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 0B3FB38AC9A; Tue, 28 Apr 2026 13:41:31 +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=1777383695; cv=none; b=f42owSQlP9yGQYloKTEy9At3ERkDrvvcNs1BJT68kutH1/Q+3cXhpSNrGZRC9SYlsnMVZstns6H+ynj+6rhRBjjFn7EsaqoebyxCVO0yebXrresnsoWBWtSWkCU659CAgA1UDYSuU0kx+mwU9ZFAMYFVqaiFbKCjZbp2n0kjo8I= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777383695; c=relaxed/simple; bh=zicy1VoOoqWTu9mSATtC6N+HthFHLjJWRfjwkNJc2OM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=fQGqMz6IlwWfcPcrf/flnfU/Hg8UMeTM7BgZD8Ss44hpvgp1i6R3wMfeLnwiSLbGfgJcPXFnJfiTlfX5q4DDNHlMaIZel9D0ArDli9GwCeyZUuL6o9aZbLbt+qFBiPKzD2L6sV3i+Q0YXXuKrmwalV4mB4/zNiB8BjKUQZvjcw0= 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 63SDfKAD018334 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NOT); Tue, 28 Apr 2026 15:41:20 +0200 (CEST) Message-ID: Date: Tue, 28 Apr 2026 15:41:20 +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> Content-Language: en-US From: Simon Schippers In-Reply-To: <20260428092150-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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. > >> 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 >>> >