Re: [PATCH net-next v8 4/4] tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present

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

On 3/24/26 02:47, Jason Wang wrote:
> On Thu, Mar 12, 2026 at 9:07 PM Simon Schippers
> <simon.schippers@tu-dortmund.de> 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.
>>
>> The existing __tun_wake_queue() function wakes the netdev queue. Races
>> between this wakeup and the queue-stop logic could leave the queue
>> stopped indefinitely. To prevent this, a memory barrier is enforced
>> (as discussed in a similar implementation in [1]), followed by a recheck
>> that wakes the queue if space is already available.
>>
>> If no qdisc is present, the previous tail-drop behavior is preserved.
> 
> I wonder if we need a dedicated TUN flag to enable this. With this new
> flag, we can even prevent TUN from using noqueue (not sure if it's
> possible or not).
> 

Except of the slight regressions because of this patchset I do not see
a reason for such a flag.

I have never seen that the driver prevents noqueue. For example you can
set noqueue to your ethernet interface and under load you soon get 

net_crit_ratelimited("Virtual device %s asks to queue packet!\n",
					     dev->name);

followed by a -ENETDOWN. And this is not prevented even though it is
clearly not something a user wants.

>>
>> Benchmarks:
>> The benchmarks show a slight regression in raw transmission performance,
>> though no packets are lost anymore.
>>
>> 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 20 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 | 1.151 Mpps   | 1.139 Mpps     | -1.1%    |
>> |            +-------------+--------------+----------------+----------+
>> |            | Lost/s      | 3.606 Mpps   | 0 pps          |          |
>> +------------+-------------+--------------+----------------+----------+
>> | TAP        | Transmitted | 3.948 Mpps   | 3.738 Mpps     | -5.3%    |
>> |            +-------------+--------------+----------------+----------+
>> | +vhost-net | Lost/s      | 496.5 Kpps   | 0 pps          |          |
>> +------------+-------------+--------------+----------------+----------+
>>
>> +--------------------------+--------------+----------------+----------+
>> | 2 threads                | Stock        | Patched with   | diff     |
>> | sending                  |              | fq_codel qdisc |          |
>> +------------+-------------+--------------+----------------+----------+
>> | TAP        | Transmitted | 1.133 Mpps   | 1.109 Mpps     | -2.1%    |
>> |            +-------------+--------------+----------------+----------+
>> |            | Lost/s      | 8.269 Mpps   | 0 pps          |          |
>> +------------+-------------+--------------+----------------+----------+
>> | TAP        | Transmitted | 3.820 Mpps   | 3.513 Mpps     | -8.0%    |
>> |            +-------------+--------------+----------------+----------+
>> | +vhost-net | Lost/s      | 4.961 Mpps   | 0 pps          |          |
>> +------------+-------------+--------------+----------------+----------+
>>
>> [1] Link: https://lore.kernel.org/all/20250424085358.75d817ae@kernel.org/
>>
>> 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 b86582cc6cb6..9b7daec69acd 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]);
>> @@ -1063,13 +1065,37 @@ 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) {
> 
> So, it's possible that the administrator is switching between noqueue
> and another qdisc. So ptr_ring_produce() can fail here, do we need to
> check that or not?
> 

Do you mean that? My thoughts:

Switching from noqueue to some qdisc can cause a 

net_crit_ratelimited("Virtual device %s asks to queue packet!\n",
					     dev->name);

followed by a return of -ENETDOWN in __dev_queue_xmit().
This is because tun_net_xmit detects some qdisc with 

qdisc_present = !qdisc_txq_has_no_queue(queue);

and returns NETDEV_TX_BUSY even though __dev_queue_xmit() did still
detect noqueue.

I am not sure how to solve this/if this has to be solved.
I do not see a proper way to avoid parallel execution of ndo_start_xmit
and a qdisc change (dev_graft_qdisc only takes qdisc_skb_head lock).

And from my understanding the veth implementation faces the same issue.


Switching from some qdisc to noqueue is no problem I think.

>> +               netif_tx_stop_queue(queue);
>> +               /* Avoid races with queue wake-ups in __tun_wake_queue by
>> +                * waking if space is available in a re-check.
>> +                * The barrier makes sure that the stop is visible before
>> +                * we re-check.
>> +                */
>> +               smp_mb__after_atomic();
> 
> Let's document which barrier is paired with this.
> 

I am basically copying the (old) logic of veth [1] proposed by
Jakub Kicinski. I must admit I am not 100% sure what it pairs with.

[1] Link: https://lore.kernel.org/all/20250424085358.75d817ae@kernel.org/

>> +               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
>>
> 
> Thanks
>