *queue;
>  	struct tun_file *tfile;
>  	int len = skb->len;
> +	bool qdisc_present;
> +	int ret;
>  
>  	rcu_read_lock();
>  	tfile = rcu_dereference(tun->tfiles[txq]);
> @@ -1065,13 +1067,37 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  
>  	nf_reset_ct(skb);
>  
> -	if (ptr_ring_produce(&tfile->tx_ring, skb)) {
> +	queue = netdev_get_tx_queue(dev, txq);
> +	qdisc_present = !qdisc_txq_has_no_queue(queue);
> +
> +	spin_lock(&tfile->tx_ring.producer_lock);
> +	ret = __ptr_ring_produce(&tfile->tx_ring, skb);
> +	if (__ptr_ring_produce_peek(&tfile->tx_ring) && qdisc_present) {
> +		netif_tx_stop_queue(queue);
> +		/* Re-peek and wake if the consumer drained the ring
> +		 * concurrently in a race. smp_mb__after_atomic() pairs
> +		 * with the test_and_clear_bit() of netif_wake_subqueue()
> +		 * in __tun_wake_queue().
> +		 */
> +		smp_mb__after_atomic();
> +		if (!__ptr_ring_produce_peek(&tfile->tx_ring))
> +			netif_tx_wake_queue(queue);
> +	}
> +	spin_unlock(&tfile->tx_ring.producer_lock);
> +
> +	if (ret) {
> +		/* If a qdisc is attached to our virtual device,
> +		 * returning NETDEV_TX_BUSY is allowed.
> +		 */
> +		if (qdisc_present) {
> +			rcu_read_unlock();
> +			return NETDEV_TX_BUSY;
> +		}
>  		drop_reason = SKB_DROP_REASON_FULL_RING;
>  		goto drop;
>  	}
>  
>  	/* dev->lltx requires to do our own update of trans_start */
> -	queue = netdev_get_tx_queue(dev, txq);
>  	txq_trans_cond_update(queue);
>  
>  	/* Notify and wake up reader process */
> -- 
> 2.43.0