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    SubjectRe: [PATCH 0/6] x86/cpu hotplug: Wake up offline CPU via mwait or nmi
    Hi Thomas,
    It's a great idea to use state machine to implement cpu online/offline.
    Actually we are trying to split the cpu online/offline flow into several stages
    so we could do error recover easily when hot-plugging physical processors.

    On 06/05/2012 04:11 AM, Thomas Gleixner wrote:
    > On Mon, 4 Jun 2012, Fenghua Yu wrote:
    >> From: Fenghua Yu <>
    >> Since offline CPU is in wmait or hlt if mwait feature is not available, it can
    >> be waken up by writing to monitored memory range or via nmi.
    >> Compared to current INIT, INIT, STARTUP wake up sequence, waking up offline CPU
    >> is faster via wmait or nmi. This is especially useful when offline CPU for
    >> power saving and shorter waking up time is desired. On one tested desktop
    >> machine, waking up time via mwait or nmi is reduced to 23% of waking up time
    >> via INIT. Waking up time is measured from the beginning of store_online() to
    >> the beginning of cpu_idle() after the CPU is waken up.
    >> Waking up offline CPU via mwait or nmi is also useful to support BSP offline/
    >> online because offline BSP can not be waken up by the INIT's sequence. The BSP
    >> offline/online patchset will be sent out seperately.
    > I understand what you are trying to do, though I completely disagree
    > with the solution.
    > The main problem of the current hotplug code is that it is an all or
    > nothing approach. You have to tear down the whole thing completely
    > instead of just taking it out of the usable set of cpus.
    > I'm working on a proper state machine driven online/offline sequence,
    > where you can put the cpu into an intermediate state which avoids
    > bringing it down completely. This is enough to get the full
    > powersaving benefits w/o having to go through all the synchronization
    > states of a full online/offline. That will shorten the onlining time
    > of an previously offlined cpu to almost nothing.
    > I really want to avoid adding more bandaids to the hotplug code before
    > we have sorted out the existing horror.
    > Thanks,
    > tglx
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