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Revert "tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present"

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This reverts commit 1d6e569b7d.

The commit stops the netdev queue when the ptr_ring is full instead of
dropping the packet. My own tests showed no relevant regression, but on
Brett Sheffield's librecast testbed an IPv6 multicast testcase got
slower. With 8 iperf3 TCP threads sending, the throughput dropped from
13.5 Gbit/s to 9.13 Gbit/s.

Reported-by: Brett Sheffield <brett@librecast.net>
Closes: https://lore.kernel.org/netdev/akVnoOYQOrt8k-Gu@karahi.librecast.net/
Signed-off-by: Simon Schippers <simon.schippers@tu-dortmund.de>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://patch.msgid.link/20260728092240.250257-2-simon.schippers@tu-dortmund.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Simon Schippers
2026-07-29 17:12:26 -07:00
committed by Jakub Kicinski
parent 82f0e98ba3
commit f11b48aa67
1 changed files with 2 additions and 23 deletions
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drivers/net/tun.c
+2 -23
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@@ -1018,7 +1018,6 @@ 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]);
@@ -1073,33 +1072,13 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
nf_reset_ct(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_check_produce(&tfile->tx_ring) == -ENOSPC) {
netif_tx_stop_queue(queue);
/* Paired with smp_mb() in __tun_wake_queue() */
smp_mb__after_atomic();
if (!__ptr_ring_check_produce(&tfile->tx_ring))
netif_tx_wake_queue(queue);
}
spin_unlock(&tfile->tx_ring.producer_lock);
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.
*/
if (ptr_ring_produce(&tfile->tx_ring, skb)) {
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 */