		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>

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 only] use this to limit hotpluggable cpus.
                        This option sets
			cpu_possible_map = cpu_present_map + additional_cpus

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
