From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 B7EB435F8B9 for ; Wed, 6 May 2026 22:28:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778106492; cv=none; b=Sl1At/Xxj1RE4t2x89SULV6PjGaRaXixAtQkXKM0wLW6iTcSJVWIoN3ZDSqqRt4hmLED6AtaFY1aUbfww90crn7UUX+BgG2p5Gz7mV4f+DxH3GmVSVTikxifqHkZIHkidoUYTA3XSXcBYu+JpJcavOq2Q6EgKfj45MQaOqkU6C0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778106492; c=relaxed/simple; bh=rtVwk1ax2o4nJ4GXyKTejUp5AmKhUCVDmo4YRJ3ERHs=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=sMrXRIE9wgCS7gJ5CMRLvHmYYyrdW6B0ZMCKEw4a/tz+wFbWDqWIAP3OdSaHSHgqhz7HG/95RXVri9OZfcwXMQBWZjlFrDM7tF0A9hh+KLDmflG3RRMmqVU8lUo9YUn91uvs2f3GjWAjQQFV0EzBhkKeULOZcR3JN/jDAwJ1xuI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=UTfuKZIf; dkim=pass (2048-bit key) header.d=redhat.com header.i=@redhat.com header.b=T7JIkOfA; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="UTfuKZIf"; dkim=pass (2048-bit key) header.d=redhat.com header.i=@redhat.com header.b="T7JIkOfA" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1778106489; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=t8i9O8vJ9NfXLrrn+cYRMOYCqXIwE6jcbZ+nzxxFXS0=; b=UTfuKZIfGm0uVdIw9KVN/gemybNl8rQfia42DnBaW6TBWSPw2oxD2wO58aWcRbz5MjBNEY f1xpSjyGCIGZrR3hJI5/hI5y8KONHILHq2hhbf6ZH/v57hdYIcB6fkLAAK/WD9KG3ACMWz PPs3wRlpCVjOWtd+Wxde3yaFtIjcUb0= Received: from mail-wr1-f70.google.com (mail-wr1-f70.google.com [209.85.221.70]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-196-tLxUMm_qNb-QqZj8fB5xLA-1; Wed, 06 May 2026 18:28:08 -0400 X-MC-Unique: tLxUMm_qNb-QqZj8fB5xLA-1 X-Mimecast-MFC-AGG-ID: tLxUMm_qNb-QqZj8fB5xLA_1778106487 Received: by mail-wr1-f70.google.com with SMTP id ffacd0b85a97d-44dad1b938fso121647f8f.1 for ; Wed, 06 May 2026 15:28:08 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=google; t=1778106487; x=1778711287; darn=vger.kernel.org; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=t8i9O8vJ9NfXLrrn+cYRMOYCqXIwE6jcbZ+nzxxFXS0=; b=T7JIkOfA6LbehnvBkk0609D5QbeP0HnzceqdAojuDwtCVjAcVhTFLIVvYYQu400PFJ iT6iTcvEK+l1WVKMU7J+LpSLVdWwTFYCO59KthyikifuNBCeKAKh6oIsWmLvw/scTFcv f2v31Ds7CwBq+6cKpkhV40tsSNLLuKtBX6aCs5Ka9cTE8bRt5M8aN2kUFNAk6WoI40IP XaNNH2A2w2RFnkJGs363Bxla86nrcpQr1Hhgn0Z5JOy87HCYkbBwddikygWd9lc4b9I7 DrN+pFOOVLsWnX/Zl8Qb8Qw1fBXgqgwXuGJ3oSqY/btk0zXApynMWU6zXELRvxMAso4p OghQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1778106487; x=1778711287; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-gg:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=t8i9O8vJ9NfXLrrn+cYRMOYCqXIwE6jcbZ+nzxxFXS0=; b=QMdgpO/nvGNwvWQDc+dFIvg06v++l9tBg0iWoNGMqCRI/V2JHj07egQc1xOc4DE5UX NDF1VaGNlYDvLqEb9/8kaQMRl9cDXIJenGIic5DrdfDXZVkLC0SNWkHzSjRtGL+FG98o xBy9A1ZudfP8+muP973OTnz0pZQNcgKVc11z8DM4IC+VEkx6toJLDxsyo1qxUYr2cNXA /0uWdVRj7S94QBTTqX6x+t07wX4/yhLXQFCSIEGr+3yUYm4rdpTN+UAKPzRFNFseQC9e 7vjxtmB/NHB5tnWFI9QAyzSJQtJ4pZeRQpspnL+ZG77cBGO82VRPJ5evuB0GMUSDScbA M9ig== X-Forwarded-Encrypted: i=1; AFNElJ+/TeF2urcC/pWywCesJUVY4cR1KFQfz1DdPlVuMVx0pJ/AHq9zFl1zgmhPq6ofKhmtEujDVSqU/pOF4es=@vger.kernel.org X-Gm-Message-State: AOJu0YyQNdNfmxUy9jQ1HKsoHAJ9f1DJA2w3EvQxOFWIpL24R64whuCY XdE6mlFuGyOZBRJI7qW0rbO2uwZ/7f33ITFoEFJoVz/0IMpJwC53tZuVHk3ZCxphBtT2/BPsMim w5bF6NKGFPGuZ7ccppeYk9HzB6Lx9TWpU1EmtZnp0I6rIl6+GuUmhYw57XeNOsE7b9A== X-Gm-Gg: AeBDievo3PSaifYJKuqbcpJnijg6vQxIfR8nPbsMhnxTuwWsKcZZF3++decLwpdR6NV FegOM/9EIfdFEezIsPHzLLMyk9y+gKJoEoHnGgyh6gP7YMMdtsd9adSBoyUPPiBgMoFho8H3OiD G0Z+dcDvRhUlX/AVp1bHDi6BznJ21/d72knELgq8lKYbzS9JxJ7o8GPw6Rz2Ik0tPlIybd4kc2/ 04V612/qAM8iKzJPSQl6HmtPKCe+gkBa0BHSxyQt8Axw2ytgr0OyK+kVvD5/gV0xMb+KWSttx8S UqbEOAjLDX1eA7VXRdsEopJNpRatoC5app53wSfrI/MIX/28tFbXtMGeOnvDvK9MFoDcRtSBSXw Rl8Pl68oI7BYgEqsSHNQzfACbU1H+rpHXzbpGgMVLIzIJwAUAI0A= X-Received: by 2002:a05:600c:8908:b0:48a:5821:5ff2 with SMTP id 5b1f17b1804b1-48e51f24340mr59778965e9.8.1778106487227; Wed, 06 May 2026 15:28:07 -0700 (PDT) X-Received: by 2002:a05:600c:8908:b0:48a:5821:5ff2 with SMTP id 5b1f17b1804b1-48e51f24340mr59778725e9.8.1778106486686; Wed, 06 May 2026 15:28:06 -0700 (PDT) Received: from redhat.com (IGLD-80-230-48-7.inter.net.il. [80.230.48.7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48e538aaa0fsm97231005e9.7.2026.05.06.15.28.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 06 May 2026 15:28:06 -0700 (PDT) Date: Wed, 6 May 2026 18:28:02 -0400 From: "Michael S. Tsirkin" To: Simon Schippers 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 Subject: Re: [PATCH net-next v10 4/4] tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present Message-ID: <20260506181909-mutt-send-email-mst@kernel.org> References: <20260506141033.180450-1-simon.schippers@tu-dortmund.de> <20260506141033.180450-5-simon.schippers@tu-dortmund.de> Precedence: bulk X-Mailing-List: linux-kernel@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: <20260506141033.180450-5-simon.schippers@tu-dortmund.de> On Wed, May 06, 2026 at 04:10:33PM +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 no qdisc is present, the previous tail-drop > behavior is preserved. > > If producing an entry fails anyways due to a race, tun_net_xmit() drops > the packet. Such races are expected because LLTX is enabled and the > transmit path operates without the usual locking. > > The __tun_wake_queue() function of the consumer races with the producer > for waking/stopping the netdev queue, which could result in a stalled > queue. Therefore, an smp_mb__after_atomic() is introduced that pairs > with the smp_mb() of the consumer. It follows the principle of store > buffering described in tools/memory-model/Documentation/recipes.txt: > > - The producer in tun_net_xmit() first sets __QUEUE_STATE_DRV_XOFF, > followed by an smp_mb__after_atomic() (= smp_mb()), and then reads the > ring with __ptr_ring_produce_peek(). > > - The consumer in __tun_wake_queue() first writes zero to the ring in > __ptr_ring_consume(), followed by an smp_mb(), and then reads the queue > status with netif_tx_queue_stopped(). > > => Following the aforementioned principle, it is impossible for the > producer to see a full ring (and therefore not wake the queue on the > re-check) while the consumer simultaneously fails to see a stopped > queue (and therefore also does not wake it). > > Benchmarks: > The benchmarks show a slight regression in raw transmission performance > when using two sending threads. Packet loss also occurs only in the > two-thread sending case; no packet loss was observed with a single > sending thread. > > 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 | Received | 1.132 Mpps | 1.133 Mpps | +0.1% | > | +-------------+--------------+----------------+----------+ > | | Lost/s | 3.765 Mpps | 0 pps | | > +------------+-------------+--------------+----------------+----------+ > | TAP | Received | 3.857 Mpps | 3.905 Mpps | +1.2% | > | +-------------+--------------+----------------+----------+ > | +vhost-net | Lost/s | 0.802 Mpps | 0 pps | | > +------------+-------------+--------------+----------------+----------+ > > +--------------------------+--------------+----------------+----------+ > | 2 threads | Stock | Patched with | diff | > | sending | | fq_codel qdisc | | > +------------+-------------+--------------+----------------+----------+ > | TAP | Received | 1.115 Mpps | 1.092 Mpps | -2.1% | > | +-------------+--------------+----------------+----------+ > | | Lost/s | 8.490 Mpps | 359 pps | | > +------------+-------------+--------------+----------------+----------+ > | TAP | Received | 3.664 Mpps | 3.549 Mpps | -3.1% | > | +-------------+--------------+----------------+----------+ > | +vhost-net | Lost/s | 5.330 Mpps | 832 pps | | > +------------+-------------+--------------+----------------+----------+ > > Co-developed-by: Tim Gebauer > Signed-off-by: Tim Gebauer > Signed-off-by: Simon Schippers > --- > drivers/net/tun.c | 25 +++++++++++++++++++++++-- > 1 file changed, 23 insertions(+), 2 deletions(-) > > diff --git a/drivers/net/tun.c b/drivers/net/tun.c > index fc358c4c355b..d9ffbf88cfd8 100644 > --- a/drivers/net/tun.c > +++ b/drivers/net/tun.c > @@ -1018,6 +1018,7 @@ 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; > + int ret; > > rcu_read_lock(); > tfile = rcu_dereference(tun->tfiles[txq]); > @@ -1072,13 +1073,33 @@ 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); > + > + spin_lock(&tfile->tx_ring.producer_lock); > + ret = __ptr_ring_produce(&tfile->tx_ring, skb); > + if (!qdisc_txq_has_no_queue(queue) && > + (__ptr_ring_produce_peek(&tfile->tx_ring) || ret)) { > + netif_tx_stop_queue(queue); > + /* Paired with smp_mb() 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); > + There's a weird corner case here when tx_queue_len is 0 but a qdisc has been configured - it looks like that currently it just drops all packets, with this change, the qdisc will get stuck permanently. I suspect just checking tx_ring.size should fix it. Or if you feel adventurous, change return code for __ptr_ring_produce to distinguish between "no ring" and "no space". > + if (ret) { > + /* This should be a rare case if 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 > + * we must drop instead. > + */ > 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