Re: [PATCH] ACPI: Add ACPI CPU hot-remove support

From: Toshi Kani
Date: Fri Jul 06 2012 - 17:03:30 EST

On Fri, 2012-07-06 at 14:09 -0600, Khalid Aziz wrote:
> On 07/06/2012 01:13 PM, Toshi Kani wrote:
> >
> > For step 2) and 4), I am wondering if they are relevant to CPU hotplug
> > these days. In ACPI namespace, a processor object represents a logical
> > processor (or a core when hyper-threading is disabled). A physical
> > processor (i.e. a socket) usually has multiple cores, and memory
> > controller and bus interface are part of the socket functionality.
> > Hence, I think step 2) and 4) belong to socket-level hot-removal
> > operation, which can be implemented as container hot-remove when a
> > socket is represented with a container object.
> What does it mean to eject just a core in that case? If there are seven
> other cores in the physical processor and you get a request to eject
> one core, what would you expect kernel to do - simply move all processes
> and interrupts off of that core, take it out of scheduling consideration
> and simply idle the core? If yes, how is that any different from simply
> offlining a core?

Yes, offlining and eject are similar operations to a core as it alone
cannot be removed physically. Ejecting a core is a logical eject
operation, which updates the status (_STA) of the object in ACPI after
offlining. The difference from the offlining is that the ejected core
is no longer assigned to the partition. Here is one example. Say, a
core is assigned to a guest partition as a dedicated resource (ex. 100%
of its CPU time is bound to the partition). Offlining this core saves
the power-consumption, but this core is still bound to the partition.
Ejecting the core removes it from the partition (logically), and allows
it to be assigned to other partition as a dedicated resource with

> If you are ejecting individual cores at a time, do you
> keep track of how many you have ejected and then eject the entire physical
> CPU along with memory and IOH associated with the socket when the last
> core is ejected?

It depends on the firmware implementation, but typically the answer is
no. _EJ0 of a core object only removes the associated core object. It
will require a separate socket hot-remove request to eject the
socket-level resources. That is, the OS may not call _EJ0 of a socket
object from the core hot-remove operation just because all children
cores are removed.


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