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    SubjectRe: [PATCH v3 0/3] cpu: pseries: Cpu offline states framework
    On Tue, Sep 15, 2009 at 08:28:34PM +0530, Balbir Singh wrote:
    > * Peter Zijlstra <a.p.zijlstra@chello.nl> [2009-09-15 14:11:41]:
    >
    > > 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
    > > > even in the absense of this driver. The CPU would call make an
    > > > rtas_stop_self() call and hand over the CPU back to the resource pool,
    > > > thereby effectively deallocating that vCPU from the LPAR.
    > > > NOTE: This would result in a configuration change to the LPAR
    > > > which is visible to the outside world.
    > > >
    > > > inactive: This cedes the vCPU to the hypervisor with a cede latency
    > > > specifier value 2.
    > > > NOTE: This option does not result in a configuration change
    > > > and the vCPU would be still entitled to the LPAR to which it earlier
    > > > belong to.
    > > >
    > > > 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.
    > >
    >
    > From a design standpoint where we stand today is
    >
    > 1. A cede indicates that the CPU is no longer needed and can be
    > reassigned (remember we do dedicated CPU partitions in power)
    > 2. What this patch is trying to do is to say "We don't need the
    > CPU, but please don't reassign, put it to sleep"

    FWIW, this sounds exactly like the same we have already on s390.
    But back then I didn't consider adding a common code infrastructure
    would make sense :)

    Besides the "online" attribute we have an additional "configure"
    attribute to which can only be written if the cpu is offline.
    Writing a "0" to it would mean that you currently won't need the cpu
    anymore and the hypervisor is free to reassign the cpu to a different
    LPAR.
    Writing a "1" to it means you want to use it. If there are enough
    resources you will get it. If not.. bad luck.


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