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

[Simon Schippers]

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

> 
> 
>>>
>>>> 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 <tim.gebauer@xxxxxxxxxxxxxx>
>>>>>> Signed-off-by: Tim Gebauer <tim.gebauer@xxxxxxxxxxxxxx>
>>>>>> Signed-off-by: Simon Schippers <simon.schippers@xxxxxxxxxxxxxx>
>>>>>> ---
>>>>>>  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
>>>>>
>>>
>