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Tsirkin" To: Markus Fohrer Cc: virtualization@lists.linux-foundation.org, jasowang@redhat.com, davem@davemloft.net, edumazet@google.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [REGRESSION] Massive virtio-net throughput drop in guest VM with Linux 6.8+ Message-ID: <20250403170543-mutt-send-email-mst@kernel.org> References: <1d388413ab9cfd765cd2c5e05b5e69cdb2ec5a10.camel@webked.de> <20250403090001-mutt-send-email-mst@kernel.org> <20250403100206-mutt-send-email-mst@kernel.org> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Thu, Apr 03, 2025 at 10:07:12PM +0200, Markus Fohrer wrote: > Am Donnerstag, dem 03.04.2025 um 10:03 -0400 schrieb Michael S. > Tsirkin: > > On Thu, Apr 03, 2025 at 03:51:01PM +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, > > > > > > > > > > I'm observing a significant performance regression in KVM guest > > > > > VMs > > > > > using virtio-net with recent Linux kernels (6.8.1+ and 6.14). > > > > > > > > > > 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 100–200 KB/s. The same guest configuration > > > > > performs > > > > > normally (~100 MB/s) when using kernel 6.8.0 or when the VM is > > > > > moved to a host with Intel NICs. > > > > > > > > > > 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 > > > > > > > > > > This issue is not present: > > > > > - On 6.8.0 > > > > > - On hosts with Intel NICs (same VM config) > > > > > > > > > > I have bisected the issue to the following upstream commit: > > > > > > > > > >   49d14b54a527 ("virtio-net: Suppress tx timeout warning for > > > > > small > > > > > tx") > > > > >   https://git.kernel.org/linus/49d14b54a527 > > > > > > > > Thanks a lot for the info! > > > > > > > > > > > > both the link and commit point at: > > > > > > > > commit 49d14b54a527289d09a9480f214b8c586322310a > > > > Author: Eric Dumazet > > > > Date:   Thu Sep 26 16:58:36 2024 +0000 > > > > > > > >     net: test for not too small csum_start in > > > > virtio_net_hdr_to_skb() > > > >     > > > > > > > > is this what you mean? > > > > > > > > I don't know which commit is "virtio-net: Suppress tx timeout > > > > warning > > > > for small tx" > > > > > > > > > > > > > > > > > Reverting this commit restores normal network performance in > > > > > affected guest VMs. > > > > > > > > > > I’m happy to provide more data or assist with testing a > > > > > potential > > > > > fix. > > > > > > > > > > Thanks, > > > > > Markus Fohrer > > > > > > > > > > > > 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: > > > > > > > > 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, > > > >   > > > >   if (!skb_partial_csum_set(skb, start, off)) > > > >   return -EINVAL; > > > > + if (skb_transport_offset(skb) < nh_min_len) > > > > + return -EINVAL; > > > >   > > > > - nh_min_len = max_t(u32, nh_min_len, > > > > skb_transport_offset(skb)); > > > > + nh_min_len = skb_transport_offset(skb); > > > >   p_off = nh_min_len + thlen; > > > >   if (!pskb_may_pull(skb, p_off)) > > > >   return -EINVAL; > > > > > > > > > > > > sticking a printk before return -EINVAL to show the offset and > > > > nh_min_len > > > > would be a good 1st step. Thanks! > > > > > > > > > > > > > Hi Eric, > > > > > > thanks a lot for the quick response — and yes, you're absolutely > > > right. > > > > > > Apologies for the confusion: I mistakenly wrote the wrong commit > > > description in my initial mail. > > > > > > The correct commit is indeed: > > > > > > commit 49d14b54a527289d09a9480f214b8c586322310a > > > Author: Eric Dumazet > > > Date:   Thu Sep 26 16:58:36 2024 +0000 > > > > > >     net: test for not too small csum_start in > > > virtio_net_hdr_to_skb() > > > > > > This is the one I bisected and which causes the performance > > > regression > > > in my environment. > > > > > > Thanks again, > > > Markus > > > > > > I'm not Eric but good to know. > > Alright, so I would start with the two items: device features and > > printk. > > > > as requested, here’s the device/feature information from the guest > running kernel 6.14 (mainline): > > Interface: ens18 > > ethtool -i ens18: > driver: virtio_net > version: 1.0.0 > firmware-version: > expansion-rom-version: > bus-info: 0000:00:12.0 > supports-statistics: yes > supports-test: no > supports-eeprom-access: no > supports-register-dump: no > supports-priv-flags: no > > > ethtool -k ens18: > Features for ens18: > rx-checksumming: on [fixed] > tx-checksumming: on > tx-checksum-ipv4: off [fixed] > tx-checksum-ip-generic: on > tx-checksum-ipv6: off [fixed] > tx-checksum-fcoe-crc: off [fixed] > tx-checksum-sctp: off [fixed] > scatter-gather: on > tx-scatter-gather: on > tx-scatter-gather-fraglist: off [fixed] > tcp-segmentation-offload: on > tx-tcp-segmentation: on > tx-tcp-ecn-segmentation: on > tx-tcp-mangleid-segmentation: off > tx-tcp6-segmentation: on > generic-segmentation-offload: on > generic-receive-offload: on > large-receive-offload: off [fixed] > rx-vlan-offload: off [fixed] > tx-vlan-offload: off [fixed] > ntuple-filters: off [fixed] > receive-hashing: off [fixed] > highdma: on [fixed] > rx-vlan-filter: on [fixed] > vlan-challenged: off [fixed] > tx-gso-robust: on [fixed] > tx-fcoe-segmentation: off [fixed] > tx-gre-segmentation: off [fixed] > tx-gre-csum-segmentation: off [fixed] > tx-ipxip4-segmentation: off [fixed] > tx-ipxip6-segmentation: off [fixed] > tx-udp_tnl-segmentation: off [fixed] > tx-udp_tnl-csum-segmentation: off [fixed] > tx-gso-partial: off [fixed] > tx-tunnel-remcsum-segmentation: off [fixed] > tx-sctp-segmentation: off [fixed] > tx-esp-segmentation: off [fixed] > tx-udp-segmentation: off > tx-gso-list: off [fixed] > tx-nocache-copy: off > loopback: off [fixed] > rx-fcs: off [fixed] > rx-all: off [fixed] > tx-vlan-stag-hw-insert: off [fixed] > rx-vlan-stag-hw-parse: off [fixed] > rx-vlan-stag-filter: off [fixed] > l2-fwd-offload: off [fixed] > hw-tc-offload: off [fixed] > esp-hw-offload: off [fixed] > esp-tx-csum-hw-offload: off [fixed] > rx-udp_tunnel-port-offload: off [fixed] > tls-hw-tx-offload: off [fixed] > tls-hw-rx-offload: off [fixed] > rx-gro-hw: on > tls-hw-record: off [fixed] > rx-gro-list: off > macsec-hw-offload: off [fixed] > rx-udp-gro-forwarding: off > hsr-tag-ins-offload: off [fixed] > hsr-tag-rm-offload: off [fixed] > hsr-fwd-offload: off [fixed] > hsr-dup-offload: off [fixed] > > ethtool ens18: > Settings for ens18: > Supported ports: [ ] > Supported link modes: Not reported > Supported pause frame use: No > Supports auto-negotiation: No > Supported FEC modes: Not reported > Advertised link modes: Not reported > Advertised pause frame use: No > Advertised auto-negotiation: No > Advertised FEC modes: Not reported > Speed: Unknown! > Duplex: Unknown! (255) > Auto-negotiation: off > Port: Other > PHYAD: 0 > Transceiver: internal > netlink error: Operation not permitted > Link detected: yes > > > Kernel log (journalctl -k): > Apr 03 19:50:37 kb-test.allod.com kernel: virtio_scsi virtio2: 4/0/0 > default/read/poll queues > Apr 03 19:50:37 kb-test.allod.com kernel: virtio_net virtio1 ens18: > renamed from eth0 > > Let me know if you’d like comparison data from kernel 6.11 or any > additional tests I think let's redo bisect first then I will suggest which traces to add.