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 A8F7014F9F9; Fri, 4 Apr 2025 20:23:19 +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=1743798204; cv=none; b=Q6jFamm1YI4i3AJnxplVVlI7jJZHJRmabXGkbfsYbK2JZiO93aT6y3q0QZ/TlLW/zu5dwWTzXiiI9HVswhXW5IDdIaPZ0pRicYSL1Cv6Y2n/fthu0CR/wyrnDcQo9oqyGxJg2CB7x9EPV6hVmv9c6Vji2mGcHkjyD2avVHjtBV4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1743798204; c=relaxed/simple; bh=KynK17m8gzCfpiirN3+RmhG9s7QXD8jjm7XlUSRzDBI=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=emNgZ1fDbVDh4BD2cgLq4En+tISGOau28KSoeLR23i2xrhhOJUi0EW110d89sLl3HFj0thmx6olqGD5jH1l1oB/uf9939YhHpCdAVrmI3G5fDU0OycgFclnWZhPpYU/1kmvBhodx1YNMKmcBx3yQPcRp9Cl+UTB7MfKXq3KxYcE= 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 AD1C061A78; Fri, 4 Apr 2025 22:23:01 +0200 (CEST) Message-ID: <62a6bf463858c443f6380bbf7ae5a3cc76b81844.camel@webked.de> Subject: Re: [REGRESSION] Massive virtio-net throughput drop in guest VM with Linux 6.8+ From: Markus Fohrer To: Willem de Bruijn , "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 22:23:01 +0200 In-Reply-To: <67eff7303df69_1ddca829490@willemb.c.googlers.com.notmuch> References: <1d388413ab9cfd765cd2c5e05b5e69cdb2ec5a10.camel@webked.de> <20250403090001-mutt-send-email-mst@kernel.org> <11c5cb52d024a5158c5b8c5e69e2e4639a055a31.camel@webked.de> <20250404042711-mutt-send-email-mst@kernel.org> <3b02f37ee12232359672a6a6c2bccaa340fbb6ff.camel@webked.de> <67eff7303df69_1ddca829490@willemb.c.googlers.com.notmuch> 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 Freitag, dem 04.04.2025 um 11:13 -0400 schrieb Willem de Bruijn: > Markus Fohrer wrote: > > Am Freitag, dem 04.04.2025 um 10:52 +0200 schrieb Markus Fohrer: > > > Am Freitag, dem 04.04.2025 um 04:29 -0400 schrieb Michael S. > > > Tsirkin: > > > > On Fri, Apr 04, 2025 at 10:16:55AM +0200, Markus Fohrer wrote: > > > > > 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 configura= tion > > > > > > > performs > > > > > > > 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_hdr_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 > > > > > > > potential > > > > > > > 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 > > > > >=20 > > > > > I added the following printk inside virtio_net_hdr_to_skb(): > > > > >=20 > > > > > =C2=A0=C2=A0=C2=A0 if (skb_transport_offset(skb) < nh_min_len){ > > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 printk(KERN_INFO "virt= io_net: 3 drop, > > > > > transport_offset=3D%u, > > > > > nh_min_len=3D%u\n", > > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0 skb_transport_offset(skb), nh_min_len); > > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return -EINVAL; > > > > > =C2=A0=C2=A0=C2=A0 } > > > > >=20 > > > > > Built and installed the kernel, then triggered a large > > > > > download > > > > > via: > > > > >=20 > > > > > =C2=A0=C2=A0=C2=A0 wget http://speedtest.belwue.net/10G > > > > >=20 > > > > > Relevant output from `dmesg -w`: > > > > >=20 > > > > > [=C2=A0=C2=A0 57.327943] virtio_net: 3 drop, transport_offset=3D3= 4, > > > > > nh_min_len=3D40=C2=A0=20 > > > > > [=C2=A0=C2=A0 57.428942] virtio_net: 3 drop, transport_offset=3D3= 4, > > > > > nh_min_len=3D40=C2=A0=20 > > > > > [=C2=A0=C2=A0 57.428962] virtio_net: 3 drop, transport_offset=3D3= 4, > > > > > nh_min_len=3D40=C2=A0=20 > > > > > [=C2=A0=C2=A0 57.553068] virtio_net: 3 drop, transport_offset=3D3= 4, > > > > > nh_min_len=3D40=C2=A0=20 > > > > > [=C2=A0=C2=A0 57.553088] virtio_net: 3 drop, transport_offset=3D3= 4, > > > > > nh_min_len=3D40=C2=A0=20 > > > > > [=C2=A0=C2=A0 57.576678] virtio_net: 3 drop, transport_offset=3D3= 4, > > > > > nh_min_len=3D40=C2=A0=20 > > > > > [=C2=A0=C2=A0 57.618438] virtio_net: 3 drop, transport_offset=3D3= 4, > > > > > nh_min_len=3D40=C2=A0=20 > > > > > [=C2=A0=C2=A0 57.618453] virtio_net: 3 drop, transport_offset=3D3= 4, > > > > > nh_min_len=3D40=C2=A0=20 > > > > > [=C2=A0=C2=A0 57.703077] virtio_net: 3 drop, transport_offset=3D3= 4, > > > > > nh_min_len=3D40=C2=A0=20 > > > > > [=C2=A0=C2=A0 57.823072] virtio_net: 3 drop, transport_offset=3D3= 4, > > > > > nh_min_len=3D40=C2=A0=20 > > > > > [=C2=A0=C2=A0 57.891982] virtio_net: 3 drop, transport_offset=3D3= 4, > > > > > nh_min_len=3D40=C2=A0=20 > > > > > [=C2=A0=C2=A0 57.946190] virtio_net: 3 drop, transport_offset=3D3= 4, > > > > > nh_min_len=3D40=C2=A0=20 > > > > > [=C2=A0=C2=A0 58.218686] virtio_net: 3 drop, transport_offset=3D3= 4, > > > > > nh_min_len=3D40=C2=A0=20 > > > >=20 > > > > Hmm indeed. And what about these values? > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0 u32 start =3D __virtio16_to_cpu(little_endian, > > > > hdr- > > > > > csum_start); > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0 u32 off =3D __virtio16_to_cpu(little_endian, hdr- > > > > > csum_offset); > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0 u32 needed =3D start + max_t(u32, thlen, off + > > > > sizeof(__sum16)); > > > > print them too? > > > >=20 > > > >=20 > > > >=20 > > > > > I would now do the test with commit > > > > > 49d14b54a527289d09a9480f214b8c586322310a and commit > > > > > 49d14b54a527289d09a9480f214b8c586322310a~1 > > > > >=20 > > > >=20 > > > > Worth checking though it seems likely now the hypervisor is > > > > doing > > > > weird > > > > things. what kind of backend is it? qemu? tun? vhost-user? > > > > vhost- > > > > net? > > > >=20 > > >=20 > > > Backend: QEMU/KVM hypervisor (Proxmox) > > >=20 > > >=20 > > > printk output: > > >=20 > > > [=C2=A0=C2=A0 58.641906] virtio_net: drop, transport_offset=3D34=C2= =A0 start=3D34, > > > off=3D16, > > > needed=3D54, nh_min_len=3D40 > > > [=C2=A0=C2=A0 58.678048] virtio_net: drop, transport_offset=3D34=C2= =A0 start=3D34, > > > off=3D16, > > > needed=3D54, nh_min_len=3D40 > > > [=C2=A0=C2=A0 58.952871] virtio_net: drop, transport_offset=3D34=C2= =A0 start=3D34, > > > off=3D16, > > > needed=3D54, nh_min_len=3D40 > > > [=C2=A0=C2=A0 58.962157] virtio_net: drop, transport_offset=3D34=C2= =A0 start=3D34, > > > off=3D16, > > > needed=3D54, nh_min_len=3D40 > > > [=C2=A0=C2=A0 59.071645] virtio_net: drop, transport_offset=3D34=C2= =A0 start=3D34, > > > off=3D16, > > > needed=3D54, nh_min_len=3D40 >=20 > So likely a TCP/IPv4 packet, but with VIRTIO_NET_HDR_GSO_TCPV6. >=20 > This is observed in the guest on the ingress path, right? In > virtnet_receive_done. Yes, all tests are done inside the guest system. Packet drops are seen when receiving traffic. I hadn't tested upload before, so I did now: - Download is slow (<200 KB/s) - Upload works fine (~190 MB/s) >=20 > Is this using vhost-net in the host for pass-through? IOW, is > the host writing the virtio_net_hdr too? Yes, the guest runs on a Proxmox host using QEMU/KVM with vhost-net. vhost_net module is loaded: # lsmod | grep vhost vhost_net 32768 30 vhost 61440 1 vhost_net vhost_iotlb 16384 1 vhost tap 28672 1 vhost_net QEMU is launched with: -netdev type=3Dtap,...,vhost=3Don -device virtio-net-pci,... >=20 > > >=20 > > >=20 > > >=20 > > >=20 > >=20 > > I just noticed that commit 17bd3bd82f9f79f3feba15476c2b2c95a9b11ff8 > > (tcp_offload.c: gso fix) also touches checksum handling and may > > affect how skb state is passed to virtio_net_hdr_to_skb(). > >=20 > > Is it possible that the regression only appears due to the > > combination > > of 17bd3bd8 and 49d14b54a5? >=20 > That patch only affects packets with SKB_GSO_FRAGLIST. Which is only > set on forwarding if NETIF_F_FRAGLIST is set. I don=20 Checked in the guest: # ethtool -k eth0 | grep frag tx-scatter-gather-fraglist: off [fixed] So fraglist offload is disabled in the guest.