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 10:32:16 -0400

On Tue, Apr 28, 2026 at 04:18:54PM +0200, Simon Schippers wrote:
> On 4/28/26 16:10, Michael S. Tsirkin wrote:
> > 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 just do use the sample script [1]:
> 
> ./pktgen_sample02_multiqueue.sh -n 100000000 ...
> 
> ... and this runs until 100_000_000 packets were sucessfully
> transmitted, independently of the lost packets/errors.
> 
> [1] Link: https://www.kernel.org/doc/html/latest/networking/pktgen.html#sample-scripts

Confused. Are you saying "transmitted" is actually "received"? And the #
of packets sent is Transmitted + Lost?

> > 
> > 
> >>>
> >>>> 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
> >>>>>
> >>>
> >