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, the tun_net_xmit returns
NETDEV_TX_BUSY, again avoiding a drop. Such failures are expected because
LLTX is enabled and the transmit path operates without the usual locking.
As a result, concurrent calls to tun_net_xmit() are not prevented.
The existing __{tun,tap}_ring_consume functions free space in the
ptr_ring and wake the netdev queue. Races between this wakeup and the
queue-stop logic could leave the queue stopped indefinitely. To prevent
this, a memory barrier is enforced (as discussed in a similar
implementation in [1]), followed by a recheck that wakes the queue if
space is already available.
If no qdisc is present, the previous tail-drop behavior is preserved.
+-------------------------+-----------+---------------+----------------+
| pktgen benchmarks to | Stock | Patched with | Patched with |
| Debian VM, i5 6300HQ, | | noqueue qdisc | fq_codel qdisc |
| 10M packets | | | |
+-----------+-------------+-----------+---------------+----------------+
| TAP | Transmitted | 196 Kpps | 195 Kpps | 185 Kpps |
| +-------------+-----------+---------------+----------------+
| | Lost | 1618 Kpps | 1556 Kpps | 0 |
+-----------+-------------+-----------+---------------+----------------+
| TAP | Transmitted | 577 Kpps | 582 Kpps | 578 Kpps |
| + +-------------+-----------+---------------+----------------+
| vhost-net | Lost | 1170 Kpps | 1109 Kpps | 0 |
+-----------+-------------+-----------+---------------+----------------+
[1] Link: https://lore.kernel.org/all/[email protected]/
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 | 31 +++++++++++++++++++++++++++++--
1 file changed, 29 insertions(+), 2 deletions(-)
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 71b6981d07d7..74d7fd09e9ba 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1008,6 +1008,8 @@ 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]);
@@ -1060,13 +1062,38 @@ 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);
+ /* Avoid races with queue wake-up in
+ * __{tun,tap}_ring_consume by waking if space is
+ * available in a re-check.
+ * The barrier makes sure that the stop is visible before
+ * we re-check.
+ */
+ 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
