Documentation/cpu-hotplug.txt - arm/linux-arm64-legacy - Git at Google

 		CPU hotplug Support in Linux(tm) Kernel

 		Maintainers:
 		CPU Hotplug Core:
 			Rusty Russell <rusty@rustycorp.com.au>
 			Srivatsa Vaddagiri <vatsa@in.ibm.com>
 		i386:
 			Zwane Mwaikambo <zwane@arm.linux.org.uk>
 		ppc64:
 			Nathan Lynch <nathanl@austin.ibm.com>
 			Joel Schopp <jschopp@austin.ibm.com>
 		ia64/x86_64:
 			Ashok Raj <ashok.raj@intel.com>
 		s390:
 			Heiko Carstens <heiko.carstens@de.ibm.com>

 Authors: Ashok Raj <ashok.raj@intel.com>
 Lots of feedback: Nathan Lynch <nathanl@austin.ibm.com>,
 	     Joel Schopp <jschopp@austin.ibm.com>

 Introduction

 Modern advances in system architectures have introduced advanced error
 reporting and correction capabilities in processors. CPU architectures permit
 partitioning support, where compute resources of a single CPU could be made
 available to virtual machine environments. There are couple OEMS that
 support NUMA hardware which are hot pluggable as well, where physical
 node insertion and removal require support for CPU hotplug.

 Such advances require CPUs available to a kernel to be removed either for
 provisioning reasons, or for RAS purposes to keep an offending CPU off
 system execution path. Hence the need for CPU hotplug support in the
 Linux kernel.

 A more novel use of CPU-hotplug support is its use today in suspend
 resume support for SMP. Dual-core and HT support makes even
 a laptop run SMP kernels which didn't support these methods. SMP support
 for suspend/resume is a work in progress.

 General Stuff about CPU Hotplug
 --------------------------------

 Command Line Switches
 ---------------------
 maxcpus=n    Restrict boot time cpus to n. Say if you have 4 cpus, using
              maxcpus=2 will only boot 2. You can choose to bring the
              other cpus later online, read FAQ's for more info.

 additional_cpus=n	[x86_64, s390 only] use this to limit hotpluggable cpus.
                           This option sets
   			cpu_possible_map = cpu_present_map + additional_cpus

 ia64 and x86_64 use the number of disabled local apics in ACPI tables MADT
 to determine the number of potentially hot-pluggable cpus. The implementation
 should only rely on this to count the #of cpus, but *MUST* not rely on the
 apicid values in those tables for disabled apics. In the event BIOS doesnt
 mark such hot-pluggable cpus as disabled entries, one could use this
 parameter "additional_cpus=x" to represent those cpus in the cpu_possible_map.


 CPU maps and such
 -----------------
 [More on cpumaps and primitive to manipulate, please check
 include/linux/cpumask.h that has more descriptive text.]

 cpu_possible_map: Bitmap of possible CPUs that can ever be available in the
 system. This is used to allocate some boot time memory for per_cpu variables
 that aren't designed to grow/shrink as CPUs are made available or removed.
 Once set during boot time discovery phase, the map is static, i.e no bits
 are added or removed anytime.  Trimming it accurately for your system needs
 upfront can save some boot time memory. See below for how we use heuristics
 in x86_64 case to keep this under check.

 cpu_online_map: Bitmap of all CPUs currently online. Its set in __cpu_up()
 after a cpu is available for kernel scheduling and ready to receive
 interrupts from devices. Its cleared when a cpu is brought down using
 __cpu_disable(), before which all OS services including interrupts are
 migrated to another target CPU.

 cpu_present_map: Bitmap of CPUs currently present in the system. Not all
 of them may be online. When physical hotplug is processed by the relevant
 subsystem (e.g ACPI) can change and new bit either be added or removed
 from the map depending on the event is hot-add/hot-remove. There are currently
 no locking rules as of now. Typical usage is to init topology during boot,
 at which time hotplug is disabled.

 You really dont need to manipulate any of the system cpu maps. They should
 be read-only for most use. When setting up per-cpu resources almost always use
 cpu_possible_map/for_each_cpu() to iterate.

 Never use anything other than cpumask_t to represent bitmap of CPUs.

 #include <linux/cpumask.h>

 for_each_cpu              - Iterate over cpu_possible_map
 for_each_online_cpu       - Iterate over cpu_online_map
 for_each_present_cpu      - Iterate over cpu_present_map
 for_each_cpu_mask(x,mask) - Iterate over some random collection of cpu mask.

 #include <linux/cpu.h>
 lock_cpu_hotplug() and unlock_cpu_hotplug():

 The above calls are used to inhibit cpu hotplug operations. While holding the
 cpucontrol mutex, cpu_online_map will not change. If you merely need to avoid
 cpus going away, you could also use preempt_disable() and preempt_enable()
 for those sections. Just remember the critical section cannot call any
 function that can sleep or schedule this process away. The preempt_disable()
 will work as long as stop_machine_run() is used to take a cpu down.

 CPU Hotplug - Frequently Asked Questions.

 Q: How to i enable my kernel to support CPU hotplug?
 A: When doing make defconfig, Enable CPU hotplug support

    "Processor type and Features" -> Support for Hotpluggable CPUs

 Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.

 You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
 as well.

 Q: What architectures support CPU hotplug?
 A: As of 2.6.14, the following architectures support CPU hotplug.

 i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64

 Q: How to test if hotplug is supported on the newly built kernel?
 A: You should now notice an entry in sysfs.

 Check if sysfs is mounted, using the "mount" command. You should notice
 an entry as shown below in the output.

 ....
 none on /sys type sysfs (rw)
 ....

 if this is not mounted, do the following.

 #mkdir /sysfs
 #mount -t sysfs sys /sys

 now you should see entries for all present cpu, the following is an example
 in a 8-way system.

 #pwd
 #/sys/devices/system/cpu
 #ls -l
 total 0
 drwxr-xr-x  10 root root 0 Sep 19 07:44 .
 drwxr-xr-x  13 root root 0 Sep 19 07:45 ..
 drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu0
 drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu1
 drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu2
 drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu3
 drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu4
 drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu5
 drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu6
 drwxr-xr-x   3 root root 0 Sep 19 07:48 cpu7

 Under each directory you would find an "online" file which is the control
 file to logically online/offline a processor.

 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_CPU_HOTPLUG 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.

 Q: How do i logically offline a CPU?
 A: Do the following.

 #echo 0 > /sys/devices/system/cpu/cpuX/online

 once the logical offline is successful, check

 #cat /proc/interrupts

 you should now not see the CPU that you removed. Also online file will report
 the state as 0 when a cpu if offline and 1 when its online.

 #To display the current cpu state.
 #cat /sys/devices/system/cpu/cpuX/online

 Q: Why cant i remove CPU0 on some systems?
 A: Some architectures may have some special dependency on a certain CPU.

 For e.g in IA64 platforms we have ability to sent platform interrupts to the
 OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
 specifications, we didn't have a way to change the target CPU. Hence if the
 current ACPI version doesn't support such re-direction, we disable that CPU
 by making it not-removable.

 In such cases you will also notice that the online file is missing under cpu0.

 Q: How do i find out if a particular CPU is not removable?
 A: Depending on the implementation, some architectures may show this by the
 absence of the "online" file. This is done if it can be determined ahead of
 time that this CPU cannot be removed.

 In some situations, this can be a run time check, i.e if you try to remove the
 last CPU, this will not be permitted. You can find such failures by
 investigating the return value of the "echo" command.

 Q: What happens when a CPU is being logically offlined?
 A: The following happen, listed in no particular order :-)

 - A notification is sent to in-kernel registered modules by sending an event
   CPU_DOWN_PREPARE
 - All process is migrated away from this outgoing CPU to a new CPU
 - All interrupts targeted to this CPU is migrated to a new CPU
 - timers/bottom half/task lets are also migrated to a new CPU
 - Once all services are migrated, kernel calls an arch specific routine
   __cpu_disable() to perform arch specific cleanup.
 - Once this is successful, an event for successful cleanup is sent by an event
   CPU_DEAD.

   "It is expected that each service cleans up when the CPU_DOWN_PREPARE
   notifier is called, when CPU_DEAD is called its expected there is nothing
   running on behalf of this CPU that was offlined"

 Q: If i have some kernel code that needs to be aware of CPU arrival and
    departure, how to i arrange for proper notification?
 A: This is what you would need in your kernel code to receive notifications.

     #include <linux/cpu.h>
     static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb,
 					    unsigned long action, void *hcpu)
 	{
 		unsigned int cpu = (unsigned long)hcpu;

 		switch (action) {
 		case CPU_ONLINE:
 			foobar_online_action(cpu);
 			break;
 		case CPU_DEAD:
 			foobar_dead_action(cpu);
 			break;
 		}
 		return NOTIFY_OK;
 	}

 	static struct notifier_block foobar_cpu_notifer =
 	{
 	   .notifier_call = foobar_cpu_callback,
 	};


 In your init function,

 	register_cpu_notifier(&foobar_cpu_notifier);

 You can fail PREPARE notifiers if something doesn't work to prepare resources.
 This will stop the activity and send a following CANCELED event back.

 CPU_DEAD should not be failed, its just a goodness indication, but bad
 things will happen if a notifier in path sent a BAD notify code.

 Q: I don't see my action being called for all CPUs already up and running?
 A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined.
    If you need to perform some action for each cpu already in the system, then

   for_each_online_cpu(i) {
 		foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
 		foobar_cpu_callback(&foobar-cpu_notifier, CPU_ONLINE, i);
   }

 Q: If i would like to develop cpu hotplug support for a new architecture,
    what do i need at a minimum?
 A: The following are what is required for CPU hotplug infrastructure to work
    correctly.

     - Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU
     - __cpu_up()        - Arch interface to bring up a CPU
     - __cpu_disable()   - Arch interface to shutdown a CPU, no more interrupts
                           can be handled by the kernel after the routine
                           returns. Including local APIC timers etc are
                           shutdown.
      - __cpu_die()      - This actually supposed to ensure death of the CPU.
                           Actually look at some example code in other arch
                           that implement CPU hotplug. The processor is taken
                           down from the idle() loop for that specific
                           architecture. __cpu_die() typically waits for some
                           per_cpu state to be set, to ensure the processor
                           dead routine is called to be sure positively.

 Q: I need to ensure that a particular cpu is not removed when there is some
    work specific to this cpu is in progress.
 A: First switch the current thread context to preferred cpu

    int my_func_on_cpu(int cpu)
    {
        cpumask_t saved_mask, new_mask = CPU_MASK_NONE;
        int curr_cpu, err = 0;

        saved_mask = current->cpus_allowed;
        cpu_set(cpu, new_mask);
        err = set_cpus_allowed(current, new_mask);

        if (err)
            return err;

        /*
         * If we got scheduled out just after the return from
         * set_cpus_allowed() before running the work, this ensures
         * we stay locked.
         */
        curr_cpu = get_cpu();

        if (curr_cpu != cpu) {
 	   err = -EAGAIN;
            goto ret;
        } else {
        	   /*
 	    * Do work : But cant sleep, since get_cpu() disables preempt
 	    */
        }
     ret:
     	put_cpu();
 	set_cpus_allowed(current, saved_mask);
 	return err;
     }


 Q: How do we determine how many CPUs are available for hotplug.
 A: There is no clear spec defined way from ACPI that can give us that
    information today. Based on some input from Natalie of Unisys,
    that the ACPI MADT (Multiple APIC Description Tables) marks those possible
    CPUs in a system with disabled status.

    Andi implemented some simple heuristics that count the number of disabled
    CPUs in MADT as hotpluggable CPUS.  In the case there are no disabled CPUS
    we assume 1/2 the number of CPUs currently present can be hotplugged.

    Caveat: Today's ACPI MADT can only provide 256 entries since the apicid field
    in MADT is only 8 bits.

 User Space Notification

 Hotplug support for devices is common in Linux today. Its being used today to
 support automatic configuration of network, usb and pci devices. A hotplug
 event can be used to invoke an agent script to perform the configuration task.

 You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
 scripts.

 	#!/bin/bash
 	# $Id: cpu.agent
 	# Kernel hotplug params include:
 	#ACTION=%s [online or offline]
 	#DEVPATH=%s
 	#
 	cd /etc/hotplug
 	. ./hotplug.functions

 	case $ACTION in
 		online)
 			echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt
 			;;
 		offline)
 			echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt
 			;;
 		*)
 			debug_mesg CPU $ACTION event not supported
         exit 1
         ;;
 	esac
	CPU hotplug Support in Linux(tm) Kernel

	Maintainers:
	CPU Hotplug Core:
	Rusty Russell <rusty@rustycorp.com.au>
	Srivatsa Vaddagiri <vatsa@in.ibm.com>
	i386:
	Zwane Mwaikambo <zwane@arm.linux.org.uk>
	ppc64:
	Nathan Lynch <nathanl@austin.ibm.com>
	Joel Schopp <jschopp@austin.ibm.com>
	ia64/x86_64:
	Ashok Raj <ashok.raj@intel.com>
	s390:
	Heiko Carstens <heiko.carstens@de.ibm.com>

	Authors: Ashok Raj <ashok.raj@intel.com>
	Lots of feedback: Nathan Lynch <nathanl@austin.ibm.com>,
	Joel Schopp <jschopp@austin.ibm.com>

	Introduction

	Modern advances in system architectures have introduced advanced error
	reporting and correction capabilities in processors. CPU architectures permit
	partitioning support, where compute resources of a single CPU could be made
	available to virtual machine environments. There are couple OEMS that
	support NUMA hardware which are hot pluggable as well, where physical
	node insertion and removal require support for CPU hotplug.

	Such advances require CPUs available to a kernel to be removed either for
	provisioning reasons, or for RAS purposes to keep an offending CPU off
	system execution path. Hence the need for CPU hotplug support in the
	Linux kernel.

	A more novel use of CPU-hotplug support is its use today in suspend
	resume support for SMP. Dual-core and HT support makes even
	a laptop run SMP kernels which didn't support these methods. SMP support
	for suspend/resume is a work in progress.

	General Stuff about CPU Hotplug
	--------------------------------

	Command Line Switches
	---------------------
	maxcpus=n Restrict boot time cpus to n. Say if you have 4 cpus, using
	maxcpus=2 will only boot 2. You can choose to bring the
	other cpus later online, read FAQ's for more info.

	additional_cpus=n [x86_64, s390 only] use this to limit hotpluggable cpus.
	This option sets
	cpu_possible_map = cpu_present_map + additional_cpus

	ia64 and x86_64 use the number of disabled local apics in ACPI tables MADT
	to determine the number of potentially hot-pluggable cpus. The implementation
	should only rely on this to count the #of cpus, but MUST not rely on the
	apicid values in those tables for disabled apics. In the event BIOS doesnt
	mark such hot-pluggable cpus as disabled entries, one could use this
	parameter "additional_cpus=x" to represent those cpus in the cpu_possible_map.


	CPU maps and such
	-----------------
	[More on cpumaps and primitive to manipulate, please check
	include/linux/cpumask.h that has more descriptive text.]

	cpu_possible_map: Bitmap of possible CPUs that can ever be available in the
	system. This is used to allocate some boot time memory for per_cpu variables
	that aren't designed to grow/shrink as CPUs are made available or removed.
	Once set during boot time discovery phase, the map is static, i.e no bits
	are added or removed anytime. Trimming it accurately for your system needs
	upfront can save some boot time memory. See below for how we use heuristics
	in x86_64 case to keep this under check.

	cpu_online_map: Bitmap of all CPUs currently online. Its set in __cpu_up()
	after a cpu is available for kernel scheduling and ready to receive
	interrupts from devices. Its cleared when a cpu is brought down using
	__cpu_disable(), before which all OS services including interrupts are
	migrated to another target CPU.

	cpu_present_map: Bitmap of CPUs currently present in the system. Not all
	of them may be online. When physical hotplug is processed by the relevant
	subsystem (e.g ACPI) can change and new bit either be added or removed
	from the map depending on the event is hot-add/hot-remove. There are currently
	no locking rules as of now. Typical usage is to init topology during boot,
	at which time hotplug is disabled.

	You really dont need to manipulate any of the system cpu maps. They should
	be read-only for most use. When setting up per-cpu resources almost always use
	cpu_possible_map/for_each_cpu() to iterate.

	Never use anything other than cpumask_t to represent bitmap of CPUs.

	#include <linux/cpumask.h>

	for_each_cpu - Iterate over cpu_possible_map
	for_each_online_cpu - Iterate over cpu_online_map
	for_each_present_cpu - Iterate over cpu_present_map
	for_each_cpu_mask(x,mask) - Iterate over some random collection of cpu mask.

	#include <linux/cpu.h>
	lock_cpu_hotplug() and unlock_cpu_hotplug():

	The above calls are used to inhibit cpu hotplug operations. While holding the
	cpucontrol mutex, cpu_online_map will not change. If you merely need to avoid
	cpus going away, you could also use preempt_disable() and preempt_enable()
	for those sections. Just remember the critical section cannot call any
	function that can sleep or schedule this process away. The preempt_disable()
	will work as long as stop_machine_run() is used to take a cpu down.

	CPU Hotplug - Frequently Asked Questions.

	Q: How to i enable my kernel to support CPU hotplug?
	A: When doing make defconfig, Enable CPU hotplug support

	"Processor type and Features" -> Support for Hotpluggable CPUs

	Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.

	You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
	as well.

	Q: What architectures support CPU hotplug?
	A: As of 2.6.14, the following architectures support CPU hotplug.

	i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64

	Q: How to test if hotplug is supported on the newly built kernel?
	A: You should now notice an entry in sysfs.

	Check if sysfs is mounted, using the "mount" command. You should notice
	an entry as shown below in the output.

	....
	none on /sys type sysfs (rw)
	....

	if this is not mounted, do the following.

	#mkdir /sysfs
	#mount -t sysfs sys /sys

	now you should see entries for all present cpu, the following is an example
	in a 8-way system.

	#pwd
	#/sys/devices/system/cpu
	#ls -l
	total 0
	drwxr-xr-x 10 root root 0 Sep 19 07:44 .
	drwxr-xr-x 13 root root 0 Sep 19 07:45 ..
	drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu0
	drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu1
	drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu2
	drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu3
	drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu4
	drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu5
	drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu6
	drwxr-xr-x 3 root root 0 Sep 19 07:48 cpu7

	Under each directory you would find an "online" file which is the control
	file to logically online/offline a processor.

	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_CPU_HOTPLUG 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.

	Q: How do i logically offline a CPU?
	A: Do the following.

	#echo 0 > /sys/devices/system/cpu/cpuX/online

	once the logical offline is successful, check

	#cat /proc/interrupts

	you should now not see the CPU that you removed. Also online file will report
	the state as 0 when a cpu if offline and 1 when its online.

	#To display the current cpu state.
	#cat /sys/devices/system/cpu/cpuX/online

	Q: Why cant i remove CPU0 on some systems?
	A: Some architectures may have some special dependency on a certain CPU.

	For e.g in IA64 platforms we have ability to sent platform interrupts to the
	OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
	specifications, we didn't have a way to change the target CPU. Hence if the
	current ACPI version doesn't support such re-direction, we disable that CPU
	by making it not-removable.

	In such cases you will also notice that the online file is missing under cpu0.

	Q: How do i find out if a particular CPU is not removable?
	A: Depending on the implementation, some architectures may show this by the
	absence of the "online" file. This is done if it can be determined ahead of
	time that this CPU cannot be removed.

	In some situations, this can be a run time check, i.e if you try to remove the
	last CPU, this will not be permitted. You can find such failures by
	investigating the return value of the "echo" command.

	Q: What happens when a CPU is being logically offlined?
	A: The following happen, listed in no particular order :-)

	- A notification is sent to in-kernel registered modules by sending an event
	CPU_DOWN_PREPARE
	- All process is migrated away from this outgoing CPU to a new CPU
	- All interrupts targeted to this CPU is migrated to a new CPU
	- timers/bottom half/task lets are also migrated to a new CPU
	- Once all services are migrated, kernel calls an arch specific routine
	__cpu_disable() to perform arch specific cleanup.
	- Once this is successful, an event for successful cleanup is sent by an event
	CPU_DEAD.

	"It is expected that each service cleans up when the CPU_DOWN_PREPARE
	notifier is called, when CPU_DEAD is called its expected there is nothing
	running on behalf of this CPU that was offlined"

	Q: If i have some kernel code that needs to be aware of CPU arrival and
	departure, how to i arrange for proper notification?
	A: This is what you would need in your kernel code to receive notifications.

	#include <linux/cpu.h>
	static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb,
	unsigned long action, void *hcpu)
	{
	unsigned int cpu = (unsigned long)hcpu;

	switch (action) {
	case CPU_ONLINE:
	foobar_online_action(cpu);
	break;
	case CPU_DEAD:
	foobar_dead_action(cpu);
	break;
	}
	return NOTIFY_OK;
	}

	static struct notifier_block foobar_cpu_notifer =
	{
	.notifier_call = foobar_cpu_callback,
	};


	In your init function,

	register_cpu_notifier(&foobar_cpu_notifier);

	You can fail PREPARE notifiers if something doesn't work to prepare resources.
	This will stop the activity and send a following CANCELED event back.

	CPU_DEAD should not be failed, its just a goodness indication, but bad
	things will happen if a notifier in path sent a BAD notify code.

	Q: I don't see my action being called for all CPUs already up and running?
	A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined.
	If you need to perform some action for each cpu already in the system, then

	for_each_online_cpu(i) {
	foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
	foobar_cpu_callback(&foobar-cpu_notifier, CPU_ONLINE, i);
	}

	Q: If i would like to develop cpu hotplug support for a new architecture,
	what do i need at a minimum?
	A: The following are what is required for CPU hotplug infrastructure to work
	correctly.

	- Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU
	- __cpu_up() - Arch interface to bring up a CPU
	- __cpu_disable() - Arch interface to shutdown a CPU, no more interrupts
	can be handled by the kernel after the routine
	returns. Including local APIC timers etc are
	shutdown.
	- __cpu_die() - This actually supposed to ensure death of the CPU.
	Actually look at some example code in other arch
	that implement CPU hotplug. The processor is taken
	down from the idle() loop for that specific
	architecture. __cpu_die() typically waits for some
	per_cpu state to be set, to ensure the processor
	dead routine is called to be sure positively.

	Q: I need to ensure that a particular cpu is not removed when there is some
	work specific to this cpu is in progress.
	A: First switch the current thread context to preferred cpu

	int my_func_on_cpu(int cpu)
	{
	cpumask_t saved_mask, new_mask = CPU_MASK_NONE;
	int curr_cpu, err = 0;

	saved_mask = current->cpus_allowed;
	cpu_set(cpu, new_mask);
	err = set_cpus_allowed(current, new_mask);

	if (err)
	return err;

	/*
	* If we got scheduled out just after the return from
	* set_cpus_allowed() before running the work, this ensures
	* we stay locked.
	*/
	curr_cpu = get_cpu();

	if (curr_cpu != cpu) {
	err = -EAGAIN;
	goto ret;
	} else {
	/*
	* Do work : But cant sleep, since get_cpu() disables preempt
	*/
	}
	ret:
	put_cpu();
	set_cpus_allowed(current, saved_mask);
	return err;
	}


	Q: How do we determine how many CPUs are available for hotplug.
	A: There is no clear spec defined way from ACPI that can give us that
	information today. Based on some input from Natalie of Unisys,
	that the ACPI MADT (Multiple APIC Description Tables) marks those possible
	CPUs in a system with disabled status.

	Andi implemented some simple heuristics that count the number of disabled
	CPUs in MADT as hotpluggable CPUS. In the case there are no disabled CPUS
	we assume 1/2 the number of CPUs currently present can be hotplugged.

	Caveat: Today's ACPI MADT can only provide 256 entries since the apicid field
	in MADT is only 8 bits.

	User Space Notification

	Hotplug support for devices is common in Linux today. Its being used today to
	support automatic configuration of network, usb and pci devices. A hotplug
	event can be used to invoke an agent script to perform the configuration task.

	You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
	scripts.

	#!/bin/bash
	# $Id: cpu.agent
	# Kernel hotplug params include:
	#ACTION=%s [online or offline]
	#DEVPATH=%s
	#
	cd /etc/hotplug
	. ./hotplug.functions

	case $ACTION in
	online)
	echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt
	;;
	offline)
	echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt
	;;
	*)
	debug_mesg CPU $ACTION event not supported
	exit 1
	;;
	esac