[libvirt] [RFC PATCH v1 0/4] Add cpu hotplug support to libvirt.

[Viktor Mihajlovski]

On 09/03/2012 08:06 AM, Tang Chen wrote:
> It seems that libvirt is not cpu hotplug aware.
> Please refer to the following problem.
>
> 1. At first, we have 2 cpus.
> # cat /cgroup/cpuset/cpuset.cpus
> 0-1
> # cat /cgroup/cpuset/libvirt/qemu/cpuset.cpus
> 0-1
>
> 2. And we have a vm1 with following configuration.
>    <cputune>
>      <vcpupin vcpu='0' cpuset='1'/>
>      <hypervisorpin cpuset='1'/>
>    </cputune>
>
> 3. Offline cpu1.
> # echo 0 > /sys/devices/system/cpu/cpu1/online
> # cat /sys/devices/system/cpu/cpu1/online
> 0
> # cat /cgroup/cpuset/cpuset.cpus
> 0
> # cat /cgroup/cpuset/libvirt/qemu/cpuset.cpus
> 0
> # cat /cgroup/cpuset/libvirt/lxc/cpuset.cpus
> 0
>
> 4. Online cpu1.
> # echo 1 > /sys/devices/system/cpu/cpu1/online
> # cat /sys/devices/system/cpu/cpu1/online
> 1
> # cat /cgroup/cpuset/cpuset.cpus
> 0-1
> # cat /cgroup/cpuset/libvirt/cpuset.cpus
> 0
> # cat /cgroup/cpuset/libvirt/qemu/cpuset.cpus
> 0
> # cat /cgroup/cpuset/libvirt/lxc/cpuset.cpus
> 0
>
> Here，cgroup updated cpuset.cpus，but not for libvirt directory，and also qemu and lxc directory.
> vm1 cannot be started again.
> # virsh start vm1
> error: Failed to start domain vm1
> error: Unable to set cpuset.cpus: Permission denied
>
> And libvird gave the following errors.
> 2012-07-17 07:30:22.478+0000: 3118: error : qemuSetupCgroupVcpuPin:498 : Unable to set cpuset.cpus: Permission denied
>
>
> These patches resolves this problem by listening on the netlink for cpu hotplug event.
> When the netlink service gets the cpu hotplug event, it will attract the cpuid in the message,
> and add it into cpuset.cpus in:
>      /cgroup/cpuset/libvirt
>      /cgroup/cpuset/libvirt/qemu
>      /cgroup/cpuset/libvirt/lxc
>

Hi,

this approach requires that libvirtd keeps running through the entire 
lifecycle of a guest. That is something that cannot be safely assumed 
and therefore hotplug events can be missed.
That means that libvirt must synchronize the hypervisors cpusets with 
the host's current CPU states. You could do that for instance when 
registering the callback.

-- 

Mit freundlichen Grüßen/Kind Regards
    Viktor Mihajlovski

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