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 protected]> > >>>>>> Signed-off-by: Tim Gebauer <[email protected]> > >>>>>> Signed-off-by: Simon Schippers <[email protected]> > >>>>>> --- > >>>>>> 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 > >>>>> > >>> > >
