Difference between logical and physical cpu hotplug

[Mulyadi Santosa]

Hi....

On Wed, Aug 17, 2011 at 12:14, Vaibhav Jain <vjoss197 at gmail.com> wrote:
> Hi,
>
> I need to know the difference between making a cpu logically offline
> and physically offline. I have read that the following command
>
> $ echo 0  > /sys/device/system/cpu/<cpu number>/online
>
> makes a cpu logically offline. It frees the cpu from interrupts and migrates
> running processes.
> But then what does it mean to make a cpu physically offline ?
> I referred to following article :
>
> http://www.kernel.org/doc/Documentation/cpu-hotplug.txt
>
> Here is the relevant excerpt :
>
> Q: Does hot-add/hot-remove refer to physical add/remove of cpus?
> A: The usage of hot-add/remove may not be very consistently used in the
> code.
> CONFIG_HOTPLUG_CPU enables logical online/offline capability in the kernel.
> To support physical addition/removal, one would need some BIOS hooks and
> the platform should have something like an attention button in PCI hotplug.
> CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of
> CPUs.

IMHO, logical cpu online capability is a foundation or baseline for
going into physical cpu offline/online. It's like disk hot swap in
RAID software. Since we can "remove" a RAID member, then removing them
is not a problem.

Spesificly about CPU, i think the clue here is "BIOS hook". Meaning?
We do cpu unplug by ... maybe pressing a button or engaging a
menu...and then, motherboard sends notification to the Linux kernel
and Linux kernel does whatever necessary (process migration, turning
off LAPIC etc) before CPU is safe to be removed.

What do you think?
-- 
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
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