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[79.177.145.168]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-47aa0960b06sm7158230f8f.28.2026.07.04.05.28.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 04 Jul 2026 05:28:51 -0700 (PDT) Date: Sat, 4 Jul 2026 08:28:48 -0400 From: "Michael S. Tsirkin" To: Simon Schippers Cc: Willem de Bruijn , Jason Wang , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , netdev@vger.kernel.org, Simon Horman , Jonathan Corbet , Shuah Khan , Andrew Lunn , Tim Gebauer , Brett Sheffield , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH net] tun/tap & vhost-net: make qdisc backpressure opt-in via IFF_BACKPRESSURE Message-ID: <20260704082746-mutt-send-email-mst@kernel.org> References: <20260704112058.95421-1-simon.schippers@tu-dortmund.de> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260704112058.95421-1-simon.schippers@tu-dortmund.de> On Sat, Jul 04, 2026 at 01:20:58PM +0200, Simon Schippers wrote: > Commit 1d6e569b7d0c ("tun/tap & vhost-net: avoid ptr_ring tail-drop > when a qdisc is present") did not show a relevant performance regression > in my testing but on Brett Sheffield's librecast testbed it shows a > significant performance drop. The regression can be pinpointed when > multiple iperf3 TCP threads are sending. For 8 threads the performance > dropped from 13.5 Gbit/s to 9.13 Gbit/s. This is the reason why this > patch makes the qdisc backpressure behavior opt-in. > > One option to accomplish the opt-in would be to set the default qdisc to > noqueue at init. However this may also break userspace as users might > have chosen a custom qdisc even though most of the qdiscs did nothing > for tun/tap in the past due to missing backpressure... > > This is the reason why in this patch, the flag IFF_BACKPRESSURE is > introduced instead which is required to enable the backpressure logic. > This means the stopping logic in tun_net_xmit() and the waking logic in > __tun_wake_queue() are skipped if the flag is disabled. > > In tun_set_iff(), netif_tx_wake_all_queues() is replaced with looping > over all tfiles in which the netdev queues are woken and cons_cnt is > reset while the consumer_lock and producer_lock are held. This is to > ensure that tun_net_xmit() can not stop the queue concurrently, avoiding > a possible stall. > > The documentation in tuntap.rst is updated accordingly. > > Fixes: 1d6e569b7d0c ("tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present") > Reported-by: Brett Sheffield > Closes: https://lore.kernel.org/netdev/akVnoOYQOrt8k-Gu@karahi.librecast.net/T/#u > Signed-off-by: Simon Schippers I don't object to this approach. At the same time - a new UAPI outside the merge window? Is this acceptable to net maintainers? > --- > Documentation/networking/tuntap.rst | 17 +++++++++++++++++ > drivers/net/tun.c | 29 +++++++++++++++++++++++------ > include/uapi/linux/if_tun.h | 1 + > tools/include/uapi/linux/if_tun.h | 1 + > 4 files changed, 42 insertions(+), 6 deletions(-) > > diff --git a/Documentation/networking/tuntap.rst b/Documentation/networking/tuntap.rst > index 4d7087f727be..599264825dd2 100644 > --- a/Documentation/networking/tuntap.rst > +++ b/Documentation/networking/tuntap.rst > @@ -206,6 +206,23 @@ enable is true we enable it, otherwise we disable it:: > return ioctl(fd, TUNSETQUEUE, (void *)&ifr); > } > > +3.4 qdisc backpressure > +---------------------- > + > +Starting with Linux 7.2, IFF_BACKPRESSURE can be set to enable qdisc > +backpressure. Without it, TX drops occur when the internal ring buffer is > +full. With it, the kernel stops the TX queue instead, letting the qdisc > +hold packets. Drops only occur as a rare race. This can benefit protocols > +like TCP that react to drops. Backpressure requires a qdisc to be > +attached and has no effect with noqueue. > + > +The TUN/TAP ring buffer size 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, 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 ffbe6f13fb1f..3bf8a73a0816 100644 > --- a/drivers/net/tun.c > +++ b/drivers/net/tun.c > @@ -98,7 +98,8 @@ static void tun_default_link_ksettings(struct net_device *dev, > #define TUN_FASYNC IFF_ATTACH_QUEUE > > #define TUN_FEATURES (IFF_NO_PI | IFF_ONE_QUEUE | IFF_VNET_HDR | \ > - IFF_MULTI_QUEUE | IFF_NAPI | IFF_NAPI_FRAGS) > + IFF_MULTI_QUEUE | IFF_NAPI | IFF_NAPI_FRAGS | \ > + IFF_BACKPRESSURE) > > #define GOODCOPY_LEN 128 > > @@ -1077,7 +1078,8 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev) > > spin_lock(&tfile->tx_ring.producer_lock); > ret = __ptr_ring_produce(&tfile->tx_ring, skb); > - if (!qdisc_txq_has_no_queue(queue) && > + 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() */ > @@ -2151,8 +2153,12 @@ static ssize_t tun_put_user(struct tun_struct *tun, > static void __tun_wake_queue(struct tun_struct *tun, > struct tun_file *tfile, int consumed) > { > - struct netdev_queue *txq = netdev_get_tx_queue(tun->dev, > - tfile->queue_index); > + struct netdev_queue *txq; > + > + if (!(tun->flags & IFF_BACKPRESSURE)) > + return; > + > + txq = netdev_get_tx_queue(tun->dev, tfile->queue_index); > > /* Paired with smp_mb__after_atomic() in tun_net_xmit() */ > smp_mb(); > @@ -2893,8 +2899,19 @@ 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 (int i = 0; i < tun->numqueues; i++) { > + struct tun_file *i_tfile; > + > + i_tfile = rtnl_dereference(tun->tfiles[i]); > + spin_lock_bh(&i_tfile->tx_ring.consumer_lock); > + spin_lock(&i_tfile->tx_ring.producer_lock); > + netif_wake_subqueue(tun->dev, i_tfile->queue_index); > + i_tfile->cons_cnt = 0; > + spin_unlock(&i_tfile->tx_ring.producer_lock); > + spin_unlock_bh(&i_tfile->tx_ring.consumer_lock); > + } > + } > > strscpy(ifr->ifr_name, tun->dev->name); > return 0; > diff --git a/include/uapi/linux/if_tun.h b/include/uapi/linux/if_tun.h > index 79d53c7a1ebd..73a77141315c 100644 > --- a/include/uapi/linux/if_tun.h > +++ b/include/uapi/linux/if_tun.h > @@ -69,6 +69,7 @@ > #define IFF_NAPI_FRAGS 0x0020 > /* Used in TUNSETIFF to bring up tun/tap without carrier */ > #define IFF_NO_CARRIER 0x0040 > +#define IFF_BACKPRESSURE 0x0080 > #define IFF_NO_PI 0x1000 > /* This flag has no real effect */ > #define IFF_ONE_QUEUE 0x2000 > diff --git a/tools/include/uapi/linux/if_tun.h b/tools/include/uapi/linux/if_tun.h > index 2ec07de1d73b..97b670f5bc0a 100644 > --- a/tools/include/uapi/linux/if_tun.h > +++ b/tools/include/uapi/linux/if_tun.h > @@ -67,6 +67,7 @@ > #define IFF_TAP 0x0002 > #define IFF_NAPI 0x0010 > #define IFF_NAPI_FRAGS 0x0020 > +#define IFF_BACKPRESSURE 0x0080 > #define IFF_NO_PI 0x1000 > /* This flag has no real effect */ > #define IFF_ONE_QUEUE 0x2000 > -- > 2.43.0