Bug 996174 - xen domU kernel does not activate all vcpus
xen domU kernel does not activate all vcpus
Product: Fedora
Classification: Fedora
Component: xen (Show other bugs)
x86_64 Linux
unspecified Severity high
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Assigned To: Michael Young
Fedora Extras Quality Assurance
Depends On:
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Reported: 2013-08-12 10:59 EDT by Carl Roth
Modified: 2015-02-17 11:43 EST (History)
5 users (show)

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Fixed In Version:
Doc Type: Bug Fix
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Last Closed: 2015-02-17 11:43:51 EST
Type: Bug
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Description Carl Roth 2013-08-12 10:59:53 EDT
Description of problem:

I have a XEN domU instance running with two configured vcpus, but only one was activated at boot.  The VM was recently upgraded to F19; the F18 version of the VM *did* activate both cpus.

Version-Release number of selected component (if applicable):

Hypervisor is running F18:

domU is running F19:

How reproducible:


Steps to Reproduce:
1. Configure VM with 2 cpus
2. boot VM

Actual results:

hypervisor shows 2 vcpus for the VM, but only one is active.  VM detects 2 cpus but only activates one of them.

Expected results:

Additional info:

Here is a snippet of the libvirt XML for the domain:

<domain type='xen' id='7'>
  <memory unit='KiB'>2097152</memory>
  <currentMemory unit='KiB'>2097152</currentMemory>
  <vcpu placement='static'>2</vcpu>
    <type arch='x86_64' machine='xenpv'>linux</type>
  <clock offset='utc' adjustment='reset'/>

# xl vcpu-list | grep " 7 "
xen-d975xbx-anon                     7     0    3   -b-   15063.3  any cpu
xen-d975xbx-anon                     7     1    -   --p       0.0  any cpu

Then, on the VM:
[root@xen-d975xbx-anon ~]# dmesg | grep cpu
[    0.000000] Initializing cgroup subsys cpuset
[    0.000000] Initializing cgroup subsys cpu
[    0.000000] Initializing cgroup subsys cpuacct
[    0.000000] setup_percpu: NR_CPUS:128 nr_cpumask_bits:128 nr_cpu_ids:2 nr_node_ids:1
[    0.000000] PERCPU: Embedded 28 pages/cpu @ffff88007fa00000 s85568 r8192 d20928 u1048576
[    0.000000] pcpu-alloc: s85568 r8192 d20928 u1048576 alloc=1*2097152
[    0.000000] pcpu-alloc: [0] 0 1
[    0.000000]  RCU restricting CPUs from NR_CPUS=128 to nr_cpu_ids=2.
[    0.000000] Xen: using vcpuop timer interface
[    0.028029] NMI watchdog: disabled (cpu0): hardware events not enabled

[root@xen-d975xbx-anon ~]# grep -c processor /proc/cpuinfo
Comment 1 Carl Roth 2013-09-26 13:59:35 EDT
I browsed over to


and did some more experimentation, I find

1. libvirt domain configuration specifies 2 cpus for the guest
2. 'xl list' shows 1 cpu after the guest is started
3. /proc/cpuinfo on the guest shows 1 cpu
4. boot messages on the guest suggest 2 cpus, but only one is brought on-line
5. /sys/devices/system/cpu on the guest only shows 1 cpu
6. (as above) 'xl vcpu-list' shows two cpus, but one is paused/offline

Then, I tried

# xl vcpu-set xen-d975xbx-anon 2

At that point, the 2nd cpu entry shows up in /sys/devices/system/cpu.

Then, on the guest:

# echo "1" > /sys/devices/system/cpu/cpu1/online

and the cpu comes on-line.  The /proc/cpuinfo is correct, as is the 'xl list' and 'xl vcpu-list' output on the host.

Perhaps something is not getting configured correctly in libxl based on the libvirt configuration.  Please advise.  What's the (intended) relationship between the 'vcpu' tag in the libvirt xml, and the boot-time vcpu settings for a guest?  This did used to "just work" for F18.

The secondary issue (why a newly-available vcpu is not automatically brought on-line on the guest) appears to be a "known issue" with some debate surrounding it, so I won't bother.
Comment 2 Carl Roth 2013-09-26 14:03:30 EDT
As per


(though the text there is a bit confusing)

should the 'current' attribute be specified to enable the vcpus at startup?
Comment 3 Fedora End Of Life 2015-01-09 14:25:13 EST
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Comment 4 Fedora End Of Life 2015-02-17 11:43:51 EST
Fedora 19 changed to end-of-life (EOL) status on 2015-01-06. Fedora 19 is
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