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

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Message-ID: <1341608402.16730.730.camel@misato.fc.hp.com>
Date:	Fri, 06 Jul 2012 15:00:02 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Khalid Aziz <khalid.aziz@...com>
Cc:	Jiang Liu <liuj97@...il.com>, lenb@...nel.org,
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ACPI: Add ACPI CPU hot-remove support

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
hot-add.

> 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. 

Thanks,
-Toshi

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