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[80.230.48.7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48e538aaa0fsm97231005e9.7.2026.05.06.15.28.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 06 May 2026 15:28:06 -0700 (PDT) Date: Wed, 6 May 2026 18:28:02 -0400 From: "Michael S. Tsirkin" To: Simon Schippers Cc: willemdebruijn.kernel@gmail.com, jasowang@redhat.com, andrew+netdev@lunn.ch, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, eperezma@redhat.com, leiyang@redhat.com, stephen@networkplumber.org, jon@nutanix.com, tim.gebauer@tu-dortmund.de, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux.dev Subject: Re: [PATCH net-next v10 4/4] tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present Message-ID: <20260506181909-mutt-send-email-mst@kernel.org> References: <20260506141033.180450-1-simon.schippers@tu-dortmund.de> <20260506141033.180450-5-simon.schippers@tu-dortmund.de> Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <20260506141033.180450-5-simon.schippers@tu-dortmund.de> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: KBXm5NbRm7ZNlm9qqaIY8bIAOidijvRssPMFJ3K3I5U_1778106487 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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 > Signed-off-by: Tim Gebauer > Signed-off-by: Simon Schippers > --- > 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". > + 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