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 <[email protected]> Signed-off-by: Tim Gebauer <[email protected]> Signed-off-by: Simon Schippers <[email protected]> --- 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); + + 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
