Re: [PATCH net-next v10 4/4] tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, May 06, 2026 at 06:28:06PM -0400, Michael S. Tsirkin wrote:
> On Wed, May 06, 2026 at 04:10:33PM +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 no qdisc is present, the previous tail-drop
> > behavior is preserved.
> > 
> > If producing an entry fails anyways due to a race, tun_net_xmit() drops
> > the packet. Such races are expected because LLTX is enabled and the
> > transmit path operates without the usual locking.
> > 
> > The __tun_wake_queue() function of the consumer races with the producer
> > for waking/stopping the netdev queue, which could result in a stalled
> > queue. Therefore, an smp_mb__after_atomic() is introduced that pairs
> > with the smp_mb() of the consumer. It follows the principle of store
> > buffering described in tools/memory-model/Documentation/recipes.txt:
> > 
> > - The producer in tun_net_xmit() first sets __QUEUE_STATE_DRV_XOFF,
> >   followed by an smp_mb__after_atomic() (= smp_mb()), and then reads the
> >   ring with __ptr_ring_produce_peek().
> > 
> > - The consumer in __tun_wake_queue() first writes zero to the ring in
> >   __ptr_ring_consume(), followed by an smp_mb(), and then reads the queue
> >   status with netif_tx_queue_stopped().
> > 
> > => Following the aforementioned principle, it is impossible for the
> >    producer to see a full ring (and therefore not wake the queue on the
> >    re-check) while the consumer simultaneously fails to see a stopped
> >    queue (and therefore also does not wake it).
> > 
> > Benchmarks:
> > The benchmarks show a slight regression in raw transmission performance
> > when using two sending threads. Packet loss also occurs only in the
> > two-thread sending case; no packet loss was observed with a single
> > sending thread.
> > 
> > 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        | Received    | 1.132 Mpps   | 1.133 Mpps     | +0.1%    |
> > |            +-------------+--------------+----------------+----------+
> > |            | Lost/s      | 3.765 Mpps   | 0 pps          |          |
> > +------------+-------------+--------------+----------------+----------+
> > | TAP        | Received    | 3.857 Mpps   | 3.905 Mpps     | +1.2%    |
> > |            +-------------+--------------+----------------+----------+
> > | +vhost-net | Lost/s      | 0.802 Mpps   | 0 pps          |          |
> > +------------+-------------+--------------+----------------+----------+
> > 
> > +--------------------------+--------------+----------------+----------+
> > | 2 threads                | Stock        | Patched with   | diff     |
> > | sending                  |              | fq_codel qdisc |          |
> > +------------+-------------+--------------+----------------+----------+
> > | TAP        | Received    | 1.115 Mpps   | 1.092 Mpps     | -2.1%    |
> > |            +-------------+--------------+----------------+----------+
> > |            | Lost/s      | 8.490 Mpps   | 359 pps        |          |
> > +------------+-------------+--------------+----------------+----------+
> > | TAP        | Received    | 3.664 Mpps   | 3.549 Mpps     | -3.1%    |
> > |            +-------------+--------------+----------------+----------+
> > | +vhost-net | Lost/s      | 5.330 Mpps   | 832 pps        |          |
> > +------------+-------------+--------------+----------------+----------+
> > 
> > Co-developed-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
> > Signed-off-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
> > Signed-off-by: Simon Schippers <simon.schippers@tu-dortmund.de>
> > ---
> >  drivers/net/tun.c | 25 +++++++++++++++++++++++--
> >  1 file changed, 23 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index fc358c4c355b..d9ffbf88cfd8 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -1018,6 +1018,7 @@ 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;
> > +	int ret;
> >  
> >  	rcu_read_lock();
> >  	tfile = rcu_dereference(tun->tfiles[txq]);
> > @@ -1072,13 +1073,33 @@ 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);
> > +
> > +	spin_lock(&tfile->tx_ring.producer_lock);
> > +	ret = __ptr_ring_produce(&tfile->tx_ring, skb);
> > +	if (!qdisc_txq_has_no_queue(queue) &&
> > +	    (__ptr_ring_produce_peek(&tfile->tx_ring) || ret)) {
> > +		netif_tx_stop_queue(queue);
> > +		/* Paired with smp_mb() 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);
> > +
> 
> There's a weird corner case here when tx_queue_len is 0
> but a qdisc has been configured - it looks like that
> currently it just drops all packets, with this change,
> the qdisc will get stuck permanently.
> 
> I suspect just checking tx_ring.size should fix it.
> Or if you feel adventurous, change return code for __ptr_ring_produce
> to distinguish between "no ring" and "no space".


__ptr_ring_produce_peek really.


> 
> > +	if (ret) {
> > +		/* This should be a rare case if a qdisc is present, but
> > +		 * can happen due to lltx.
> > +		 * Since skb_tx_timestamp(), skb_orphan(),
> > +		 * run_ebpf_filter() and pskb_trim() could have tinkered
> > +		 * with the SKB, returning NETDEV_TX_BUSY is unsafe and
> > +		 * we must drop instead.
> > +		 */
> >  		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