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 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?

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
>