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. > >> 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 >>> >
