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+=========================
+CPU hotplug in the Kernel
+=========================
+
+:Date: December, 2016
+: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>
+
+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 fourV 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 CPUs the kernel will support. If the number
+  supplied here is lower than the number of physically available CPUs than
+  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.
+
+``cede_offline={"off","on"}``
+  Use this option to disable/enable putting offlined processors to an extended
+  ``H_CEDE`` state on supported pseries platforms. If nothing is specified,
+  ``cede_offline`` is set to "on".
+
+  This option is limited to the PowerPC 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.
+
+Using the hotplug API
+---------------------
+It is possible to receive notifications once a CPU is offline or onlined. This
+might be important to certain drivers which need to perform some kind of setup
+or clean up functions based on the number of available CPUs: ::
+
+  #include <linux/cpuhotplug.h>
+
+  ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "X/Y:online",
+                          Y_online, Y_prepare_down);
+
+*X* is the subsystem and *Y* the particular driver. The *Y_online* callback
+will be invoked during registration on all online CPUs. If an error
+occurs during the online callback the *Y_prepare_down* callback will be
+invoked on all CPUs on which the online callback was previously invoked.
+After registration completed, the *Y_online* callback will be invoked
+once a CPU is brought online and *Y_prepare_down* will be invoked when a
+CPU is shutdown. All resources which were previously allocated in
+*Y_online* should be released in *Y_prepare_down*.
+The return value *ret* is negative if an error occurred during the
+registration process. Otherwise a positive value is returned which
+contains the allocated hotplug for dynamically allocated states
+(*CPUHP_AP_ONLINE_DYN*). It will return zero for predefined states.
+
+The callback can be remove by invoking ``cpuhp_remove_state()``. In case of a
+dynamically allocated state (*CPUHP_AP_ONLINE_DYN*) use the returned state.
+During the removal of a hotplug state the teardown callback will be invoked.
+
+Multiple instances
+~~~~~~~~~~~~~~~~~~
+If a driver has multiple instances and each instance needs to perform the
+callback independently then it is likely that a ''multi-state'' should be used.
+First a multi-state state needs to be registered: ::
+
+  ret = cpuhp_setup_state_multi(CPUHP_AP_ONLINE_DYN, "X/Y:online,
+                                Y_online, Y_prepare_down);
+  Y_hp_online = ret;
+
+The ``cpuhp_setup_state_multi()`` behaves similar to ``cpuhp_setup_state()``
+except it prepares the callbacks for a multi state and does not invoke
+the callbacks. This is a one time setup.
+Once a new instance is allocated, you need to register this new instance: ::
+
+  ret = cpuhp_state_add_instance(Y_hp_online, &d->node);
+
+This function will add this instance to your previously allocated
+*Y_hp_online* state and invoke the previously registered callback
+(*Y_online*) on all online CPUs. The *node* element is a ``struct
+hlist_node`` member of your per-instance data structure.
+
+On removal of the instance: ::
+  cpuhp_state_remove_instance(Y_hp_online, &d->node)
+
+should be invoked which will invoke the teardown callback on all online
+CPUs.
+
+Manual setup
+~~~~~~~~~~~~
+Usually it is handy to invoke setup and teardown callbacks on registration or
+removal of a state because usually the operation needs to performed once a CPU
+goes online (offline) and during initial setup (shutdown) of the driver. However
+each registration and removal function is also available with a ``_nocalls``
+suffix which does not invoke the provided callbacks if the invocation of the
+callbacks is not desired. During the manual setup (or teardown) the functions
+``get_online_cpus()`` and ``put_online_cpus()`` should be used to inhibit CPU
+hotplug operations.
+
+
+The ordering of the events
+--------------------------
+The hotplug states are defined in ``include/linux/cpuhotplug.h``:
+
+* The states *CPUHP_OFFLINE* … *CPUHP_AP_OFFLINE* are invoked before the
+  CPU is up.
+* The states *CPUHP_AP_OFFLINE* … *CPUHP_AP_ONLINE* are invoked
+  just the after the CPU has been brought up. The interrupts are off and
+  the scheduler is not yet active on this CPU. Starting with *CPUHP_AP_OFFLINE*
+  the callbacks are invoked on the target CPU.
+* The states between *CPUHP_AP_ONLINE_DYN* and *CPUHP_AP_ONLINE_DYN_END* are
+  reserved for the dynamic allocation.
+* The states are invoked in the reverse order on CPU shutdown starting with
+  *CPUHP_ONLINE* and stopping at *CPUHP_OFFLINE*. Here the callbacks are
+  invoked on the CPU that will be shutdown until *CPUHP_AP_OFFLINE*.
+
+A dynamically allocated state via *CPUHP_AP_ONLINE_DYN* is often enough.
+However if an earlier invocation during the bring up or shutdown is required
+then an explicit state should be acquired. An explicit state might also be
+required if the hotplug event requires specific ordering in respect to
+another hotplug event.
+
+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 callbac 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