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    SubjectRe: [PATCH v3 0/3] cpu: pseries: Cpu offline states framework
    On Wed, Sep 16, 2009 at 05:32:51PM +0200, Peter Zijlstra wrote:
    > On Wed, 2009-09-16 at 20:58 +0530, Dipankar Sarma wrote:
    > > On Tue, Sep 15, 2009 at 02:11:41PM +0200, Peter Zijlstra wrote:
    > > > On Tue, 2009-09-15 at 17:36 +0530, Gautham R Shenoy wrote:
    > > > > This patchset contains the offline state driver implemented for
    > > > > pSeries. For pSeries, we define three available_hotplug_states. They are:
    > > > >
    > > > > online: The processor is online.
    > > > >
    > > > > offline: This is the the default behaviour when the cpu is offlined
    > > > >
    > > > > inactive: This cedes the vCPU to the hypervisor with a cede latency
    > > > >
    > > > > Any feedback on the patchset will be immensely valuable.
    > > >
    > > > I still think its a layering violation... its the hypervisor manager
    > > > that should be bothered in what state an off-lined cpu is in.
    > >
    > > The problem is that all hypervisor managers cannot figure out what sort
    > > of latency guest OS can tolerate under the situation. They wouldn't know
    > > from what context guest OS has ceded the vcpu. It has to have
    > > some sort of hint, which is what the guest OS provides.
    >
    > I'm missing something here, hot-unplug is a slow path and should not
    > ever be latency critical..?

    You aren't, I did :)

    No, for this specific case, latency isn't an issue. The issue is -
    how do we cede unused vcpus to hypervisor for better energy management ?
    Yes, it can be done by a hypervisor manager telling the kernel to
    offline and make a bunch of vcpus "inactive". It does have to choose
    offline (release vcpu) vs. inactive (cede but guranteed if needed).
    The problem is that long ago we exported a lot of hotplug stuff to
    userspace through the sysfs interface and we cannot do something
    inside the kernel without keeping the sysfs stuff consistent.
    This seems like a sane way to do that without undoing all the
    virtual cpu hotplug infrastructure in different supporting archs.

    Thanks
    Dipankar


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