=========================
CPU hotplug in the Kernel
=========================

:Date: September, 2021
:Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
         Rusty Russell <rusty@rustcorp.com.au>,
         Srivatsa Vaddagiri <vatsa@in.ibm.com>,
         Ashok Raj <ashok.raj@intel.com>,
         Joel Schopp <jschopp@austin.ibm.com>,
	 Thomas Gleixner <tglx@linutronix.de>

Introduction
============

Modern advances in system architectures have introduced advanced error
reporting and correction capabilities in processors. 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.


Command Line Switches
=====================
``maxcpus=n``
  Restrict boot time CPUs to *n*. Say if you have four CPUs, using
  ``maxcpus=2`` will only boot two. You can choose to bring the
  other CPUs later online.

``nr_cpus=n``
  Restrict the total amount of CPUs the kernel will support. If the number
  supplied here is lower than the number of physically available CPUs, then
  those CPUs can not be brought online later.

``additional_cpus=n``
  Use this to limit hotpluggable CPUs. This option sets
  ``cpu_possible_mask = cpu_present_mask + additional_cpus``

  This option is limited to the IA64 architecture.

``possible_cpus=n``
  This option sets ``possible_cpus`` bits in ``cpu_possible_mask``.

  This option is limited to the X86 and S390 architecture.

``cpu0_hotplug``
  Allow to shutdown CPU0.

  This option is limited to the X86 architecture.

CPU maps
========

``cpu_possible_mask``
  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.

``cpu_online_mask``
  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_mask``
  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 don't 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_mask`` or ``for_each_possible_cpu()`` to iterate. To macro
``for_each_cpu()`` can be used to iterate over a custom CPU mask.

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


Using CPU hotplug
=================

The kernel option *CONFIG_HOTPLUG_CPU* needs to be enabled. It is currently
available on multiple architectures including ARM, MIPS, PowerPC and X86. The
configuration is done via the sysfs interface::

 $ ls -lh /sys/devices/system/cpu
 total 0
 drwxr-xr-x  9 root root    0 Dec 21 16:33 cpu0
 drwxr-xr-x  9 root root    0 Dec 21 16:33 cpu1
 drwxr-xr-x  9 root root    0 Dec 21 16:33 cpu2
 drwxr-xr-x  9 root root    0 Dec 21 16:33 cpu3
 drwxr-xr-x  9 root root    0 Dec 21 16:33 cpu4
 drwxr-xr-x  9 root root    0 Dec 21 16:33 cpu5
 drwxr-xr-x  9 root root    0 Dec 21 16:33 cpu6
 drwxr-xr-x  9 root root    0 Dec 21 16:33 cpu7
 drwxr-xr-x  2 root root    0 Dec 21 16:33 hotplug
 -r--r--r--  1 root root 4.0K Dec 21 16:33 offline
 -r--r--r--  1 root root 4.0K Dec 21 16:33 online
 -r--r--r--  1 root root 4.0K Dec 21 16:33 possible
 -r--r--r--  1 root root 4.0K Dec 21 16:33 present

The files *offline*, *online*, *possible*, *present* represent the CPU masks.
Each CPU folder contains an *online* file which controls the logical on (1) and
off (0) state. To logically shutdown CPU4::

 $ echo 0 > /sys/devices/system/cpu/cpu4/online
  smpboot: CPU 4 is now offline

Once the CPU is shutdown, it will be removed from */proc/interrupts*,
*/proc/cpuinfo* and should also not be shown visible by the *top* command. To
bring CPU4 back online::

 $ echo 1 > /sys/devices/system/cpu/cpu4/online
 smpboot: Booting Node 0 Processor 4 APIC 0x1

The CPU is usable again. This should work on all CPUs. CPU0 is often special
and excluded from CPU hotplug. On X86 the kernel option
*CONFIG_BOOTPARAM_HOTPLUG_CPU0* has to be enabled in order to be able to
shutdown CPU0. Alternatively the kernel command option *cpu0_hotplug* can be
used. Some known dependencies of CPU0:

* Resume from hibernate/suspend. Hibernate/suspend will fail if CPU0 is offline.
* PIC interrupts. CPU0 can't be removed if a PIC interrupt is detected.

Please let Fenghua Yu <fenghua.yu@intel.com> know if you find any dependencies
on CPU0.

The CPU hotplug coordination
============================

The offline case
----------------

Once a CPU has been logically shutdown the teardown callbacks of registered
hotplug states will be invoked, starting with ``CPUHP_ONLINE`` and terminating
at state ``CPUHP_OFFLINE``. This includes:

* If tasks are frozen due to a suspend operation then *cpuhp_tasks_frozen*
  will be set to true.
* 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 are migrated to a new CPU
* timers 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.


The CPU hotplug API
===================

CPU hotplug state machine
-------------------------

CPU hotplug uses a trivial state machine with a linear state space from
CPUHP_OFFLINE to CPUHP_ONLINE. Each state has a startup and a teardown
callback.

When a CPU is onlined, the startup callbacks are invoked sequentially until
the state CPUHP_ONLINE is reached. They can also be invoked when the
callbacks of a state are set up or an instance is added to a multi-instance
state.

When a CPU is offlined the teardown callbacks are invoked in the reverse
order sequentially until the state CPUHP_OFFLINE is reached. They can also
be invoked when the callbacks of a state are removed or an instance is
removed from a multi-instance state.

If a usage site requires only a callback in one direction of the hotplug
operations (CPU online or CPU offline) then the other not-required callback
can be set to NULL when the state is set up.

The state space is divided into three sections:

* The PREPARE section

  The PREPARE section covers the state space from CPUHP_OFFLINE to
  CPUHP_BRINGUP_CPU.

  The startup callbacks in this section are invoked before the CPU is
  started during a CPU online operation. The teardown callbacks are invoked
  after the CPU has become dysfunctional during a CPU offline operation.

  The callbacks are invoked on a control CPU as they can't obviously run on
  the hotplugged CPU which is either not yet started or has become
  dysfunctional already.

  The startup callbacks are used to setup resources which are required to
  bring a CPU successfully online. The teardown callbacks are used to free
  resources or to move pending work to an online CPU after the hotplugged
  CPU became dysfunctional.

  The startup callbacks are allowed to fail. If a callback fails, the CPU
  online operation is aborted and the CPU is brought down to the previous
  state (usually CPUHP_OFFLINE) again.

  The teardown callbacks in this section are not allowed to fail.

* The STARTING section

  The STARTING section covers the state space between CPUHP_BRINGUP_CPU + 1
  and CPUHP_AP_ONLINE.

  The startup callbacks in this section are invoked on the hotplugged CPU
  with interrupts disabled during a CPU online operation in the early CPU
  setup code. The teardown callbacks are invoked with interrupts disabled
  on the hotplugged CPU during a CPU offline operation shortly before the
  CPU is completely shut down.

  The callbacks in this section are not allowed to fail.

  The callbacks are used for low level hardware initialization/shutdown and
  for core subsystems.

* The ONLINE section

  The ONLINE section covers the state space between CPUHP_AP_ONLINE + 1 and
  CPUHP_ONLINE.

  The startup callbacks in this section are invoked on the hotplugged CPU
  during a CPU online operation. The teardown callbacks are invoked on the
  hotplugged CPU during a CPU offline operation.

  The callbacks are invoked in the context of the per CPU hotplug thread,
  which is pinned on the hotplugged CPU. The callbacks are invoked with
  interrupts and preemption enabled.

  The callbacks are allowed to fail. When a callback fails the hotplug
  operation is aborted and the CPU is brought back to the previous state.

CPU online/offline operations
-----------------------------

A successful online operation looks like this::

  [CPUHP_OFFLINE]
  [CPUHP_OFFLINE + 1]->startup()       -> success
  [CPUHP_OFFLINE + 2]->startup()       -> success
  [CPUHP_OFFLINE + 3]                  -> skipped because startup == NULL
  ...
  [CPUHP_BRINGUP_CPU]->startup()       -> success
  === End of PREPARE section
  [CPUHP_BRINGUP_CPU + 1]->startup()   -> success
  ...
  [CPUHP_AP_ONLINE]->startup()         -> success
  === End of STARTUP section
  [CPUHP_AP_ONLINE + 1]->startup()     -> success
  ...
  [CPUHP_ONLINE - 1]->startup()        -> success
  [CPUHP_ONLINE]

A successful offline operation looks like this::

  [CPUHP_ONLINE]
  [CPUHP_ONLINE - 1]->teardown()       -> success
  ...
  [CPUHP_AP_ONLINE + 1]->teardown()    -> success
  === Start of STARTUP section
  [CPUHP_AP_ONLINE]->teardown()        -> success
  ...
  [CPUHP_BRINGUP_ONLINE - 1]->teardown()
  ...
  === Start of PREPARE section
  [CPUHP_BRINGUP_CPU]->teardown()
  [CPUHP_OFFLINE + 3]->teardown()
  [CPUHP_OFFLINE + 2]                  -> skipped because teardown == NULL
  [CPUHP_OFFLINE + 1]->teardown()
  [CPUHP_OFFLINE]

A failed online operation looks like this::

  [CPUHP_OFFLINE]
  [CPUHP_OFFLINE + 1]->startup()       -> success
  [CPUHP_OFFLINE + 2]->startup()       -> success
  [CPUHP_OFFLINE + 3]                  -> skipped because startup == NULL
  ...
  [CPUHP_BRINGUP_CPU]->startup()       -> success
  === End of PREPARE section
  [CPUHP_BRINGUP_CPU + 1]->startup()   -> success
  ...
  [CPUHP_AP_ONLINE]->startup()         -> success
  === End of STARTUP section
  [CPUHP_AP_ONLINE + 1]->startup()     -> success
  ---
  [CPUHP_AP_ONLINE + N]->startup()     -> fail
  [CPUHP_AP_ONLINE + (N - 1)]->teardown()
  ...
  [CPUHP_AP_ONLINE + 1]->teardown()
  === Start of STARTUP section
  [CPUHP_AP_ONLINE]->teardown()
  ...
  [CPUHP_BRINGUP_ONLINE - 1]->teardown()
  ...
  === Start of PREPARE section
  [CPUHP_BRINGUP_CPU]->teardown()
  [CPUHP_OFFLINE + 3]->teardown()
  [CPUHP_OFFLINE + 2]                  -> skipped because teardown == NULL
  [CPUHP_OFFLINE + 1]->teardown()
  [CPUHP_OFFLINE]

A failed offline operation looks like this::

  [CPUHP_ONLINE]
  [CPUHP_ONLINE - 1]->teardown()       -> success
  ...
  [CPUHP_ONLINE - N]->teardown()       -> fail
  [CPUHP_ONLINE - (N - 1)]->startup()
  ...
  [CPUHP_ONLINE - 1]->startup()
  [CPUHP_ONLINE]

Recursive failures cannot be handled sensibly. Look at the following
example of a recursive fail due to a failed offline operation: ::

  [CPUHP_ONLINE]
  [CPUHP_ONLINE - 1]->teardown()       -> success
  ...
  [CPUHP_ONLINE - N]->teardown()       -> fail
  [CPUHP_ONLINE - (N - 1)]->startup()  -> success
  [CPUHP_ONLINE - (N - 2)]->startup()  -> fail

The CPU hotplug state machine stops right here and does not try to go back
down again because that would likely result in an endless loop::

  [CPUHP_ONLINE - (N - 1)]->teardown() -> success
  [CPUHP_ONLINE - N]->teardown()       -> fail
  [CPUHP_ONLINE - (N - 1)]->startup()  -> success
  [CPUHP_ONLINE - (N - 2)]->startup()  -> fail
  [CPUHP_ONLINE - (N - 1)]->teardown() -> success
  [CPUHP_ONLINE - N]->teardown()       -> fail

Lather, rinse and repeat. In this case the CPU left in state::

  [CPUHP_ONLINE - (N - 1)]

which at least lets the system make progress and gives the user a chance to
debug or even resolve the situation.

Allocating a state
------------------

There are two ways to allocate a CPU hotplug state:

* Static allocation

  Static allocation has to be used when the subsystem or driver has
  ordering requirements versus other CPU hotplug states. E.g. the PERF core
  startup callback has to be invoked before the PERF driver startup
  callbacks during a CPU online operation. During a CPU offline operation
  the driver teardown callbacks have to be invoked before the core teardown
  callback. The statically allocated states are described by constants in
  the cpuhp_state enum which can be found in include/linux/cpuhotplug.h.

  Insert the state into the enum at the proper place so the ordering
  requirements are fulfilled. The state constant has to be used for state
  setup and removal.

  Static allocation is also required when the state callbacks are not set
  up at runtime and are part of the initializer of the CPU hotplug state
  array in kernel/cpu.c.

* Dynamic allocation

  When there are no ordering requirements for the state callbacks then
  dynamic allocation is the preferred method. The state number is allocated
  by the setup function and returned to the caller on success.

  Only the PREPARE and ONLINE sections provide a dynamic allocation
  range. The STARTING section does not as most of the callbacks in that
  section have explicit ordering requirements.

Setup of a CPU hotplug state
----------------------------

The core code provides the following functions to setup a state:

* cpuhp_setup_state(state, name, startup, teardown)
* cpuhp_setup_state_nocalls(state, name, startup, teardown)
* cpuhp_setup_state_cpuslocked(state, name, startup, teardown)
* cpuhp_setup_state_nocalls_cpuslocked(state, name, startup, teardown)

For cases where a driver or a subsystem has multiple instances and the same
CPU hotplug state callbacks need to be invoked for each instance, the CPU
hotplug core provides multi-instance support. The advantage over driver
specific instance lists is that the instance related functions are fully
serialized against CPU hotplug operations and provide the automatic
invocations of the state callbacks on add and removal. To set up such a
multi-instance state the following function is available:

* cpuhp_setup_state_multi(state, name, startup, teardown)

The @state argument is either a statically allocated state or one of the
constants for dynamically allocated states - CPUHP_PREPARE_DYN,
CPUHP_ONLINE_DYN - depending on the state section (PREPARE, ONLINE) for
which a dynamic state should be allocated.

The @name argument is used for sysfs output and for instrumentation. The
naming convention is "subsys:mode" or "subsys/driver:mode",
e.g. "perf:mode" or "perf/x86:mode". The common mode names are:

======== =======================================================
prepare  For states in the PREPARE section

dead     For states in the PREPARE section which do not provide
         a startup callback

starting For states in the STARTING section

dying    For states in the STARTING section which do not provide
         a startup callback

online   For states in the ONLINE section

offline  For states in the ONLINE section which do not provide
         a startup callback
======== =======================================================

As the @name argument is only used for sysfs and instrumentation other mode
descriptors can be used as well if they describe the nature of the state
better than the common ones.

Examples for @name arguments: "perf/online", "perf/x86:prepare",
"RCU/tree:dying", "sched/waitempty"

The @startup argument is a function pointer to the callback which should be
invoked during a CPU online operation. If the usage site does not require a
startup callback set the pointer to NULL.

The @teardown argument is a function pointer to the callback which should
be invoked during a CPU offline operation. If the usage site does not
require a teardown callback set the pointer to NULL.

The functions differ in the way how the installed callbacks are treated:

  * cpuhp_setup_state_nocalls(), cpuhp_setup_state_nocalls_cpuslocked()
    and cpuhp_setup_state_multi() only install the callbacks

  * cpuhp_setup_state() and cpuhp_setup_state_cpuslocked() install the
    callbacks and invoke the @startup callback (if not NULL) for all online
    CPUs which have currently a state greater than the newly installed
    state. Depending on the state section the callback is either invoked on
    the current CPU (PREPARE section) or on each online CPU (ONLINE
    section) in the context of the CPU's hotplug thread.

    If a callback fails for CPU N then the teardown callback for CPU
    0 .. N-1 is invoked to rollback the operation. The state setup fails,
    the callbacks for the state are not installed and in case of dynamic
    allocation the allocated state is freed.

The state setup and the callback invocations are serialized against CPU
hotplug operations. If the setup function has to be called from a CPU
hotplug read locked region, then the _cpuslocked() variants have to be
used. These functions cannot be used from within CPU hotplug callbacks.

The function return values:
  ======== ===================================================================
  0        Statically allocated state was successfully set up

  >0       Dynamically allocated state was successfully set up.

           The returned number is the state number which was allocated. If
           the state callbacks have to be removed later, e.g. module
           removal, then this number has to be saved by the caller and used
           as @state argument for the state remove function. For
           multi-instance states the dynamically allocated state number is
           also required as @state argument for the instance add/remove
           operations.

  <0	   Operation failed
  ======== ===================================================================

Removal of a CPU hotplug state
------------------------------

To remove a previously set up state, the following functions are provided:

* cpuhp_remove_state(state)
* cpuhp_remove_state_nocalls(state)
* cpuhp_remove_state_nocalls_cpuslocked(state)
* cpuhp_remove_multi_state(state)

The @state argument is either a statically allocated state or the state
number which was allocated in the dynamic range by cpuhp_setup_state*(). If
the state is in the dynamic range, then the state number is freed and
available for dynamic allocation again.

The functions differ in the way how the installed callbacks are treated:

  * cpuhp_remove_state_nocalls(), cpuhp_remove_state_nocalls_cpuslocked()
    and cpuhp_remove_multi_state() only remove the callbacks.

  * cpuhp_remove_state() removes the callbacks and invokes the teardown
    callback (if not NULL) for all online CPUs which have currently a state
    greater than the removed state. Depending on the state section the
    callback is either invoked on the current CPU (PREPARE section) or on
    each online CPU (ONLINE section) in the context of the CPU's hotplug
    thread.

    In order to complete the removal, the teardown callback should not fail.

The state removal and the callback invocations are serialized against CPU
hotplug operations. If the remove function has to be called from a CPU
hotplug read locked region, then the _cpuslocked() variants have to be
used. These functions cannot be used from within CPU hotplug callbacks.

If a multi-instance state is removed then the caller has to remove all
instances first.

Multi-Instance state instance management
----------------------------------------

Once the multi-instance state is set up, instances can be added to the
state:

  * cpuhp_state_add_instance(state, node)
  * cpuhp_state_add_instance_nocalls(state, node)

The @state argument is either a statically allocated state or the state
number which was allocated in the dynamic range by cpuhp_setup_state_multi().

The @node argument is a pointer to an hlist_node which is embedded in the
instance's data structure. The pointer is handed to the multi-instance
state callbacks and can be used by the callback to retrieve the instance
via container_of().

The functions differ in the way how the installed callbacks are treated:

  * cpuhp_state_add_instance_nocalls() and only adds the instance to the
    multi-instance state's node list.

  * cpuhp_state_add_instance() adds the instance and invokes the startup
    callback (if not NULL) associated with @state for all online CPUs which
    have currently a state greater than @state. The callback is only
    invoked for the to be added instance. Depending on the state section
    the callback is either invoked on the current CPU (PREPARE section) or
    on each online CPU (ONLINE section) in the context of the CPU's hotplug
    thread.

    If a callback fails for CPU N then the teardown callback for CPU
    0 .. N-1 is invoked to rollback the operation, the function fails and
    the instance is not added to the node list of the multi-instance state.

To remove an instance from the state's node list these functions are
available:

  * cpuhp_state_remove_instance(state, node)
  * cpuhp_state_remove_instance_nocalls(state, node)

The arguments are the same as for the the cpuhp_state_add_instance*()
variants above.

The functions differ in the way how the installed callbacks are treated:

  * cpuhp_state_remove_instance_nocalls() only removes the instance from the
    state's node list.

  * cpuhp_state_remove_instance() removes the instance and invokes the
    teardown callback (if not NULL) associated with @state for all online
    CPUs which have currently a state greater than @state.  The callback is
    only invoked for the to be removed instance.  Depending on the state
    section the callback is either invoked on the current CPU (PREPARE
    section) or on each online CPU (ONLINE section) in the context of the
    CPU's hotplug thread.

    In order to complete the removal, the teardown callback should not fail.

The node list add/remove operations and the callback invocations are
serialized against CPU hotplug operations. These functions cannot be used
from within CPU hotplug callbacks and CPU hotplug read locked regions.

Examples
--------

Setup and teardown a statically allocated state in the STARTING section for
notifications on online and offline operations::

   ret = cpuhp_setup_state(CPUHP_SUBSYS_STARTING, "subsys:starting", subsys_cpu_starting, subsys_cpu_dying);
   if (ret < 0)
        return ret;
   ....
   cpuhp_remove_state(CPUHP_SUBSYS_STARTING);

Setup and teardown a dynamically allocated state in the ONLINE section
for notifications on offline operations::

   state = cpuhp_setup_state(CPUHP_ONLINE_DYN, "subsys:offline", NULL, subsys_cpu_offline);
   if (state < 0)
       return state;
   ....
   cpuhp_remove_state(state);

Setup and teardown a dynamically allocated state in the ONLINE section
for notifications on online operations without invoking the callbacks::

   state = cpuhp_setup_state_nocalls(CPUHP_ONLINE_DYN, "subsys:online", subsys_cpu_online, NULL);
   if (state < 0)
       return state;
   ....
   cpuhp_remove_state_nocalls(state);

Setup, use and teardown a dynamically allocated multi-instance state in the
ONLINE section for notifications on online and offline operation::

   state = cpuhp_setup_state_multi(CPUHP_ONLINE_DYN, "subsys:online", subsys_cpu_online, subsys_cpu_offline);
   if (state < 0)
       return state;
   ....
   ret = cpuhp_state_add_instance(state, &inst1->node);
   if (ret)
        return ret;
   ....
   ret = cpuhp_state_add_instance(state, &inst2->node);
   if (ret)
        return ret;
   ....
   cpuhp_remove_instance(state, &inst1->node);
   ....
   cpuhp_remove_instance(state, &inst2->node);
   ....
   remove_multi_state(state);


Testing of hotplug states
=========================

One way to verify whether a custom state is working as expected or not is to
shutdown a CPU and then put it online again. It is also possible to put the CPU
to certain state (for instance *CPUHP_AP_ONLINE*) and then go back to
*CPUHP_ONLINE*. This would simulate an error one state after *CPUHP_AP_ONLINE*
which would lead to rollback to the online state.

All registered states are enumerated in ``/sys/devices/system/cpu/hotplug/states`` ::

 $ tail /sys/devices/system/cpu/hotplug/states
 138: mm/vmscan:online
 139: mm/vmstat:online
 140: lib/percpu_cnt:online
 141: acpi/cpu-drv:online
 142: base/cacheinfo:online
 143: virtio/net:online
 144: x86/mce:online
 145: printk:online
 168: sched:active
 169: online

To rollback CPU4 to ``lib/percpu_cnt:online`` and back online just issue::

  $ cat /sys/devices/system/cpu/cpu4/hotplug/state
  169
  $ echo 140 > /sys/devices/system/cpu/cpu4/hotplug/target
  $ cat /sys/devices/system/cpu/cpu4/hotplug/state
  140

It is important to note that the teardown callback of state 140 have been
invoked. And now get back online::

  $ echo 169 > /sys/devices/system/cpu/cpu4/hotplug/target
  $ cat /sys/devices/system/cpu/cpu4/hotplug/state
  169

With trace events enabled, the individual steps are visible, too::

  #  TASK-PID   CPU#    TIMESTAMP  FUNCTION
  #     | |       |        |         |
      bash-394  [001]  22.976: cpuhp_enter: cpu: 0004 target: 140 step: 169 (cpuhp_kick_ap_work)
   cpuhp/4-31   [004]  22.977: cpuhp_enter: cpu: 0004 target: 140 step: 168 (sched_cpu_deactivate)
   cpuhp/4-31   [004]  22.990: cpuhp_exit:  cpu: 0004  state: 168 step: 168 ret: 0
   cpuhp/4-31   [004]  22.991: cpuhp_enter: cpu: 0004 target: 140 step: 144 (mce_cpu_pre_down)
   cpuhp/4-31   [004]  22.992: cpuhp_exit:  cpu: 0004  state: 144 step: 144 ret: 0
   cpuhp/4-31   [004]  22.993: cpuhp_multi_enter: cpu: 0004 target: 140 step: 143 (virtnet_cpu_down_prep)
   cpuhp/4-31   [004]  22.994: cpuhp_exit:  cpu: 0004  state: 143 step: 143 ret: 0
   cpuhp/4-31   [004]  22.995: cpuhp_enter: cpu: 0004 target: 140 step: 142 (cacheinfo_cpu_pre_down)
   cpuhp/4-31   [004]  22.996: cpuhp_exit:  cpu: 0004  state: 142 step: 142 ret: 0
      bash-394  [001]  22.997: cpuhp_exit:  cpu: 0004  state: 140 step: 169 ret: 0
      bash-394  [005]  95.540: cpuhp_enter: cpu: 0004 target: 169 step: 140 (cpuhp_kick_ap_work)
   cpuhp/4-31   [004]  95.541: cpuhp_enter: cpu: 0004 target: 169 step: 141 (acpi_soft_cpu_online)
   cpuhp/4-31   [004]  95.542: cpuhp_exit:  cpu: 0004  state: 141 step: 141 ret: 0
   cpuhp/4-31   [004]  95.543: cpuhp_enter: cpu: 0004 target: 169 step: 142 (cacheinfo_cpu_online)
   cpuhp/4-31   [004]  95.544: cpuhp_exit:  cpu: 0004  state: 142 step: 142 ret: 0
   cpuhp/4-31   [004]  95.545: cpuhp_multi_enter: cpu: 0004 target: 169 step: 143 (virtnet_cpu_online)
   cpuhp/4-31   [004]  95.546: cpuhp_exit:  cpu: 0004  state: 143 step: 143 ret: 0
   cpuhp/4-31   [004]  95.547: cpuhp_enter: cpu: 0004 target: 169 step: 144 (mce_cpu_online)
   cpuhp/4-31   [004]  95.548: cpuhp_exit:  cpu: 0004  state: 144 step: 144 ret: 0
   cpuhp/4-31   [004]  95.549: cpuhp_enter: cpu: 0004 target: 169 step: 145 (console_cpu_notify)
   cpuhp/4-31   [004]  95.550: cpuhp_exit:  cpu: 0004  state: 145 step: 145 ret: 0
   cpuhp/4-31   [004]  95.551: cpuhp_enter: cpu: 0004 target: 169 step: 168 (sched_cpu_activate)
   cpuhp/4-31   [004]  95.552: cpuhp_exit:  cpu: 0004  state: 168 step: 168 ret: 0
      bash-394  [005]  95.553: cpuhp_exit:  cpu: 0004  state: 169 step: 140 ret: 0

As it an be seen, CPU4 went down until timestamp 22.996 and then back up until
95.552. All invoked callbacks including their return codes are visible in the
trace.

Architecture's requirements
===========================

The following functions and configurations are required:

``CONFIG_HOTPLUG_CPU``
  This entry needs to be enabled in Kconfig

``__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. This includes the shutdown of the timer.

``__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.

User Space Notification
=======================

After CPU successfully onlined or offline udev events are sent. A udev rule like::

  SUBSYSTEM=="cpu", DRIVERS=="processor", DEVPATH=="/devices/system/cpu/*", RUN+="the_hotplug_receiver.sh"

will receive all events. A script like::

  #!/bin/sh

  if [ "${ACTION}" = "offline" ]
  then
      echo "CPU ${DEVPATH##*/} offline"

  elif [ "${ACTION}" = "online" ]
  then
      echo "CPU ${DEVPATH##*/} online"

  fi

can process the event further.

Kernel Inline Documentations Reference
======================================

.. kernel-doc:: include/linux/cpuhotplug.h