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 (*)	Use this to limit hotpluggable cpus. This option sets

    			cpu_possible_map = cpu_present_map + additional_cpus

  (*) Option valid only for following architectures

  - ia64

  ia64 uses 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 doesn't 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.

  possible_cpus=n		[s390,x86_64] use this to set hotpluggable cpus.

  			This option sets possible_cpus bits in
  			cpu_possible_map. Thus keeping the numbers of bits set
  			constant even if the machine gets rebooted.

  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_possible_cpu() to iterate.

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

  	#include <linux/cpumask.h>

  	for_each_possible_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>

  	get_online_cpus() and put_online_cpus():

  The above calls are used to inhibit cpu hotplug operations. While the
  cpu_hotplug.refcount is non zero, the 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 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_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.
  
  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 or CPU_DOWN_PREPARE_FROZEN, depending on whether or not the
    CPU is being offlined while tasks are frozen due to a suspend operation in
    progress

  - All processes are migrated away from this outgoing CPU to new CPUs.
    The new CPU is chosen from each process' current cpuset, which may be
    a subset of all online CPUs.

  - 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 (or CPU_DEAD_FROZEN if tasks are frozen due to a suspend while the
    CPU is being offlined).

  
    "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:

  		case CPU_ONLINE_FROZEN:

  			foobar_online_action(cpu);
  			break;
  		case CPU_DEAD:

  		case CPU_DEAD_FROZEN:

  			foobar_dead_action(cpu);
  			break;
  		}
  		return NOTIFY_OK;
  	}

  	static struct notifier_block __cpuinitdata foobar_cpu_notifer =

  	{
  	   .notifier_call = foobar_cpu_callback,
  	};

  You need to call register_cpu_notifier() from your init function.
  Init functions could be of two types:
  1. early init (init function called when only the boot processor is online).
  2. late init (init function called _after_ all the CPUs are online).

  For the first case, you should add the following to your init function

  
  	register_cpu_notifier(&foobar_cpu_notifier);

  For the second case, you should add the following to your init function
  
  	register_hotcpu_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