From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from unimail.uni-dortmund.de (mx1.hrz.uni-dortmund.de [129.217.128.51]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CA3B62D7DEA; Tue, 14 Jul 2026 07:01:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=129.217.128.51 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784012476; cv=none; b=BwA3tGNzspaVKPsek5ahQDBpkEg2Sah1mE8/rVCypOfAm2K50ruIiqsUw9n1K9f/XDM51y7tB1Uw2704yzGJX5LL91HYy4ADFuuED7ip/vnBEuPsYM1ngKB2Rc+d51PRbd/DmSP872/h7/9LRun/omrYjV3O68YiKVT+zbzyBlg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784012476; c=relaxed/simple; bh=HDLfuRPyooaC7kV9g4qUYU8Otats0QUvY0yWG5tV3m0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=mGz6/tmcRgL7QLUm43ARtSiewAaNYlvwBHF+koAGnoegFdd1dI2y9a2hyC4Gm3anolI78HaX8rs3kNtGbAp/hbJtB/uKTxdd4OfU5H1WHoOTayuN6pdl16kwt96ab9nJtRWB6sDYv94IOOtAhSgiYLeBXdiAG64zhFGIbtqZHdQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=tu-dortmund.de; spf=pass smtp.mailfrom=tu-dortmund.de; dkim=pass (1024-bit key) header.d=tu-dortmund.de header.i=@tu-dortmund.de header.b=tf0ym95v; arc=none smtp.client-ip=129.217.128.51 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=tu-dortmund.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=tu-dortmund.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=tu-dortmund.de header.i=@tu-dortmund.de header.b="tf0ym95v" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=tu-dortmund.de; s=unimail; t=1784012266; bh=Xecf86T3mJpwBCTCSSWygTpgD1boPi3ryJlIGJT4UBc=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=tf0ym95vkXa67xMlCq1kDI8Ne4JYzHmYpJLQFEsM2WwGlx2/64S28mtIAJs4VsSEY 8d4M2K9S3vWPjjXUFU9SehNnaTyHT5vvFufAFZ8npQh08EehGGPaM5AasoMyiUuwuz m63g4MvfvY6xbZJMohS28a4m4686PQ6wnooqtk/Y= Received: from [129.217.186.229] ([129.217.186.229]) (authenticated bits=0) by unimail.uni-dortmund.de (8.19.0.2/8.19.0.2) with ESMTPSA id 66E6vjsg029473 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NOT); Tue, 14 Jul 2026 08:57:45 +0200 (CEST) Message-ID: <08b841f6-2d47-4dc6-86e7-252fe7a82cc9@tu-dortmund.de> Date: Tue, 14 Jul 2026 08:57:45 +0200 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net v3] tun/tap & vhost-net: make qdisc backpressure opt-in via IFF_BACKPRESSURE To: "Michael S . Tsirkin" Cc: Simon Horman , Jonathan Corbet , Shuah Khan , Andrew Lunn , Tim Gebauer , Brett Sheffield , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Willem de Bruijn , Jason Wang , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , netdev@vger.kernel.org References: <20260709095511.168235-1-simon.schippers@tu-dortmund.de> Content-Language: en-US From: Simon Schippers In-Reply-To: <20260709095511.168235-1-simon.schippers@tu-dortmund.de> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/9/26 11:55, 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 in a IPv6 multicast testcase. 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. Setting > IFF_BACKPRESSURE makes an attached qdisc effective by stopping the queue > instead of tail-dropping when the internal ring is full. > > To avoid a possible stall due to disabling IFF_BACKPRESSURE, the new > helper tun_force_wake_queue() is implemented. The helper safely wakes the > respective netdev queue and resets cons_cnt while the consumer_lock and > the producer_lock of the ring are held. The helper is run in tun_attach() > when a queue (re)attaches, in tun_set_iff() for attached tfiles, and > in tun_queue_resize(). > > 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 > --- > V2 -> V3: > - As suggested by MST: Clarify in tuntap.rst and the UAPI header what > enabling IFF_BACKPRESSURE opts into: an attached qdisc becomes effective > instead of the driver tail-dropping when the internal ring is full. > - Avoid lines over 75 characters. > - Update comment in tun_net_xmit() to include IFF_BACKPRESSURE. > - Brett: Update in commit message that the referenced tests were TCP. > > V1 -> V2: > - Sashiko: Ensure detached queues are woken on re-attach by calling the > new tun_force_wake_queue() helper from tun_attach(), and reuse it > across the existing wake paths. > - Specify the failing test case in the commit message. > > V1: https://lore.kernel.org/netdev/20260704112058.95421-1-simon.schippers@tu-dortmund.de/T/#u > V2: https://lore.kernel.org/netdev/20260706094242.115992-1-simon.schippers@tu-dortmund.de/T/#u > > Documentation/networking/tuntap.rst | 22 +++++++++++++ > drivers/net/tun.c | 51 ++++++++++++++++++++--------- > include/uapi/linux/if_tun.h | 4 +++ > tools/include/uapi/linux/if_tun.h | 1 + > 4 files changed, 62 insertions(+), 16 deletions(-) > > diff --git a/Documentation/networking/tuntap.rst b/Documentation/networking/tuntap.rst > index 4d7087f727be..5921a924c2ae 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 > +---------------------- > + > +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, 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 ffbe6f13fb1f..5941e8f302ea 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 > > @@ -694,6 +695,20 @@ static void tun_detach_all(struct net_device *dev) > module_put(THIS_MODULE); > } > > +static void tun_force_wake_queue(struct tun_struct *tun, > + struct tun_file *tfile) > +{ > + /* Ensure that the producer can not stop the > + * queue concurrently by taking locks. > + */ > + spin_lock_bh(&tfile->tx_ring.consumer_lock); > + spin_lock(&tfile->tx_ring.producer_lock); > + netif_wake_subqueue(tun->dev, tfile->queue_index); > + tfile->cons_cnt = 0; > + spin_unlock(&tfile->tx_ring.producer_lock); > + spin_unlock_bh(&tfile->tx_ring.consumer_lock); > +} > + > static int tun_attach(struct tun_struct *tun, struct file *file, > bool skip_filter, bool napi, bool napi_frags, > bool publish_tun) > @@ -737,11 +752,9 @@ static int tun_attach(struct tun_struct *tun, struct file *file, > goto out; > } > > - spin_lock(&tfile->tx_ring.consumer_lock); > - tfile->cons_cnt = 0; > - spin_unlock(&tfile->tx_ring.consumer_lock); > tfile->queue_index = tun->numqueues; > tfile->socket.sk->sk_shutdown &= ~RCV_SHUTDOWN; > + tun_force_wake_queue(tun, tfile); > > if (tfile->detached) { > /* Re-attach detached tfile, updating XDP queue_index */ > @@ -1077,7 +1090,8 @@ 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() */ > @@ -1088,8 +1102,8 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev) > 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. > + /* 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 > @@ -2151,8 +2165,12 @@ 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(); > @@ -2764,7 +2782,7 @@ 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; > @@ -2893,8 +2911,12 @@ 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; > @@ -3693,10 +3715,7 @@ static int tun_queue_resize(struct tun_struct *tun) > if (!ret) { > for (i = 0; i < tun->numqueues; i++) { > tfile = rtnl_dereference(tun->tfiles[i]); > - spin_lock(&tfile->tx_ring.consumer_lock); > - netif_wake_subqueue(tun->dev, tfile->queue_index); > - tfile->cons_cnt = 0; > - spin_unlock(&tfile->tx_ring.consumer_lock); > + tun_force_wake_queue(tun, tfile); > } > } > > diff --git a/include/uapi/linux/if_tun.h b/include/uapi/linux/if_tun.h > index 79d53c7a1ebd..a0ddc50a7534 100644 > --- a/include/uapi/linux/if_tun.h > +++ b/include/uapi/linux/if_tun.h > @@ -69,6 +69,10 @@ > #define IFF_NAPI_FRAGS 0x0020 > /* Used in TUNSETIFF to bring up tun/tap without carrier */ > #define IFF_NO_CARRIER 0x0040 > +/* Stop the queue instead of dropping when the internal ring is full, so an > + * attached qdisc applies backpressure instead of being bypassed. > + */ > +#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 Hi Michael, WDYT? Thanks!