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

From: Markus Fohrer
Date: Fri Apr 04 2025 - 16:23:39 EST

Next message: Josh Poimboeuf: "Re: [PATCH v2] objtool: Fix SYSCALL instruction handling and INSN_CONTEXT_SWITCH"
Previous message: kernel test robot: "[tip:timers/cleanups 1/11] drivers/usb/host/xhci.c:1074:4: error: implicit declaration of function 'timer_delele_sync'; did you mean 'timer_delete_sync'?"
In reply to: Willem de Bruijn: "Re: [REGRESSION] Massive virtio-net throughput drop in guest VM with Linux 6.8+"
Next in thread: Ilya Maximets: "Re: [REGRESSION] Massive virtio-net throughput drop in guest VM with Linux 6.8+"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]

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,
> > > > > > >
> > > > > > > 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@xxxxxxxxxx>
> > > > > > 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!
> > > > > >
> > > > >
> > > > > 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=%u,
> > > > > nh_min_len=%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=34,
> > > > > nh_min_len=40
> > > > > [   57.428942] virtio_net: 3 drop, transport_offset=34,
> > > > > nh_min_len=40
> > > > > [   57.428962] virtio_net: 3 drop, transport_offset=34,
> > > > > nh_min_len=40
> > > > > [   57.553068] virtio_net: 3 drop, transport_offset=34,
> > > > > nh_min_len=40
> > > > > [   57.553088] virtio_net: 3 drop, transport_offset=34,
> > > > > nh_min_len=40
> > > > > [   57.576678] virtio_net: 3 drop, transport_offset=34,
> > > > > nh_min_len=40
> > > > > [   57.618438] virtio_net: 3 drop, transport_offset=34,
> > > > > nh_min_len=40
> > > > > [   57.618453] virtio_net: 3 drop, transport_offset=34,
> > > > > nh_min_len=40
> > > > > [   57.703077] virtio_net: 3 drop, transport_offset=34,
> > > > > nh_min_len=40
> > > > > [   57.823072] virtio_net: 3 drop, transport_offset=34,
> > > > > nh_min_len=40
> > > > > [   57.891982] virtio_net: 3 drop, transport_offset=34,
> > > > > nh_min_len=40
> > > > > [   57.946190] virtio_net: 3 drop, transport_offset=34,
> > > > > nh_min_len=40
> > > > > [   58.218686] virtio_net: 3 drop, transport_offset=34,
> > > > > nh_min_len=40
> > > >
> > > > Hmm indeed. And what about these values?
> > > >                 u32 start = __virtio16_to_cpu(little_endian,
> > > > hdr-
> > > > > csum_start);
> > > >                 u32 off = __virtio16_to_cpu(little_endian, hdr-
> > > > > csum_offset);
> > > >                 u32 needed = start + max_t(u32, thlen, off +
> > > > sizeof(__sum16));
> > > > print them too?
> > > >
> > > >
> > > >
> > > > > I would now do the test with commit
> > > > > 49d14b54a527289d09a9480f214b8c586322310a and commit
> > > > > 49d14b54a527289d09a9480f214b8c586322310a~1
> > > > >
> > > >
> > > > 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?
> > > >
> > >
> > > Backend: QEMU/KVM hypervisor (Proxmox)
> > >
> > >
> > > printk output:
> > >
> > > [   58.641906] virtio_net: drop, transport_offset=34 start=34,
> > > off=16,
> > > needed=54, nh_min_len=40
> > > [   58.678048] virtio_net: drop, transport_offset=34 start=34,
> > > off=16,
> > > needed=54, nh_min_len=40
> > > [   58.952871] virtio_net: drop, transport_offset=34 start=34,
> > > off=16,
> > > needed=54, nh_min_len=40
> > > [   58.962157] virtio_net: drop, transport_offset=34 start=34,
> > > off=16,
> > > needed=54, nh_min_len=40
> > > [   59.071645] virtio_net: drop, transport_offset=34 start=34,
> > > off=16,
> > > needed=54, nh_min_len=40
>
> So likely a TCP/IPv4 packet, but with VIRTIO_NET_HDR_GSO_TCPV6.
>
> 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)

>
> 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=tap,...,vhost=on
-device virtio-net-pci,...

>
> > >
> > >
> > >
> > >
> >
> > 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().
> >
> > Is it possible that the regression only appears due to the
> > combination
> > of 17bd3bd8 and 49d14b54a5?
>
> That patch only affects packets with SKB_GSO_FRAGLIST. Which is only
> set on forwarding if NETIF_F_FRAGLIST is set. I don

Checked in the guest:

# ethtool -k eth0 | grep frag
tx-scatter-gather-fraglist: off [fixed]

So fraglist offload is disabled in the guest.

Next message: Josh Poimboeuf: "Re: [PATCH v2] objtool: Fix SYSCALL instruction handling and INSN_CONTEXT_SWITCH"
Previous message: kernel test robot: "[tip:timers/cleanups 1/11] drivers/usb/host/xhci.c:1074:4: error: implicit declaration of function 'timer_delele_sync'; did you mean 'timer_delete_sync'?"
In reply to: Willem de Bruijn: "Re: [REGRESSION] Massive virtio-net throughput drop in guest VM with Linux 6.8+"
Next in thread: Ilya Maximets: "Re: [REGRESSION] Massive virtio-net throughput drop in guest VM with Linux 6.8+"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]