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 10:10:05 -0400

On Tue, Apr 28, 2026 at 03:41:20PM +0200, Simon Schippers wrote:
> On 4/28/26 15:22, Michael S. Tsirkin wrote:
> > On Tue, Apr 28, 2026 at 03:10:44PM +0200, Simon Schippers wrote:
> >> On 4/28/26 14:50, Michael S. Tsirkin wrote:
> >>> 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. 
> >>>
> >>
> >> Do you mean the number of packets received by the VM?
> >> They should just be the same as the number sent (shown below), right?
> > 
> > Minus the loss? Which this is about, right?
> 
> Yes. I simply calculated "Lost/s":
> 
> elapsed_time = 100e6 / sent_pps
> Lost/s = total_errors / elapsed_time
> 
> 
> To get back total_errors for example for TAP
> 1 thread sending:
> 
> elapsed_time = 100e6 / 1.136Mpps = 88s
> 
> 3758 Mpps = total_errors / 88s
> <=> total_errors = 331 million packets
> 
> So, out of 431 million packets sent, 100 million were successfully
> delivered and 331 million were lost.

That is my issue.

I kind of have trouble mapping that to the table below.
For example:

 | TAP        | Transmitted | 1.136 Mpps   | 1.130 Mpps     | -0.6%    |
 |            +-------------+--------------+----------------+----------+
 |            | Lost/s      | 3.758 Mpps   | 0 pps          |          |

how can # of lost packets exceed the # of transmitted packets?

Thanks!

> > 
> >> I assume they would be visible as RX-DRP for TAP.
> >> For TAP + vhost-net I would have to rewrite the XDP drop
> >> program to count the number of dropped packets...
> >> And I would have to automate it...
> >>
> >>>>
> >>>> 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
> >>>
> >