From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from webmail.webked.de (webmail.webked.de [159.69.203.94]) (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 ADFDA14831E; Fri, 4 Apr 2025 08:17:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=159.69.203.94 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1743754638; cv=none; b=WKusyiCYEFkNImxC+1gluV7N7frSyxXKPb6D3ILZbL1SJ0dlNF764osFpEsXvuRvD4ZKQOdFcA0Bvb+i+l4GJ9w5yNY6mExJ0sYM14NO8qtg7ccXyvBwwthVwnlM34pSHBWPHNb2YGXi79JJsIGdLmKu5O4vYYA9yUT3P+X05bw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1743754638; c=relaxed/simple; bh=Cum2HBj0wq3wG0kYJgWZyERuNJx/Px6KBEHhIMFuuDw=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=Y3Ra5kbsxoLRT8JG0vCC/Z/bJW5135JyHfZvu8BjFeXGrdR3S23IcDzSoUKV63F49jwCNJAxJFndFCrAdhfBvOuYSVQ4X1iOt6h+t99kfRKrfON7cWileOW8AEPBGdjgUJh201S6sAzX6g0+pW2fJxsy+TE6xs7yLhiFXf8shS4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=webked.de; spf=pass smtp.mailfrom=webked.de; arc=none smtp.client-ip=159.69.203.94 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=webked.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=webked.de Received: from [127.0.0.1] (localhost [127.0.0.1]) by localhost (Mailerdaemon) with ESMTPSA id 08FD562BA4; Fri, 4 Apr 2025 10:16:55 +0200 (CEST) Message-ID: <11c5cb52d024a5158c5b8c5e69e2e4639a055a31.camel@webked.de> Subject: Re: [REGRESSION] Massive virtio-net throughput drop in guest VM with Linux 6.8+ From: Markus Fohrer To: "Michael S. Tsirkin" Cc: virtualization@lists.linux-foundation.org, jasowang@redhat.com, davem@davemloft.net, edumazet@google.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Date: Fri, 04 Apr 2025 10:16:55 +0200 In-Reply-To: <20250403090001-mutt-send-email-mst@kernel.org> References: <1d388413ab9cfd765cd2c5e05b5e69cdb2ec5a10.camel@webked.de> <20250403090001-mutt-send-email-mst@kernel.org> Organization: WEBKED IT Markus Fohrer Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.52.3-0ubuntu1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Last-TLS-Session-Version: TLSv1.3 Am Donnerstag, dem 03.04.2025 um 09:04 -0400 schrieb Michael S. Tsirkin: > On Wed, Apr 02, 2025 at 11:12:07PM +0200, Markus Fohrer wrote: > > Hi, > >=20 > > I'm observing a significant performance regression in KVM guest VMs > > using virtio-net with recent Linux kernels (6.8.1+ and 6.14). > >=20 > > When running on a host system equipped with a Broadcom NetXtreme-E > > (bnxt_en) NIC and AMD EPYC CPUs, the network throughput in the > > guest drops to 100=E2=80=93200 KB/s. The same guest configuration perfo= rms > > normally (~100 MB/s) when using kernel 6.8.0 or when the VM is > > moved to a host with Intel NICs. > >=20 > > Test environment: > > - Host: QEMU/KVM, Linux 6.8.1 and 6.14.0 > > - Guest: Linux with virtio-net interface > > - NIC: Broadcom BCM57416 (bnxt_en driver, no issues at host level) > > - CPU: AMD EPYC > > - Storage: virtio-scsi > > - VM network: virtio-net, virtio-scsi (no CPU or IO bottlenecks) > > - Traffic test: iperf3, scp, wget consistently slow in guest > >=20 > > This issue is not present: > > - On 6.8.0=20 > > - On hosts with Intel NICs (same VM config) > >=20 > > I have bisected the issue to the following upstream commit: > >=20 > > =C2=A0 49d14b54a527 ("virtio-net: Suppress tx timeout warning for small > > tx") > > =C2=A0 https://git.kernel.org/linus/49d14b54a527 >=20 > Thanks a lot for the info! >=20 >=20 > both the link and commit point at: >=20 > commit 49d14b54a527289d09a9480f214b8c586322310a > Author: Eric Dumazet > Date:=C2=A0=C2=A0 Thu Sep 26 16:58:36 2024 +0000 >=20 > =C2=A0=C2=A0=C2=A0 net: test for not too small csum_start in virtio_net_h= dr_to_skb() > =C2=A0=C2=A0=C2=A0=20 >=20 > is this what you mean? >=20 > I don't know which commit is "virtio-net: Suppress tx timeout warning > for small tx" >=20 >=20 >=20 > > Reverting this commit restores normal network performance in > > affected guest VMs. > >=20 > > I=E2=80=99m happy to provide more data or assist with testing a potenti= al > > fix. > >=20 > > Thanks, > > Markus Fohrer >=20 >=20 > Thanks! First I think it's worth checking what is the setup, e.g. > which offloads are enabled. > Besides that, I'd start by seeing what's doing on. Assuming I'm right > about > Eric's patch: >=20 > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h > index 276ca543ef44d8..02a9f4dc594d02 100644 > --- a/include/linux/virtio_net.h > +++ b/include/linux/virtio_net.h > @@ -103,8 +103,10 @@ static inline int virtio_net_hdr_to_skb(struct > sk_buff *skb, > =C2=A0 > =C2=A0 if (!skb_partial_csum_set(skb, start, off)) > =C2=A0 return -EINVAL; > + if (skb_transport_offset(skb) < nh_min_len) > + return -EINVAL; > =C2=A0 > - nh_min_len =3D max_t(u32, nh_min_len, > skb_transport_offset(skb)); > + nh_min_len =3D skb_transport_offset(skb); > =C2=A0 p_off =3D nh_min_len + thlen; > =C2=A0 if (!pskb_may_pull(skb, p_off)) > =C2=A0 return -EINVAL; >=20 >=20 > sticking a printk before return -EINVAL to show the offset and > nh_min_len > would be a good 1st step. Thanks! >=20 I added the following printk inside virtio_net_hdr_to_skb(): if (skb_transport_offset(skb) < nh_min_len){ printk(KERN_INFO "virtio_net: 3 drop, transport_offset=3D%u, nh_min_len=3D%u\n", skb_transport_offset(skb), nh_min_len); return -EINVAL; } Built and installed the kernel, then triggered a large download via: wget http://speedtest.belwue.net/10G Relevant output from `dmesg -w`: [ 57.327943] virtio_net: 3 drop, transport_offset=3D34, nh_min_len=3D40 = =20 [ 57.428942] virtio_net: 3 drop, transport_offset=3D34, nh_min_len=3D40 = =20 [ 57.428962] virtio_net: 3 drop, transport_offset=3D34, nh_min_len=3D40 = =20 [ 57.553068] virtio_net: 3 drop, transport_offset=3D34, nh_min_len=3D40 = =20 [ 57.553088] virtio_net: 3 drop, transport_offset=3D34, nh_min_len=3D40 = =20 [ 57.576678] virtio_net: 3 drop, transport_offset=3D34, nh_min_len=3D40 = =20 [ 57.618438] virtio_net: 3 drop, transport_offset=3D34, nh_min_len=3D40 = =20 [ 57.618453] virtio_net: 3 drop, transport_offset=3D34, nh_min_len=3D40 = =20 [ 57.703077] virtio_net: 3 drop, transport_offset=3D34, nh_min_len=3D40 = =20 [ 57.823072] virtio_net: 3 drop, transport_offset=3D34, nh_min_len=3D40 = =20 [ 57.891982] virtio_net: 3 drop, transport_offset=3D34, nh_min_len=3D40 = =20 [ 57.946190] virtio_net: 3 drop, transport_offset=3D34, nh_min_len=3D40 = =20 [ 58.218686] virtio_net: 3 drop, transport_offset=3D34, nh_min_len=3D40 = =20 I would now do the test with commit 49d14b54a527289d09a9480f214b8c586322310a and commit 49d14b54a527289d09a9480f214b8c586322310a~1