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Chapter 13. Using virsh emulatorpin in virtual environments with NFV

Use this procedure to determine the impact of using virsh emulatorpin in Red Hat OpenStack Platform with NFV.

13.1. Symptom
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You experience packet loss in Red Hat OpenStack Platform {vernum} NFV environment, and have not configured emulator thread pinning.

Note

In Red Hat OpenStack Platform 10, customers need a support exception to pin emulator threads. However, pinning emulator threads is strongly recommended by Red Hat in almost all NFV cases. You can significantly increase performance by changing the default emulator thread configuration. Open a ticket with Red Hat support and request a support exception if needed.

13.2. Solution
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Use this section to investigate and configure emulator thread pinning.

13.2.1. qemu-kvm Emulator Threads
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Emulator threads handle interrupt requests and non-blocking processes for virtual machine hardware emulation. Threads not running vCPUs are qemu-kvm emulator threads. See the following example. 

[root@overcloud-compute-0 ~]# ps -Tp `pgrep -f instance-00000009`
    PID    SPID TTY          TIME CMD
 364936  364936 ?        00:00:02 qemu-kvm
 364936  364946 ?        00:00:00 qemu-kvm
 364936  364952 ?        00:00:52 CPU 0/KVM
 364936  364953 ?        00:00:26 CPU 1/KVM
 364936  364954 ?        00:00:30 CPU 2/KVM
 364936  364956 ?        00:00:00 vnc_worker
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Due to the Linux CFS (completely fair scheduler), emulator threads normally move periodically from one pCPU to another, within the defined in libvirt’s emulator pin set.

In NFV contexts, you might experience issues if you configure emulator threads when using the isolcpus parameter, because this kernel configuration disables the CFS scheduling on those CPUs. If you are not using the isolcpus parameter, you can experience packet loss when the emulator threads interrupt CPUs that are processing packets.

Examples of emulator threads include:

  • qemu-kvm threads
  • vnc_worker threads
  • vhost-<qemu-kvm PID> kernel threads (When virtio-net is used (kernel networking on the hypervisor)

By default, nova will configure an emulator thread pin set which spans the pCPUs assigned to all vCPUs. If you are not using the isolcpus parameter, then emulator threads can be scheduled on any pCPU, and will periodically move from one pCPU to another. 

virsh dumpxml instance-0000001d
(...)
  <vcpu placement='static'>4</vcpu>
  <cputune>
    <shares>4096</shares>
    <vcpupin vcpu='0' cpuset='34'/>
    <vcpupin vcpu='1' cpuset='14'/>
    <vcpupin vcpu='2' cpuset='10'/>
    <vcpupin vcpu='3' cpuset='30'/>
    <emulatorpin cpuset='10,14,30,34'/>
  </cputune>
(...)
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[root@overcloud-compute-0 ~]# virsh dumpxml instance-00000009
(...)
        <nova:vcpus>3</nova:vcpus>
  <vcpu placement='static'>3</vcpu>
    <vcpupin vcpu='0' cpuset='1'/>
    <vcpupin vcpu='1' cpuset='2'/>
    <vcpupin vcpu='2' cpuset='3'/>
(...)
<emulatorpin cpuset='1-3'/>
(...)
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Therefore any of these CPUs can be preempted by qemu’s emulator threads, risking packet drops.

In Red Hat OpenStack Platforms 10, there is no officially supported way to pin emulator threads. Temporarily, you can move emulator threads to a set of pCPUs by using virsh emulatorpin (…​) --live, as shown in the following example. 

# to pin emulator threads of instance instance-0000001d to CPU 34
virsh emulatorpin instance-0000001d 34 -live
# to pin emulator threads of instance instance-0000001d to CPUs 32,34
virsh emulatorpin instance-0000001d 32,34 --live
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These changes only last for the runtime of the instance. Permanent modifications require an external mechanism, such as a cron job, bash script or Ansible task. This has to be the subject of a support exception.

When isolcpus is used, CFS scheduler is disabled and all emulator threads will run on the first available, lowest indexed pCPU. As a consequence, without intervention or further configuration, one vCPU of the instance runs a high risk for resource contention with the emulator threads.

Further details can be found at Kernel.org Bugzilla – Bug 116701.

Use the following algorithm to determine which vCPU the emulator threads are using: 

PCPU=MIN([EMULATORPINSET])
VCPU=REVERSE_CPUSET(PCPU)

REVERSE_CPUSET :=  SELECT pcpu from `virsh dumpxml <instance name> | grep "cpuset=$PCPU"`
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For example, in this instance, all emulator threads and children inherit affinity 1-3 from the default emulator pin set: 

[root@overcloud-compute-0 ~]# taskset -a -c -p `pgrep -f instance-00000009`
pid 364936's current affinity list: 1-3
pid 364946's current affinity list: 1-3
pid 364952's current affinity list: 1
pid 364953's current affinity list: 2
pid 364954's current affinity list: 3
pid 364956's current affinity list: 1-3
[root@overcloud-compute-0 ~]# ps -Tp `pgrep -f instance-00000009`
    PID    SPID TTY          TIME CMD
 364936  364936 ?        00:00:02 qemu-kvm
 364936  364946 ?        00:00:00 qemu-kvm
 364936  364952 ?        00:00:51 CPU 0/KVM
 364936  364953 ?        00:00:26 CPU 1/KVM
 364936  364954 ?        00:00:30 CPU 2/KVM
 364936  364956 ?        00:00:00 vnc_worker
[root@overcloud-compute-0 ~]# pgrep -f vhost- | xargs -I {} taskset -a -c -p {}
pid 364948's current affinity list: 1-3
pid 364949's current affinity list: 1-3
pid 364950's current affinity list: 1-3
[root@overcloud-compute-0 ~]#
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In combination with isolcpus, all emulator threads and the vhost-* threads execute on pCPU1 and are never rescheduled: 

cat /proc/sched_debug | sed '/^cpu#/,/^runnable/{//!d}' | grep vhost -C3
(...)
cpu#1, 2099.998 MHz
runnable tasks:
            task   PID         tree-key  switches  prio     wait-time             sum-exec        sum-sleep
----------------------------------------------------------------------------------------------------------
      watchdog/1    11        -2.995579    410285     0         0.000000      5025.887998         0.000000 0 /
     migration/1    12         0.000000        79     0         0.000000         3.375060         0.000000 0 /
     ksoftirqd/1    13   5172444.259776        54   120         0.000000         0.570500         0.000000 0 /
     kworker/1:0    14   5188475.472257       370   120         0.000000        14.707114         0.000000 0 /
    kworker/1:0H    15      8360.049510        10   100         0.000000         0.150151         0.000000 0 /
     kworker/1:1  2707   5045807.055876     16370   120         0.000000       793.611916         0.000000 0 /
    kworker/1:1H  2763   5187682.987749     11755   100         0.000000       191.949725         0.000000 0 /
        qemu-kvm 364936      3419.522791     50276   120         0.000000      2476.880384         0.000000 0 /machine.slice/machine-qemu\x2d6\x2dinstance\x2d00000009.scope/emulator
        qemu-kvm 364946      1270.815296       102   120         0.000000        23.204111         0.000000 0 /machine.slice/machine-qemu\x2d6\x2dinstance\x2d00000009.scope/emulator
       CPU 0/KVM 364952     52703.660314     53709   120         0.000000     52715.105472         0.000000 0 /machine.slice/machine-qemu\x2d6\x2dinstance\x2d00000009.scope/vcpu0
      vnc_worker 364956       123.609634         1   120         0.000000         0.016849         0.000000 0 /machine.slice/machine-qemu\x2d6\x2dinstance\x2d00000009.scope/emulator
    vhost-364936 364948      3410.527677      1039   120         0.000000        84.254772         0.000000 0 /machine.slice/machine-qemu\x2d6\x2dinstance\x2d00000009.scope/emulator
    vhost-364936 364949      3407.341502        55   120         0.000000         2.894394         0.000000 0 /machine.slice/machine-qemu\x2d6\x2dinstance\x2d00000009.scope/emulator
    vhost-364936 364950      3410.395220       174   120         0.000000        10.969077         0.000000 0 /machine.slice/machine-qemu\x2d6\x2dinstance\x2d00000009.scope/emulator

cpu#2, 2099.998 MHz
runnable tasks:
            task   PID         tree-key  switches  prio     wait-time             sum-exec        sum-sleep
----------------------------------------------------------------------------------------------------------
      watchdog/2    16        -5.995418    410285     0         0.000000      5197.571153         0.000000 0 /
     migration/2    17         0.000000        79     0         0.000000         3.384688         0.000000 0 /
     ksoftirqd/2    18        -7.031102         3   120         0.000000         0.019079         0.000000 0 /
     kworker/2:0    19         0.119413        39   120         0.000000         0.588589         0.000000 0 /
    kworker/2:0H    20        -1.047613         8   100         0.000000         0.086272         0.000000 0 /
     kworker/2:1  2734   1475469.236026     11322   120         0.000000       241.388582         0.000000 0 /
       CPU 1/KVM 364953     27258.370583     33294   120         0.000000     27269.017017         0.000000 0 /machine.slice/machine-qemu\x2d6\x2dinstance\x2d00000009.scope/vcpu1

cpu#3, 2099.998 MHz
runnable tasks:
            task   PID         tree-key  switches  prio     wait-time             sum-exec        sum-sleep
----------------------------------------------------------------------------------------------------------
      watchdog/3    21        -5.996592    410285     0         0.000000      4970.777439         0.000000 0 /
     migration/3    22         0.000000        79     0         0.000000         3.886799         0.000000 0 /
     ksoftirqd/3    23        -7.035295         3   120         0.000000         0.014677         0.000000 0 /
     kworker/3:0    24        17.758583        38   120         0.000000         0.637152         0.000000 0 /
    kworker/3:0H    25        -1.047727         8   100         0.000000         0.077141         0.000000 0 /
     kworker/3:1 362530    154177.523420        83   120         0.000000         6.544285         0.000000 0 /
       CPU 2/KVM 364954     32456.061889     25966   120         0.000000     32466.719084         0.000000 0 /machine.slice/machine-qemu\x2d6\x2dinstance\x2d00000009.scope/vcpu2
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13.2.5. Optimal Location of Emulator Threads
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This section provides descriptions for placing emulator threads on the following networks:

  • DPDK networking within the instance and netdev datapath in Open vSwitch
  • DPDK networking within the instance, system datapath in Open vSwitch and kernel space networking on the hypervisor
  • Kernel networking within the instance and netdev datapath in Open vSwitch

If DPDK runs within the instance, packet processing is done entirely in the user space. Do not schedule PMDs to run on vCPU0, as this should remain for the OS and interrupt handling. Because PMD CPUs within the instance run an active loop and need 100% of the CPU, they should not be preempted. Packet loss can occur if one of these vCPUs is preempted. Thus, the emulatorpin cpuset needs to be configured in such a way that it does not overlap with the physical CPUs that handle the virtual CPUs numbered 1 and above.

With DPDK networking within the instance, the optimal location for emulator threads is either the pCPU that is handling vCPU 0 or a dedicated physical CPU that is not handling any virtual CPUs at all.

If OVS-DPDK is used on the hypervisor and DPDK within the instance, place the emulator thread on vCPU 0.

If kernel space networking is used on the hypervisor, then packet processing on the hypervisor is executed within the kernel.

With DPDK networking within the instance, the optimal location for emulator threads is either the pCPU that is handling vCPU 0, or a dedicated physical CPU that is not handling any virtual CPUs.

Note that in this scenario, packet processing for the vNIC queues is executed within vhost-<qemu-kvm PID> kernel threads of the hypervisor. Under high traffic, these kernel threads can generate a significant CPU load. The optimal location of the emulator threads needs to be determined on a case-by-case basis. 

[root@overcloud-compute-0 ~]# ps aux | grep vhost-
root      364948  0.0  0.0      0     0 ?        S    20:32   0:00 [vhost-364936]
root      364949  0.0  0.0      0     0 ?        S    20:32   0:00 [vhost-364936]
root      364950  0.0  0.0      0     0 ?        S    20:32   0:00 [vhost-364936]
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With kernel networking within the instance, there are two options:

  • Optimize the interrupt distribution, for example, softirqs within the instance. In such a case, you do not have to allocate an additional pCPU for emulator threads and can assign the emulator threads to a pCPU that is not handling any network interrupts.
  • Use a dedicated pCPU on the same NUMA node for emulator threads.

Due to the complexity of the first option, the second option is recommended.

13.3. Diagnosis
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13.3.1. The Demonstration Environment
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The demonstration environment runs one instance: instance-0000001d. Its associated qemu-kvm thread has the following PID: 

[root@overcloud-compute-0 ~]# pidof qemu-kvm
73517
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13.3.2. How Emulatorpin works
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By default, a Red Hat OpenStack Platform deployment uses the following settings: 

virsh dumpxml instance-0000001d
(...)
  <vcpu placement='static'>4</vcpu>
  <cputune>
    <shares>4096</shares>
    <vcpupin vcpu='0' cpuset='34'/>
    <vcpupin vcpu='1' cpuset='14'/>
    <vcpupin vcpu='2' cpuset='10'/>
    <vcpupin vcpu='3' cpuset='30'/>
    <emulatorpin cpuset='10,14,30,34'/>
  </cputune>
(...)
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This leads to an unpredictable allocation of the emulator threads, such as qemu-kvm, vnc_worker, and so on: 

[root@overcloud-compute-0 ~]# ps -T -p 73517
    PID    SPID TTY          TIME CMD
  73517   73517 ?        00:00:00 qemu-kvm
  73517   73527 ?        00:00:00 qemu-kvm
  73517   73535 ?        00:00:06 CPU 0/KVM
  73517   73536 ?        00:00:02 CPU 1/KVM
  73517   73537 ?        00:00:03 CPU 2/KVM
  73517   73538 ?        00:00:02 CPU 3/KVM
  73517   73540 ?        00:00:00 vnc_worker
[root@overcloud-compute-0 ~]# taskset -apc 73517
pid 73517's current affinity list: 10,14,30,34
pid 73527's current affinity list: 10,14,30,34
pid 73535's current affinity list: 34
pid 73536's current affinity list: 14
pid 73537's current affinity list: 10
pid 73538's current affinity list: 30
pid 73540's current affinity list: 10,14,30,34
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[root@overcloud-compute-0 ~]# virsh vcpupin instance-0000001d | awk '$NF~/[0-9]+/ {print $NF}' | sort -n | while read CPU; do sed '/cpu#/,/runnable task/{//!d}' /proc/sched_debug | sed -n "/^cpu#${CPU},/,/^$/p" ; done
cpu#10, 2197.477 MHz
runnable tasks:
            task   PID         tree-key  switches  prio     wait-time             sum-exec        sum-sleep
----------------------------------------------------------------------------------------------------------
    migration/10    64         0.000000       107     0         0.000000        90.232791         0.000000 0 /
    ksoftirqd/10    65       -13.045337         3   120         0.000000         0.004679         0.000000 0 /
    kworker/10:0    66       -12.892617        40   120         0.000000         0.157359         0.000000 0 /
   kworker/10:0H    67        -9.320550         8   100         0.000000         0.015065         0.000000 0 /
    kworker/10:1 17996      9695.675528        23   120         0.000000         0.222805         0.000000 0 /
        qemu-kvm 73517      1994.534332     27105   120         0.000000       886.203254         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/emulator
        qemu-kvm 73527       722.347466        84   120         0.000000        18.236155         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/emulator
       CPU 2/KVM 73537      3356.749162     18051   120         0.000000      3370.045619         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/vcpu2
      vnc_worker 73540       354.007735         1   120         0.000000         0.047002         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/emulator
          worker 74584      1970.499537         5   120         0.000000         0.130143         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/emulator
          worker 74585      1970.492700         4   120         0.000000         0.071887         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/emulator
          worker 74586      1982.467246         3   120         0.000000         0.033604         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/emulator
          worker 74587      1994.520768         1   120         0.000000         0.076039         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/emulator
          worker 74588      2006.500153         1   120         0.000000         0.004878         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/emulator

cpu#14, 2197.477 MHz
runnable tasks:
            task   PID         tree-key  switches  prio     wait-time             sum-exec        sum-sleep
----------------------------------------------------------------------------------------------------------
    migration/14    88         0.000000       107     0         0.000000        90.107596         0.000000 0 /
    ksoftirqd/14    89       -13.045376         3   120         0.000000         0.004782         0.000000 0 /
    kworker/14:0    90       -12.921990        40   120         0.000000         0.128166         0.000000 0 /
   kworker/14:0H    91        -9.321186         8   100         0.000000         0.016870         0.000000 0 /
    kworker/14:1 17999      6247.571171         5   120         0.000000         0.028576         0.000000 0 /
       CPU 1/KVM 73536      2274.381281      6679   120         0.000000      2287.691654         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/vcpu1

cpu#30, 2197.477 MHz
runnable tasks:
            task   PID         tree-key  switches  prio     wait-time             sum-exec        sum-sleep
----------------------------------------------------------------------------------------------------------
    migration/30   180         0.000000       107     0         0.000000        89.206960         0.000000 0 /
    ksoftirqd/30   181       -13.045892         3   120         0.000000         0.003828         0.000000 0 /
    kworker/30:0   182       -12.929272        40   120         0.000000         0.120754         0.000000 0 /
   kworker/30:0H   183        -9.321056         8   100         0.000000         0.018042         0.000000 0 /
    kworker/30:1 18012      6234.935501         5   120         0.000000         0.026505         0.000000 0 /
       CPU 3/KVM 73538      2474.183301     12595   120         0.000000      2487.479666         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/vcpu3

cpu#34, 2197.477 MHz
runnable tasks:
            task   PID         tree-key  switches  prio     wait-time             sum-exec        sum-sleep
----------------------------------------------------------------------------------------------------------
    migration/34   204         0.000000       107     0         0.000000        89.067908         0.000000 0 /
    ksoftirqd/34   205       -13.046824         3   120         0.000000         0.002884         0.000000 0 /
    kworker/34:0   206       -12.922407        40   120         0.000000         0.127423         0.000000 0 /
   kworker/34:0H   207        -9.320822         8   100         0.000000         0.017381         0.000000 0 /
    kworker/34:1 18016     10788.797590         7   120         0.000000         0.042631         0.000000 0 /
       CPU 0/KVM 73535      5969.227225     14233   120         0.000000      5983.425363         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/vcpu0
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The emulator threads can be moved by using virsh emulatorpin: 

virsh emulatorpin instance-0000001d 34
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Note that the affinity for all non-CPU threads changes. 

[root@overcloud-compute-0 ~]# ps -T -p 73517
    PID    SPID TTY          TIME CMD
  73517   73517 ?        00:00:00 qemu-kvm
  73517   73527 ?        00:00:00 qemu-kvm
  73517   73535 ?        00:00:06 CPU 0/KVM
  73517   73536 ?        00:00:02 CPU 1/KVM
  73517   73537 ?        00:00:03 CPU 2/KVM
  73517   73538 ?        00:00:02 CPU 3/KVM
  73517   73540 ?        00:00:00 vnc_worker
[root@overcloud-compute-0 ~]# taskset -apc 73517
pid 73517's current affinity list: 34
pid 73527's current affinity list: 34
pid 73535's current affinity list: 34
pid 73536's current affinity list: 14
pid 73537's current affinity list: 10
pid 73538's current affinity list: 30
pid 73540's current affinity list: 34
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Note the number of switches in the historic data in /proc/sched_debug. In the following example, PID 73517 already moved to cpu#34. The other emulator workers did not run since the last output, and therefore still show on cpu#10: 

[root@overcloud-compute-0 ~]# virsh vcpupin instance-0000001d | awk '$NF~/[0-9]+/ {print $NF}' | sort -n | while read CPU; do sed '/cpu#/,/runnable task/{//!d}' /proc/sched_debug | sed -n "/^cpu#${CPU},/,/^$/p" ; done
cpu#10, 2197.477 MHz
runnable tasks:
            task   PID         tree-key  switches  prio     wait-time             sum-exec        sum-sleep
----------------------------------------------------------------------------------------------------------
    migration/10    64         0.000000       107     0         0.000000        90.232791         0.000000 0 /
    ksoftirqd/10    65       -13.045337         3   120         0.000000         0.004679         0.000000 0 /
    kworker/10:0    66       -12.892617        40   120         0.000000         0.157359         0.000000 0 /
   kworker/10:0H    67        -9.320550         8   100         0.000000         0.015065         0.000000 0 /
    kworker/10:1 17996      9747.429082        26   120         0.000000         0.255547         0.000000 0 /
        qemu-kvm 73527       722.347466        84   120         0.000000        18.236155         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/emulator
       CPU 2/KVM 73537      3424.520709     21610   120         0.000000      3437.817166         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/vcpu2
      vnc_worker 73540       354.007735         1   120         0.000000         0.047002         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/emulator

cpu#14, 2197.477 MHz
runnable tasks:
            task   PID         tree-key  switches  prio     wait-time             sum-exec        sum-sleep
----------------------------------------------------------------------------------------------------------
    migration/14    88         0.000000       107     0         0.000000        90.107596         0.000000 0 /
    ksoftirqd/14    89       -13.045376         3   120         0.000000         0.004782         0.000000 0 /
    kworker/14:0    90       -12.921990        40   120         0.000000         0.128166         0.000000 0 /
   kworker/14:0H    91        -9.321186         8   100         0.000000         0.016870         0.000000 0 /
    kworker/14:1 17999      6247.571171         5   120         0.000000         0.028576         0.000000 0 /
       CPU 1/KVM 73536      2283.094453      7028   120         0.000000      2296.404826         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/vcpu1

cpu#30, 2197.477 MHz
runnable tasks:
            task   PID         tree-key  switches  prio     wait-time             sum-exec        sum-sleep
----------------------------------------------------------------------------------------------------------
    migration/30   180         0.000000       107     0         0.000000        89.206960         0.000000 0 /
    ksoftirqd/30   181       -13.045892         3   120         0.000000         0.003828         0.000000 0 /
    kworker/30:0   182       -12.929272        40   120         0.000000         0.120754         0.000000 0 /
   kworker/30:0H   183        -9.321056         8   100         0.000000         0.018042         0.000000 0 /
    kworker/30:1 18012      6234.935501         5   120         0.000000         0.026505         0.000000 0 /
       CPU 3/KVM 73538      2521.828931     14047   120         0.000000      2535.125296         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/vcpu3

cpu#34, 2197.477 MHz
runnable tasks:
            task   PID         tree-key  switches  prio     wait-time             sum-exec        sum-sleep
----------------------------------------------------------------------------------------------------------
    migration/34   204         0.000000       107     0         0.000000        89.067908         0.000000 0 /
    ksoftirqd/34   205       -13.046824         3   120         0.000000         0.002884         0.000000 0 /
    kworker/34:0   206       -12.922407        40   120         0.000000         0.127423         0.000000 0 /
   kworker/34:0H   207        -9.320822         8   100         0.000000         0.017381         0.000000 0 /
    kworker/34:1 18016     10788.797590         7   120         0.000000         0.042631         0.000000 0 /
        qemu-kvm 73517         2.613794     27706   120         0.000000       941.839262         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/emulator
       CPU 0/KVM 73535      5994.533905     15169   120         0.000000      6008.732043         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/vcpu0
Copy to Clipboard Toggle word wrap

Note how thread 73517 moves to cpu#34. If you now interact with a VNC session, you can see that /proc/sched_debug shows the vnc_worker threads on cpu#34 as well. 

[root@overcloud-compute-0 ~]# virsh vcpupin instance-0000001d | awk '$NF~/[0-9]+/ {print $NF}' | sort -n | while read CPU; do sed '/cpu#/,/runnable task/{//!d}' /proc/sched_debug | sed -n "/^cpu#${CPU},/,/^$/p" ; done
cpu#10, 2197.477 MHz
runnable tasks:
            task   PID         tree-key  switches  prio     wait-time             sum-exec        sum-sleep
----------------------------------------------------------------------------------------------------------
    migration/10    64         0.000000       107     0         0.000000        90.232791         0.000000 0 /
    ksoftirqd/10    65       -13.045337         3   120         0.000000         0.004679         0.000000 0 /
    kworker/10:0    66       -12.892617        40   120         0.000000         0.157359         0.000000 0 /
   kworker/10:0H    67        -9.320550         8   100         0.000000         0.015065         0.000000 0 /
    kworker/10:1 17996      9963.300958        27   120         0.000000         0.273007         0.000000 0 /
        qemu-kvm 73527       722.347466        84   120         0.000000        18.236155         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/emulator
       CPU 2/KVM 73537      3563.793234     26162   120         0.000000      3577.089691         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/vcpu2

cpu#14, 2197.477 MHz
runnable tasks:
            task   PID         tree-key  switches  prio     wait-time             sum-exec        sum-sleep
----------------------------------------------------------------------------------------------------------
    migration/14    88         0.000000       107     0         0.000000        90.107596         0.000000 0 /
    ksoftirqd/14    89       -13.045376         3   120         0.000000         0.004782         0.000000 0 /
    kworker/14:0    90       -12.921990        40   120         0.000000         0.128166         0.000000 0 /
   kworker/14:0H    91        -9.321186         8   100         0.000000         0.016870         0.000000 0 /
    kworker/14:1 17999      6247.571171         5   120         0.000000         0.028576         0.000000 0 /
       CPU 1/KVM 73536      2367.789075      9648   120         0.000000      2381.099448         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/vcpu1

cpu#30, 2197.477 MHz
runnable tasks:
            task   PID         tree-key  switches  prio     wait-time             sum-exec        sum-sleep
----------------------------------------------------------------------------------------------------------
    migration/30   180         0.000000       107     0         0.000000        89.206960         0.000000 0 /
    ksoftirqd/30   181       -13.045892         3   120         0.000000         0.003828         0.000000 0 /
    kworker/30:0   182       -12.929272        40   120         0.000000         0.120754         0.000000 0 /
   kworker/30:0H   183        -9.321056         8   100         0.000000         0.018042         0.000000 0 /
    kworker/30:1 18012      6234.935501         5   120         0.000000         0.026505         0.000000 0 /
       CPU 3/KVM 73538      2789.628278     24788   120         0.000000      2802.924643         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/vcpu3

cpu#34, 2197.477 MHz
runnable tasks:
            task   PID         tree-key  switches  prio     wait-time             sum-exec        sum-sleep
----------------------------------------------------------------------------------------------------------
    migration/34   204         0.000000       107     0         0.000000        89.067908         0.000000 0 /
    ksoftirqd/34   205       -13.046824         3   120         0.000000         0.002884         0.000000 0 /
    kworker/34:0   206       -12.922407        40   120         0.000000         0.127423         0.000000 0 /
   kworker/34:0H   207        -9.320822         8   100         0.000000         0.017381         0.000000 0 /
    kworker/34:1 18016     11315.391422        25   120         0.000000         0.196078         0.000000 0 /
        qemu-kvm 73517       471.930276     30975   120         0.000000      1295.543576         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/emulator
       CPU 0/KVM 73535      6160.062172     19201   120         0.000000      6174.260310         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/vcpu0
      vnc_worker 73540       459.653524        38   120         0.000000         7.535037         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/emulator
          worker 78703       449.098251         2   120         0.000000         0.120313         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/emulator
          worker 78704       449.131175         3   120         0.000000         0.066961         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/emulator
          worker 78705       461.100994         4   120         0.000000         0.022897         0.000000 0 /machine.slice/machine-qemu\x2d1\x2dinstance\x2d0000001d.scope/emulator
Copy to Clipboard Toggle word wrap
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