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

[Simon Schippers <simon.schippers@tu-dortmund.de>]

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.

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