This commit prevents tail-drop when IFF_BACKPRESSURE is set, a qdisc is present and the ptr_ring becomes full. Once the ring reaches capacity after a produce attempt, the netdev queue is stopped instead of dropping subsequent packets. Without the flag, or if no qdisc is present, the previous tail-drop behavior is preserved.
The unconditional version of this behavior was reverted because it caused a significant throughput drop in an IPv6 multicast testcase on Brett Sheffield's librecast testbed: with 8 iperf3 TCP threads sending, the throughput dropped from 13.5 Gbit/s to 9.13 Gbit/s. This is why the queue stopping is now gated on IFF_BACKPRESSURE. If producing an entry fails anyway due to a race, tun_net_xmit() drops the packet. Such rare races are expected because LLTX is enabled and the transmit path operates without the usual locking. Since the flag can be cleared again by a later TUNSETIFF, tun_set_iff() calls tun_force_wake_queue() for the attached tfiles, so that no queue stays stopped without a consumer that would wake it. 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_check_produce(). - 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). The documentation in tuntap.rst is updated accordingly. Benchmarks: My own 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 | 1.123 Mpps | -0.8% | | +-------------+--------------+----------------+----------+ | | Lost/s | 3.765 Mpps | 0 pps | | +------------+-------------+--------------+----------------+----------+ | TAP | Received | 3.857 Mpps | 3.901 Mpps | +1.1% | | +-------------+--------------+----------------+----------+ | +vhost-net | Lost/s | 0.802 Mpps | 0 pps | | +------------+-------------+--------------+----------------+----------+ +--------------------------+--------------+----------------+----------+ | 2 threads | Stock | Patched with | diff | | sending | | fq_codel qdisc | | +------------+-------------+--------------+----------------+----------+ | TAP | Received | 1.115 Mpps | 1.081 Mpps | -3.0% | | +-------------+--------------+----------------+----------+ | | Lost/s | 8.490 Mpps | 391 pps | | +------------+-------------+--------------+----------------+----------+ | TAP | Received | 3.664 Mpps | 3.555 Mpps | -3.0% | | +-------------+--------------+----------------+----------+ | +vhost-net | Lost/s | 5.330 Mpps | 938 pps | | +------------+-------------+--------------+----------------+----------+ Co-developed-by: Tim Gebauer <[email protected]> Signed-off-by: Tim Gebauer <[email protected]> Link: https://lore.kernel.org/netdev/[email protected]/ Signed-off-by: Simon Schippers <[email protected]> --- Documentation/networking/tuntap.rst | 22 ++++++++++++++++++ drivers/net/tun.c | 36 +++++++++++++++++++++++++---- 2 files changed, 53 insertions(+), 5 deletions(-) diff --git a/Documentation/networking/tuntap.rst b/Documentation/networking/tuntap.rst index 4d7087f727be..04155ef98e5b 100644 --- a/Documentation/networking/tuntap.rst +++ b/Documentation/networking/tuntap.rst @@ -206,6 +206,28 @@ enable is true we enable it, otherwise we disable it:: return ioctl(fd, TUNSETQUEUE, (void *)&ifr); } +3.4 qdisc backpressure +---------------------- + +IFF_BACKPRESSURE can be set to enable qdisc backpressure. Without it, TX +drops occur when the internal ring buffer is full, so any attached qdisc +is effectively bypassed and applications only learn about congestion +through those drops. + +With it, the kernel stops instead, letting the qdisc hold and schedule +packets, so its AQM, shaping and fairness actually apply. This helps +protocols like TCP, which cut throughput in reaction to packet drops. +With IFF_BACKPRESSURE, drops then only occur as a rare race. Backpressure +requires a qdisc to be attached and has no effect with noqueue. + +The txqueuelen can be reduced alongside this flag to further shift +buffering into the qdisc and reduce bufferbloat, but comes at possible +performance cost. + +When running multiple network streams in parallel through a single +TUN/TAP queue, the flag may reduce performance due to the extra overhead +of the backpressure mechanism. + Universal TUN/TAP device driver Frequently Asked Question ========================================================= diff --git a/drivers/net/tun.c b/drivers/net/tun.c index 04275dcac456..5a927bbbda2f 100644 --- a/drivers/net/tun.c +++ b/drivers/net/tun.c @@ -1034,6 +1034,7 @@ 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]); @@ -1088,13 +1089,34 @@ 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 ((tun->flags & IFF_BACKPRESSURE) && + !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 IFF_BACKPRESSURE is enabled and + * 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 */ @@ -2767,7 +2789,7 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr) struct tun_struct *tun; struct tun_file *tfile = file->private_data; struct net_device *dev; - int err; + int err, i; if (tfile->detached) return -EINVAL; @@ -2896,8 +2918,12 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr) /* Make sure persistent devices do not get stuck in * xoff state. */ - if (netif_running(tun->dev)) - netif_tx_wake_all_queues(tun->dev); + if (netif_running(tun->dev)) { + for (i = 0; i < tun->numqueues; i++) { + tfile = rtnl_dereference(tun->tfiles[i]); + tun_force_wake_queue(tun, tfile); + } + } strscpy(ifr->ifr_name, tun->dev->name); return 0; -- 2.43.0
