Difference between logical and physical cpu hotplug

On Wed, Aug 17, 2011 at 9:14 AM, Vaibhav Jain [off-list ref] 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.

Please help!

Thanks
Vaibhav Jain

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Hi Vaibhav!

I think physical CPU hot-plug is a capability to plug in and out
processing device (CPU) on running machine. Logical CPU hot-plug is a
capability to add on and remove out already available processing units
(CPU s or cores) under SMP domain of OS.

P.S. Sorry for my weak English.