qemu process consumes 100% host CPU after reverting snapshot

[Chun Feng Wu]

Hi,

I am testing throttle filter chain(multiple throttle-groups on disk) with the following steps:
1. start guest vm(chained throttle filters applied on disk per https://github.com/qemu/qemu/blob/master/docs/throttle.txt)
2. take snapshot
3. revert snapshot

after step3, I noticed qemu process in host consumes 100% cpu, and after I login guest vm, vm cannot(or slowly) response my cmd (it works well before reverting).

    PID     USER      PR  NI    VIRT    RES         SHR    S  %CPU  %MEM     TIME+ COMMAND
   65455 root      20   0 9659924 891328  20132 R 100.3   5.4    29:39.93 qemu-system-x86

Does anybody know why such issue happens?  is it a bug or I misunderstand something?

my cmd:

qemu-system-x86_64 \
  -name ubuntu-20.04-vm,debug-threads=on \
  -machine pc-i440fx-jammy,usb=off,dump-guest-core=off \
  -accel kvm \
  -cpu Broadwell-IBRS,vme=on,ss=on,vmx=on,f16c=on,rdrand=on,hypervisor=on,arat=on,tsc-adjust=on,md-clear=on,stibp=on,ssbd=on,xsaveopt=on,pdpe1gb=on,abm=on,tsx-ctrl=off,hle=off,rtm=off \
  -m 8192 \
  -overcommit mem-lock=off \
  -smp 2,sockets=1,dies=1,cores=1,threads=2 \
  -numa node,nodeid=0,cpus=0-1,memdev=ram \
  -object memory-backend-ram,id=ram,size=8192M \
  -uuid d2d68f5d-bff0-4167-bbc3-643e3566b8fb \
  -display none \
  -nodefaults \
  -monitor stdio \
  -rtc base=utc,driftfix=slew \
  -no-shutdown \
  -boot strict=on \
  -device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 \
  -object '{"qom-type":"throttle-group","id":"limit0","limits":{"bps-total":0,"bps-read":0,"bps-write":0,"iops-total":200,"iops-read":0,"iops-write":0,"bps-total-max":0,"bps-read-max":0,"bps-write-max":0,"iops-total-max":200,"iops-read-max":0,"iops-write-max":0,"iops-size":0,"bps-total-max-length":1,"bps-read-max-length":1,"bps-write-max-length":1,"iops-total-max-length":1,"iops-read-max-length":1,"iops-write-max-length":1}}' \
    -object '{"qom-type":"throttle-group","id":"limit1","limits":{"bps-total":0,"bps-read":0,"bps-write":0,"iops-total":250,"iops-read":0,"iops-write":0,"bps-total-max":0,"bps-read-max":0,"bps-write-max":0,"iops-total-max":250,"iops-read-max":0,"iops-write-max":0,"iops-size":0,"bps-total-max-length":1,"bps-read-max-length":1,"bps-write-max-length":1,"iops-total-max-length":1,"iops-read-max-length":1,"iops-write-max-length":1}}' \
    -object '{"qom-type":"throttle-group","id":"limit2","limits":{"bps-total":0,"bps-read":0,"bps-write":0,"iops-total":300,"iops-read":0,"iops-write":0,"bps-total-max":0,"bps-read-max":0,"bps-write-max":0,"iops-total-max":300,"iops-read-max":0,"iops-write-max":0,"iops-size":0,"bps-total-max-length":1,"bps-read-max-length":1,"bps-write-max-length":1,"iops-total-max-length":1,"iops-read-max-length":1,"iops-write-max-length":1}}' \
    -object '{"qom-type":"throttle-group","id":"limit012","limits":{"bps-total":0,"bps-read":0,"bps-write":0,"iops-total":400,"iops-read":0,"iops-write":0,"bps-total-max":0,"bps-read-max":0,"bps-write-max":0,"iops-total-max":400,"iops-read-max":0,"iops-write-max":0,"iops-size":0,"bps-total-max-length":1,"bps-read-max-length":1,"bps-write-max-length":1,"iops-total-max-length":1,"iops-read-max-length":1,"iops-write-max-length":1}}' \
  -blockdev '{"driver":"file","filename":"/virt/images/focal-server-cloudimg-amd64.img","node-name":"libvirt-4-storage","auto-read-only":true,"discard":"unmap"}' \
  -blockdev '{"node-name":"libvirt-4-format","read-only":false,"driver":"qcow2","file":"libvirt-4-storage","backing":null}' \
  -device virtio-blk-pci,bus=pci.0,addr=0x4,drive=libvirt-4-format,id=virtio-disk0,bootindex=1 \
  -blockdev '{"driver":"file","filename":"/virt/disks/vm1_disk_1.qcow2","node-name":"libvirt-3-storage","auto-read-only":true,"discard":"unmap"}' \
  -blockdev '{"node-name":"libvirt-3-format","read-only":false,"driver":"qcow2","file":"libvirt-3-storage","backing":null}' \
  -blockdev '{"driver":"throttle","node-name":"libvirt-5-filter","throttle-group":"limit0","file":"libvirt-3-format"}' \
  -blockdev '{"driver":"throttle","node-name":"libvirt-6-filter","throttle-group":"limit012","file":"libvirt-5-filter"}' \
  -device virtio-blk-pci,bus=pci.0,addr=0x5,drive=libvirt-6-filter,id=virtio-disk1 \
  -blockdev '{"driver":"file","filename":"/virt/disks/vm1_disk_2.qcow2","node-name":"libvirt-2-storage","auto-read-only":true,"discard":"unmap"}' \
  -blockdev '{"node-name":"libvirt-2-format","read-only":false,"driver":"qcow2","file":"libvirt-2-storage","backing":null}' \
  -blockdev '{"driver":"throttle","node-name":"libvirt-3-filter","throttle-group":"limit1","file":"libvirt-2-format"}' \
  -blockdev '{"driver":"throttle","node-name":"libvirt-4-filter","throttle-group":"limit012","file":"libvirt-3-filter"}' \
  -device virtio-blk-pci,bus=pci.0,addr=0x6,drive=libvirt-4-filter,id=virtio-disk2 \
  -blockdev '{"driver":"file","filename":"/virt/disks/vm1_disk_3.qcow2","node-name":"libvirt-1-storage","auto-read-only":true,"discard":"unmap"}' \
  -blockdev '{"node-name":"libvirt-1-format","read-only":false,"driver":"qcow2","file":"libvirt-1-storage","backing":null}' \
  -blockdev '{"driver":"throttle","node-name":"libvirt-1-filter","throttle-group":"limit2","file":"libvirt-1-format"}' \
  -blockdev '{"driver":"throttle","node-name":"libvirt-2-filter","throttle-group":"limit012","file":"libvirt-1-filter"}' \
  -device virtio-blk-pci,bus=pci.0,addr=0x7,drive=libvirt-2-filter,id=virtio-disk3 \
  -netdev user,id=user0,hostfwd=tcp::2222-:22 \
  -device virtio-net-pci,netdev=user0,id=net0,mac=52:54:00:12:34:56,bus=pci.0,addr=0x3 \
  -chardev pty,id=charserial0 \
  -device isa-serial,chardev=charserial0,id=serial0,index=0 \
  -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x2 \
  -sandbox on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny \
  -msg timestamp=on

snapshot and reverting:

(qemu) info status
VM status: running
(qemu) savevm snapshot1
(qemu) loadvm snapshot1

my env:
:~# qemu-system-x86_64 --version
QEMU emulator version 6.2.0 (Debian 1:6.2+dfsg-2ubuntu6.17)
Copyright (c) 2003-2021 Fabrice Bellard and the QEMU Project developers

:~# lsb_release -a
No LSB modules are available.
Distributor ID:    Ubuntu
Description:    Ubuntu 22.04.4 LTS
Release:    22.04
Codename:    jammy

-- 
Thanks and Regards,

Wu