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 <[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 >>>>> >>> >
