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 5BF2D359A66; Tue, 24 Mar 2026 10:15:10 +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=1774347314; cv=none; b=VJoUAJzJVJb5/4p1PU85wdWnRdDMNdCXNtxbE+/QdANzNxfFLys/6Stca6mlPceP6O6hcoDWqXuyN5T+V1wZh+2oIA1h5juSmGtEd6ga58j8LHeu/v5+63ljbj4xQ3aZyS9SKiXniRUHw28NohoRBC1O7VipDRDUmnRzXbD7cTQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774347314; c=relaxed/simple; bh=EeWxqb0HgRENSXG2d8Fk2RYIq4kncRyI4/Iu6SENreg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=HUYyLcDG0CrLvFJYF0AGd4Oq3esyRgisFIIlZ25AFNvgtyY5ctTchUpLK+clNatKZv2g352G8BO7Sb/KkO5JnOrhX2PkS6fAt0czqTQOG8cK7FE72NeWF0EXW3AA0yGLz2kXC3fvJMs2n7S+Tb1vPf/hM7h9WnVS6mf1pmOWsCg= 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=RM7R4tQr; 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="RM7R4tQr" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=tu-dortmund.de; s=unimail; t=1774347297; bh=EeWxqb0HgRENSXG2d8Fk2RYIq4kncRyI4/Iu6SENreg=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=RM7R4tQri46kneVGg7+K2sNbwiOtJh+Yany9dmcKhLoJf52nTRyXCZUWuKx1wEwBU aD3P0rpfqGS2eriGMQys7f/VbAkAArMFE6Wk6g5Nssf9y7T1y8b7UVWK4BGk/+KlQW wi+9RO07qPOuv7I/xxaeR/83bB5qH35m6DHLUcPc= Received: from [192.168.178.143] (p5dc880d2.dip0.t-ipconnect.de [93.200.128.210]) (authenticated bits=0) by unimail.uni-dortmund.de (8.18.2/8.18.2) with ESMTPSA id 62OAEtkX003138 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NOT); Tue, 24 Mar 2026 11:14:56 +0100 (CET) Message-ID: <0908392d-6314-4141-b908-6c9a880ba0a4@tu-dortmund.de> Date: Tue, 24 Mar 2026 11:14:55 +0100 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-next v8 4/4] tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present To: Jason Wang 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 References: <20260312130639.138988-1-simon.schippers@tu-dortmund.de> <20260312130639.138988-5-simon.schippers@tu-dortmund.de> Content-Language: en-US From: Simon Schippers In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 3/24/26 02:47, Jason Wang wrote: > On Thu, Mar 12, 2026 at 9:07 PM 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). > Except of the slight regressions because of this patchset I do not see a reason for such a flag. I have never seen that the driver prevents noqueue. For example you can set noqueue to your ethernet interface and under load you soon get net_crit_ratelimited("Virtual device %s asks to queue packet!\n", dev->name); followed by a -ENETDOWN. And this is not prevented even though it is clearly not something a user wants. >> >> 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 = skb->len; >> + bool qdisc_present; >> + int ret; >> >> rcu_read_lock(); >> tfile = rcu_dereference(tun->tfiles[txq]); >> @@ -1063,13 +1065,37 @@ 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); >> + qdisc_present = !qdisc_txq_has_no_queue(queue); >> + >> + spin_lock(&tfile->tx_ring.producer_lock); >> + ret = __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? > Do you mean that? My thoughts: Switching from noqueue to some qdisc can cause a net_crit_ratelimited("Virtual device %s asks to queue packet!\n", dev->name); followed by a return of -ENETDOWN in __dev_queue_xmit(). This is because tun_net_xmit detects some qdisc with qdisc_present = !qdisc_txq_has_no_queue(queue); and returns NETDEV_TX_BUSY even though __dev_queue_xmit() did still detect noqueue. I am not sure how to solve this/if this has to be solved. I do not see a proper way to avoid parallel execution of ndo_start_xmit and a qdisc change (dev_graft_qdisc only takes qdisc_skb_head lock). And from my understanding the veth implementation faces the same issue. Switching from some qdisc to noqueue is no problem I think. >> + 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. > I am basically copying the (old) logic of veth [1] proposed by Jakub Kicinski. I must admit I am not 100% sure what it pairs with. [1] Link: https://lore.kernel.org/all/20250424085358.75d817ae@kernel.org/ >> + 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 = 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 >> > > Thanks >