Re: [PATCH net-next v9 4/4] tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present

[Michael S. Tsirkin]

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?

> 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 <tim.gebauer@xxxxxxxxxxxxxx>
> >> Signed-off-by: Tim Gebauer <tim.gebauer@xxxxxxxxxxxxxx>
> >> Signed-off-by: Simon Schippers <simon.schippers@xxxxxxxxxxxxxx>
> >> ---
> >>  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
> >