Re: [REGRESSION] Massive virtio-net throughput drop in guest VM with Linux 6.8+

[Markus Fohrer <markus.fohrer@webked.de>]

Am Donnerstag, dem 03.04.2025 um 18:05 -0400 schrieb Michael S.
Tsirkin:
> On Thu, Apr 03, 2025 at 11:24:43PM +0200, Markus Fohrer wrote:
> > Am Donnerstag, dem 03.04.2025 um 17:06 -0400 schrieb Michael S.
> > Tsirkin:
> > > 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 <edumazet@google.com>
> > > > > > > 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 <edumazet@google.com>
> > > > > > 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.
> > > 
> > 
> > The build with the added printk is currently running. I’ll test it
> > shortly and report the results.
> > 
> > Should the bisect be done between v6.11 and v6.12, or v6.11 and
> > v6.14?
> 
> The commit you showed is between 6.11 and 6.12. Having said that,
> you can manually checkout 49d14b54a527289d09a9480f214b8c586322310a
> and 49d14b54a527289d09a9480f214b8c586322310a~1 and record
> the results with git bisect bad/good and if it works
> then git bisect will stop immediately for you.
> 


I built and tested:
- 49d14b54a527289d09a9480f214b8c586322310a -> bad
- 49d14b54a527289d09a9480f214b8c586322310a~1 -> good

git bisect result:
49d14b54a527289d09a9480f214b8c586322310a is the first bad commit


Log:
git bisect start
# status: waiting for both good and bad commits
# bad: [49d14b54a527289d09a9480f214b8c586322310a] net: test for not too
small csum_start in virtio_net_hdr_to_skb()
git bisect bad 49d14b54a527289d09a9480f214b8c586322310a
# status: waiting for good commit(s), bad commit known
# good: [17bd3bd82f9f79f3feba15476c2b2c95a9b11ff8] net: gso: fix tcp
fraglist segmentation after pull from frag_list
git bisect good 17bd3bd82f9f79f3feba15476c2b2c95a9b11ff8
# first bad commit: [49d14b54a527289d09a9480f214b8c586322310a] net:
test for not too small csum_start in virtio_net_hdr_to_skb()