On 4/28/26 16:32, Michael S. Tsirkin wrote: > 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?
Sorry for my confusing answer. Yes, "transmitted" in the table should be changed to "received". And yes, as you said, the real # transmitted then is: Received + Lost = 1.136 + 3.758 = 4.894 Mpps. > >>> >>> >>>>> >>>>>> 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 >>>>>>> >>>>> >>> >
