[systemd-devel] machined: after CPU offline then online, vcpupin KVM guest failed to start

[Dr. Werner Fink]

Hi,

problem with v228 (and I guess this is also later AFAICS from logs of
current git) that repeating CPU hotplug events (offline/online). The
root cause is that cpuset.cpus become not restored by machined.
Please note that libvirt can not do this as it is not allowed to do so.

Steps to reproduce:

  1. Configure vCPU pinning.
    # virsh vcpupin guest-os 0 0-3 --config
  2. Boot the guest.
    # virsh start guest-os
  3. Shutdown the guest.
    # virsh shutdown guest-os
  4. Offline one of host CPUs.
    # echo 0 > /sys/devices/system/cpu/cpu3/online
  5. Online the host CPU again.
    # echo 1 > /sys/devices/system/cpu/cpu3/online
  6. Boot the guest again.
    # virsh start guest-os

Actual result:

  error: Failed to start domain guest-os
  error: Unable to write to '/sys/fs/cgroup/cpuset/machine.slice/machine-qemu\x2d2\x2dsles12sp2\x2dbeta3.scope/vcpu0/cpuset.cpus': Permission denied

Expected result:

  The KVM guest may boot without errors.

this could be done by hand with

  # echo 0-31 > /sys/fs/cgroup/cpuset/machine.slice/cpuset.cpus

as libvirt can't touch the cpuset in machine.slice scope since
this one is owned by machined.

This problem is also discussed at upstream of libvirt

  https://www.redhat.com/archives/libvir-list/2012-April/msg00777.html

and seems to be a well know problem not only here:

  https://bugzilla.redhat.com/show_bug.cgi?id=838070

from the kernel's side this seems to a behavior by design, from right before
the cpuset_hotplug_workfn():

  /*
   * [...]
   * Non-root cpusets are only affected by offlining.  If any CPUs or memory
   * nodes have been taken down, cpuset_hotplug_update_tasks() is invoked on
   * all descendants.
   * [...]
   */

Werner

PS: Using https://github.com/systemd/systemd/issues/new seems to be very limited
with > *NOTE: Do not submit bug reports about anything but the two most recently
     > released systemd versions upstream!*
-- 
  "Having a smoking section in a restaurant is like having
          a peeing section in a swimming pool." -- Edward Burr
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