Re: [PATCH v2 0/2] cpu: pseries: Offline state framework.

On Thu, 2009-09-24 at 18:38 +1000, Benjamin Herrenschmidt wrote:
On Thu, 2009-09-24 at 09:51 +0200, Peter Zijlstra wrote:
I don't quite follow your logic here. This is useful for more than just
hypervisors. For example, take the HV out of the picture for a moment
and imagine that the HW has the ability to offline CPU in various power
levels, with varying latencies to bring them back.

cpu-hotplug is an utter slow path, anybody saying latency and hotplug in
the same sentence doesn't seem to grasp either or both concepts.

Let's forget about latency then. Let's imagine I want to set a CPU
offline to save power, vs. setting it offline -and- opening the back
door of the machine to actually physically replace it :-)

If the hardware is capable of physical hotplug, then surely powering the
socket down saves most power and is the preferred mode?

In any case, I don't see the added feature as being problematic, and
not such a "layering violation" as you seem to imply it is. It's a
convenient way to atomically take the CPU out -and- convey some
information about the "intent" to the hypervisor, and I really fail
to see why you have so strong objections about it.

Ignorance on my part probably :-)

I'm simply not seeing a use case for it, except for the virt case, which
I think we should bug the virt interface with and not the cpu-hotplug
interface.



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