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Mon, 23 Mar 2026 18:47:56 -0700 (PDT) X-Received: by 2002:a17:90b:4c52:b0:35b:e529:2d60 with SMTP id 98e67ed59e1d1-35be5292d67mr7009867a91.8.1774316875980; Mon, 23 Mar 2026 18:47:55 -0700 (PDT) Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <20260312130639.138988-1-simon.schippers@tu-dortmund.de> <20260312130639.138988-5-simon.schippers@tu-dortmund.de> In-Reply-To: <20260312130639.138988-5-simon.schippers@tu-dortmund.de> From: Jason Wang Date: Tue, 24 Mar 2026 09:47:43 +0800 X-Gm-Features: AaiRm53PVQkrGcklsZUJS-7XPhfayXnRAumXm5C_-oefvGMMXn3FtKJ6Q_kdnQ0 Message-ID: Subject: Re: [PATCH net-next v8 4/4] tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present To: Simon Schippers Cc: willemdebruijn.kernel@gmail.com, andrew+netdev@lunn.ch, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, mst@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 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: B1YLUlRlyF7uW6RTosdb59k9e9xpSaNg0gsszPUTko4_1774316876 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, Mar 12, 2026 at 9:07=E2=80=AFPM 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 producing an entry fails anyways due to a race, tun_net_xmit returns > NETDEV_TX_BUSY, again avoiding a drop. Such races are expected because > LLTX is enabled and the transmit path operates without the usual locking. > > The existing __tun_wake_queue() function wakes 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. I wonder if we need a dedicated TUN flag to enable this. With this new flag, we can even prevent TUN from using noqueue (not sure if it's possible or not). > > Benchmarks: > The benchmarks show a slight regression in raw transmission performance, > though no packets are lost anymore. > > The previously introduced threshold to only wake after the queue stopped > and half of the ring was consumed showed to be a descent choice: > Waking the queue whenever a consume made space in the ring strongly > degrades performance for tap, while waking only when the ring is empty > is too late and also hurts throughput for tap & tap+vhost-net. > Other ratios (3/4, 7/8) showed similar results (not shown here), so > 1/2 was chosen for the sake of simplicity for both tun/tap and > tun/tap+vhost-net. > > Test setup: > AMD Ryzen 5 5600X at 4.3 GHz, 3200 MHz RAM, isolated QEMU threads; > Average over 20 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 | Transmitted | 1.151 Mpps | 1.139 Mpps | -1.1% | > | +-------------+--------------+----------------+----------+ > | | Lost/s | 3.606 Mpps | 0 pps | | > +------------+-------------+--------------+----------------+----------+ > | TAP | Transmitted | 3.948 Mpps | 3.738 Mpps | -5.3% | > | +-------------+--------------+----------------+----------+ > | +vhost-net | Lost/s | 496.5 Kpps | 0 pps | | > +------------+-------------+--------------+----------------+----------+ > > +--------------------------+--------------+----------------+----------+ > | 2 threads | Stock | Patched with | diff | > | sending | | fq_codel qdisc | | > +------------+-------------+--------------+----------------+----------+ > | TAP | Transmitted | 1.133 Mpps | 1.109 Mpps | -2.1% | > | +-------------+--------------+----------------+----------+ > | | Lost/s | 8.269 Mpps | 0 pps | | > +------------+-------------+--------------+----------------+----------+ > | TAP | Transmitted | 3.820 Mpps | 3.513 Mpps | -8.0% | > | +-------------+--------------+----------------+----------+ > | +vhost-net | Lost/s | 4.961 Mpps | 0 pps | | > +------------+-------------+--------------+----------------+----------+ > > [1] Link: https://lore.kernel.org/all/20250424085358.75d817ae@kernel.org/ > > Co-developed-by: Tim Gebauer > Signed-off-by: Tim Gebauer > Signed-off-by: Simon Schippers > --- > drivers/net/tun.c | 30 ++++++++++++++++++++++++++++-- > 1 file changed, 28 insertions(+), 2 deletions(-) > > diff --git a/drivers/net/tun.c b/drivers/net/tun.c > index b86582cc6cb6..9b7daec69acd 100644 > --- a/drivers/net/tun.c > +++ b/drivers/net/tun.c > @@ -1011,6 +1011,8 @@ 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 =3D skb->len; > + bool qdisc_present; > + int ret; > > rcu_read_lock(); > tfile =3D rcu_dereference(tun->tfiles[txq]); > @@ -1063,13 +1065,37 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *s= kb, struct net_device *dev) > > nf_reset_ct(skb); > > - if (ptr_ring_produce(&tfile->tx_ring, skb)) { > + queue =3D netdev_get_tx_queue(dev, txq); > + qdisc_present =3D !qdisc_txq_has_no_queue(queue); > + > + spin_lock(&tfile->tx_ring.producer_lock); > + ret =3D __ptr_ring_produce(&tfile->tx_ring, skb); > + if (__ptr_ring_produce_peek(&tfile->tx_ring) && qdisc_present) { So, it's possible that the administrator is switching between noqueue and another qdisc. So ptr_ring_produce() can fail here, do we need to check that or not? > + netif_tx_stop_queue(queue); > + /* Avoid races with queue wake-ups in __tun_wake_queue 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(); Let's document which barrier is paired with this. > + 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 =3D SKB_DROP_REASON_FULL_RING; > goto drop; > } > > /* dev->lltx requires to do our own update of trans_start */ > - queue =3D netdev_get_tx_queue(dev, txq); > txq_trans_cond_update(queue); > > /* Notify and wake up reader process */ > -- > 2.43.0 > Thanks