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 5D72837267C; Tue, 28 Apr 2026 14:19:07 +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=1777385955; cv=none; b=gKx9u+A8kjU85p/16FxZe/0+ArJdgVcZSzKjtoFzZgQPUrFr6n3QeyIzj92j1OM2woppRXAtiY9XiouzrJxFJoaVFmAU76a6etjyM0GDN2PBWS8eGxPXmlTXmL08pv/2Pn6ZT6dNhMAgFTK1BCPki4JDsVi+kxgDF2iBBMdR7UQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777385955; c=relaxed/simple; bh=9SRwVzIOSqtIlM7CMwZtUX2d9LlVYwl6nFJymCoJbrI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=BD624h0g40LYJX7ATpDb/KH1uhq6NAlNFv6InbDpeTh7mRPM3ffDWndiYiPZ/O0QGhje1Po5hjZDACHfRrP/+gLS/Q7Z2E2NwdxqTd0i9elBnaQUanzyyDr8gNERP36wTu+rwoSmU5IsDgOQBghsEt75PyU+M9x9J6ZyP91q4hs= 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; 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 Received: from [129.217.186.62] ([129.217.186.62]) (authenticated bits=0) by unimail.uni-dortmund.de (8.18.2/8.18.2) with ESMTPSA id 63SEItGP029771 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NOT); Tue, 28 Apr 2026 16:18:55 +0200 (CEST) Message-ID: Date: Tue, 28 Apr 2026 16:18:54 +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-next v9 4/4] tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present To: "Michael S. Tsirkin" 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 References: <20260428123859.19578-1-simon.schippers@tu-dortmund.de> <20260428123859.19578-5-simon.schippers@tu-dortmund.de> <20260428084851-mutt-send-email-mst@kernel.org> <20260428092150-mutt-send-email-mst@kernel.org> <20260428100731-mutt-send-email-mst@kernel.org> Content-Language: en-US From: Simon Schippers In-Reply-To: <20260428100731-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/28/26 16:10, Michael S. Tsirkin wrote: > On Tue, Apr 28, 2026 at 03:41:20PM +0200, Simon Schippers wrote: >> On 4/28/26 15:22, Michael S. Tsirkin wrote: >>> On Tue, Apr 28, 2026 at 03:10:44PM +0200, Simon Schippers wrote: >>>> On 4/28/26 14:50, Michael S. Tsirkin wrote: >>>>> On Tue, Apr 28, 2026 at 02:38:59PM +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 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. >>>>>> >>>>>> If no qdisc is present, the previous tail-drop behavior is preserved. >>>>>> >>>>>> The existing __tun_wake_queue() function of the consumer races with the >>>>>> producer for waking/stopping the netdev queue: the consumer may drain >>>>>> the ring just as the producer stops the queue, leading to a permanent >>>>>> stall. To avoid this, the producer re-checks the ring after stopping >>>>>> and wakes the queue itself if space was just made. An >>>>>> smp_mb__after_atomic() is required so the re-peek of the ring sees any >>>>>> drain that the consumer performed. >>>>>> smp_mb__after_atomic() pairs with the test_and_clear_bit() inside of >>>>>> netif_wake_subqueue(): >>>>>> >>>>>> Consumer CPU Producer CPU >>>>>> ======================== ========================= >>>>>> __ptr_ring_consume() >>>>>> netif_wake_subqueue() netif_tx_stop_queue() >>>>>> /\ smp_mb__after_atomic() >>>>>> || __ptr_ring_produce_peek() >>>>>> contains RMW operation >>>>>> test_and_clear_bit() >>>>>> /\ >>>>>> || >>>>>> "Fully ordered RMW: >>>>>> smp_mb() before + after" >>>>>> - atomic_t.txt >>>>>> >>>>>> Benchmarks: >>>>>> The benchmarks show a slight regression in raw transmission performance, >>>>>> though no packets are lost anymore. >>>>> >>>>> Could you include the packets received as well? >>>>> To demonstrate the gains/lack of loss. >>>>> >>>> >>>> Do you mean the number of packets received by the VM? >>>> They should just be the same as the number sent (shown below), right? >>> >>> Minus the loss? Which this is about, right? >> >> Yes. I simply calculated "Lost/s": >> >> elapsed_time = 100e6 / sent_pps >> Lost/s = total_errors / elapsed_time >> >> >> To get back total_errors for example for TAP >> 1 thread sending: >> >> elapsed_time = 100e6 / 1.136Mpps = 88s >> >> 3758 Mpps = total_errors / 88s >> <=> total_errors = 331 million packets >> >> So, out of 431 million packets sent, 100 million were successfully >> delivered and 331 million were lost. > > That is my issue. > > I kind of have trouble mapping that to the table below. > For example: > > | TAP | Transmitted | 1.136 Mpps | 1.130 Mpps | -0.6% | > | +-------------+--------------+----------------+----------+ > | | Lost/s | 3.758 Mpps | 0 pps | | > > how can # of lost packets exceed the # of transmitted packets? > > Thanks! I just do use the sample script [1]: ./pktgen_sample02_multiqueue.sh -n 100000000 ... ... and this runs until 100_000_000 packets were sucessfully transmitted, independently of the lost packets/errors. [1] Link: https://www.kernel.org/doc/html/latest/networking/pktgen.html#sample-scripts > > >>> >>>> I assume they would be visible as RX-DRP for TAP. >>>> For TAP + vhost-net I would have to rewrite the XDP drop >>>> program to count the number of dropped packets... >>>> And I would have to automate it... >>>> >>>>>> >>>>>> 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 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 | Transmitted | 1.136 Mpps | 1.130 Mpps | -0.6% | >>>>>> | +-------------+--------------+----------------+----------+ >>>>>> | | Lost/s | 3.758 Mpps | 0 pps | | >>>>>> +------------+-------------+--------------+----------------+----------+ >>>>>> | TAP | Transmitted | 3.858 Mpps | 3.816 Mpps | -1.1% | >>>>>> | +-------------+--------------+----------------+----------+ >>>>>> | +vhost-net | Lost/s | 789.8 Kpps | 0 pps | | >>>>>> +------------+-------------+--------------+----------------+----------+ >>>>>> >>>>>> +--------------------------+--------------+----------------+----------+ >>>>>> | 2 threads | Stock | Patched with | diff | >>>>>> | sending | | fq_codel qdisc | | >>>>>> +------------+-------------+--------------+----------------+----------+ >>>>>> | TAP | Transmitted | 1.117 Mpps | 1.087 Mpps | -2.7% | >>>>>> | +-------------+--------------+----------------+----------+ >>>>>> | | Lost/s | 8.476 Mpps | 0 pps | | >>>>>> +------------+-------------+--------------+----------------+----------+ >>>>>> | TAP | Transmitted | 3.679 Mpps | 3.464 Mpps | -5.8% | >>>>>> | +-------------+--------------+----------------+----------+ >>>>>> | +vhost-net | Lost/s | 5.306 Mpps | 0 pps | | >>>>>> +------------+-------------+--------------+----------------+----------+ >>>>>> >>>>>> 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 efe809597622..c2a1618cc9db 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]); >>>>>> @@ -1065,13 +1067,37 @@ 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) { >>>>>> + netif_tx_stop_queue(queue); >>>>>> + /* Re-peek and wake if the consumer drained the ring >>>>>> + * concurrently in a race. smp_mb__after_atomic() pairs >>>>>> + * with the test_and_clear_bit() of netif_wake_subqueue() >>>>>> + * 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); >>>>>> + >>>>>> + 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 >>>>> >>> >