1                CPU hotplug Support in Linux(tm) Kernel
   3                Maintainers:
   4                CPU Hotplug Core:
   5                        Rusty Russell <>
   6                        Srivatsa Vaddagiri <>
   7                i386:
   8                        Zwane Mwaikambo <>
   9                ppc64:
  10                        Nathan Lynch <>
  11                        Joel Schopp <>
  12                ia64/x86_64:
  13                        Ashok Raj <>
  14                s390:
  15                        Heiko Carstens <>
  17Authors: Ashok Raj <>
  18Lots of feedback: Nathan Lynch <>,
  19             Joel Schopp <>
  23Modern advances in system architectures have introduced advanced error
  24reporting and correction capabilities in processors. CPU architectures permit
  25partitioning support, where compute resources of a single CPU could be made
  26available to virtual machine environments. There are couple OEMS that
  27support NUMA hardware which are hot pluggable as well, where physical
  28node insertion and removal require support for CPU hotplug.
  30Such advances require CPUs available to a kernel to be removed either for
  31provisioning reasons, or for RAS purposes to keep an offending CPU off
  32system execution path. Hence the need for CPU hotplug support in the
  33Linux kernel.
  35A more novel use of CPU-hotplug support is its use today in suspend
  36resume support for SMP. Dual-core and HT support makes even
  37a laptop run SMP kernels which didn't support these methods. SMP support
  38for suspend/resume is a work in progress.
  40General Stuff about CPU Hotplug
  43Command Line Switches
  45maxcpus=n    Restrict boot time cpus to n. Say if you have 4 cpus, using
  46             maxcpus=2 will only boot 2. You can choose to bring the
  47             other cpus later online, read FAQ's for more info.
  49additional_cpus=n (*)   Use this to limit hotpluggable cpus. This option sets
  50                        cpu_possible_mask = cpu_present_mask + additional_cpus
  52cede_offline={"off","on"}  Use this option to disable/enable putting offlined
  53                            processors to an extended H_CEDE state on
  54                            supported pseries platforms.
  55                            If nothing is specified,
  56                            cede_offline is set to "on".
  58(*) Option valid only for following architectures
  59- ia64
  61ia64 uses the number of disabled local apics in ACPI tables MADT to
  62determine the number of potentially hot-pluggable cpus. The implementation
  63should only rely on this to count the # of cpus, but *MUST* not rely
  64on the apicid values in those tables for disabled apics. In the event
  65BIOS doesn't mark such hot-pluggable cpus as disabled entries, one could
  66use this parameter "additional_cpus=x" to represent those cpus in the
  69possible_cpus=n         [s390,x86_64] use this to set hotpluggable cpus.
  70                        This option sets possible_cpus bits in
  71                        cpu_possible_mask. Thus keeping the numbers of bits set
  72                        constant even if the machine gets rebooted.
  74CPU maps and such
  76[More on cpumaps and primitive to manipulate, please check
  77include/linux/cpumask.h that has more descriptive text.]
  79cpu_possible_mask: Bitmap of possible CPUs that can ever be available in the
  80system. This is used to allocate some boot time memory for per_cpu variables
  81that aren't designed to grow/shrink as CPUs are made available or removed.
  82Once set during boot time discovery phase, the map is static, i.e no bits
  83are added or removed anytime.  Trimming it accurately for your system needs
  84upfront can save some boot time memory. See below for how we use heuristics
  85in x86_64 case to keep this under check.
  87cpu_online_mask: Bitmap of all CPUs currently online. Its set in __cpu_up()
  88after a cpu is available for kernel scheduling and ready to receive
  89interrupts from devices. Its cleared when a cpu is brought down using
  90__cpu_disable(), before which all OS services including interrupts are
  91migrated to another target CPU.
  93cpu_present_mask: Bitmap of CPUs currently present in the system. Not all
  94of them may be online. When physical hotplug is processed by the relevant
  95subsystem (e.g ACPI) can change and new bit either be added or removed
  96from the map depending on the event is hot-add/hot-remove. There are currently
  97no locking rules as of now. Typical usage is to init topology during boot,
  98at which time hotplug is disabled.
 100You really dont need to manipulate any of the system cpu maps. They should
 101be read-only for most use. When setting up per-cpu resources almost always use
 102cpu_possible_mask/for_each_possible_cpu() to iterate.
 104Never use anything other than cpumask_t to represent bitmap of CPUs.
 106        #include <linux/cpumask.h>
 108        for_each_possible_cpu     - Iterate over cpu_possible_mask
 109        for_each_online_cpu       - Iterate over cpu_online_mask
 110        for_each_present_cpu      - Iterate over cpu_present_mask
 111        for_each_cpu_mask(x,mask) - Iterate over some random collection of cpu mask.
 113        #include <linux/cpu.h>
 114        get_online_cpus() and put_online_cpus():
 116The above calls are used to inhibit cpu hotplug operations. While the
 117cpu_hotplug.refcount is non zero, the cpu_online_mask will not change.
 118If you merely need to avoid cpus going away, you could also use
 119preempt_disable() and preempt_enable() for those sections.
 120Just remember the critical section cannot call any
 121function that can sleep or schedule this process away. The preempt_disable()
 122will work as long as stop_machine_run() is used to take a cpu down.
 124CPU Hotplug - Frequently Asked Questions.
 126Q: How to enable my kernel to support CPU hotplug?
 127A: When doing make defconfig, Enable CPU hotplug support
 129   "Processor type and Features" -> Support for Hotpluggable CPUs
 131Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.
 133You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
 134as well.
 136Q: What architectures support CPU hotplug?
 137A: As of 2.6.14, the following architectures support CPU hotplug.
 139i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
 141Q: How to test if hotplug is supported on the newly built kernel?
 142A: You should now notice an entry in sysfs.
 144Check if sysfs is mounted, using the "mount" command. You should notice
 145an entry as shown below in the output.
 147        ....
 148        none on /sys type sysfs (rw)
 149        ....
 151If this is not mounted, do the following.
 153         #mkdir /sysfs
 154        #mount -t sysfs sys /sys
 156Now you should see entries for all present cpu, the following is an example
 157in a 8-way system.
 159        #pwd
 160        #/sys/devices/system/cpu
 161        #ls -l
 162        total 0
 163        drwxr-xr-x  10 root root 0 Sep 19 07:44 .
 164        drwxr-xr-x  13 root root 0 Sep 19 07:45 ..
 165        drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu0
 166        drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu1
 167        drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu2
 168        drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu3
 169        drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu4
 170        drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu5
 171        drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu6
 172        drwxr-xr-x   3 root root 0 Sep 19 07:48 cpu7
 174Under each directory you would find an "online" file which is the control
 175file to logically online/offline a processor.
 177Q: Does hot-add/hot-remove refer to physical add/remove of cpus?
 178A: The usage of hot-add/remove may not be very consistently used in the code.
 179CONFIG_HOTPLUG_CPU enables logical online/offline capability in the kernel.
 180To support physical addition/removal, one would need some BIOS hooks and
 181the platform should have something like an attention button in PCI hotplug.
 182CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.
 184Q: How do i logically offline a CPU?
 185A: Do the following.
 187        #echo 0 > /sys/devices/system/cpu/cpuX/online
 189Once the logical offline is successful, check
 191        #cat /proc/interrupts
 193You should now not see the CPU that you removed. Also online file will report
 194the state as 0 when a cpu if offline and 1 when its online.
 196        #To display the current cpu state.
 197        #cat /sys/devices/system/cpu/cpuX/online
 199Q: Why can't i remove CPU0 on some systems?
 200A: Some architectures may have some special dependency on a certain CPU.
 202For e.g in IA64 platforms we have ability to sent platform interrupts to the
 203OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
 204specifications, we didn't have a way to change the target CPU. Hence if the
 205current ACPI version doesn't support such re-direction, we disable that CPU
 206by making it not-removable.
 208In such cases you will also notice that the online file is missing under cpu0.
 210Q: Is CPU0 removable on X86?
 211A: Yes. If kernel is compiled with CONFIG_BOOTPARAM_HOTPLUG_CPU0=y, CPU0 is
 212removable by default. Otherwise, CPU0 is also removable by kernel option
 215But some features depend on CPU0. Two known dependencies are:
 2171. Resume from hibernate/suspend depends on CPU0. Hibernate/suspend will fail if
 218CPU0 is offline and you need to online CPU0 before hibernate/suspend can
 2202. PIC interrupts also depend on CPU0. CPU0 can't be removed if a PIC interrupt
 221is detected.
 223It's said poweroff/reboot may depend on CPU0 on some machines although I haven't
 224seen any poweroff/reboot failure so far after CPU0 is offline on a few tested
 227Please let me know if you know or see any other dependencies of CPU0.
 229If the dependencies are under your control, you can turn on CPU0 hotplug feature
 230either by CONFIG_BOOTPARAM_HOTPLUG_CPU0 or by kernel parameter cpu0_hotplug.
 232--Fenghua Yu <>
 234Q: How do i find out if a particular CPU is not removable?
 235A: Depending on the implementation, some architectures may show this by the
 236absence of the "online" file. This is done if it can be determined ahead of
 237time that this CPU cannot be removed.
 239In some situations, this can be a run time check, i.e if you try to remove the
 240last CPU, this will not be permitted. You can find such failures by
 241investigating the return value of the "echo" command.
 243Q: What happens when a CPU is being logically offlined?
 244A: The following happen, listed in no particular order :-)
 246- A notification is sent to in-kernel registered modules by sending an event
 247  CPU_DOWN_PREPARE or CPU_DOWN_PREPARE_FROZEN, depending on whether or not the
 248  CPU is being offlined while tasks are frozen due to a suspend operation in
 249  progress
 250- All processes are migrated away from this outgoing CPU to new CPUs.
 251  The new CPU is chosen from each process' current cpuset, which may be
 252  a subset of all online CPUs.
 253- All interrupts targeted to this CPU is migrated to a new CPU
 254- timers/bottom half/task lets are also migrated to a new CPU
 255- Once all services are migrated, kernel calls an arch specific routine
 256  __cpu_disable() to perform arch specific cleanup.
 257- Once this is successful, an event for successful cleanup is sent by an event
 258  CPU_DEAD (or CPU_DEAD_FROZEN if tasks are frozen due to a suspend while the
 259  CPU is being offlined).
 261  "It is expected that each service cleans up when the CPU_DOWN_PREPARE
 262  notifier is called, when CPU_DEAD is called its expected there is nothing
 263  running on behalf of this CPU that was offlined"
 265Q: If i have some kernel code that needs to be aware of CPU arrival and
 266   departure, how to i arrange for proper notification?
 267A: This is what you would need in your kernel code to receive notifications.
 269        #include <linux/cpu.h>
 270        static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb,
 271                                            unsigned long action, void *hcpu)
 272        {
 273                unsigned int cpu = (unsigned long)hcpu;
 275                switch (action) {
 276                case CPU_ONLINE:
 277                case CPU_ONLINE_FROZEN:
 278                        foobar_online_action(cpu);
 279                        break;
 280                case CPU_DEAD:
 281                case CPU_DEAD_FROZEN:
 282                        foobar_dead_action(cpu);
 283                        break;
 284                }
 285                return NOTIFY_OK;
 286        }
 288        static struct notifier_block __cpuinitdata foobar_cpu_notifer =
 289        {
 290           .notifier_call = foobar_cpu_callback,
 291        };
 293You need to call register_cpu_notifier() from your init function.
 294Init functions could be of two types:
 2951. early init (init function called when only the boot processor is online).
 2962. late init (init function called _after_ all the CPUs are online).
 298For the first case, you should add the following to your init function
 300        register_cpu_notifier(&foobar_cpu_notifier);
 302For the second case, you should add the following to your init function
 304        register_hotcpu_notifier(&foobar_cpu_notifier);
 306You can fail PREPARE notifiers if something doesn't work to prepare resources.
 307This will stop the activity and send a following CANCELED event back.
 309CPU_DEAD should not be failed, its just a goodness indication, but bad
 310things will happen if a notifier in path sent a BAD notify code.
 312Q: I don't see my action being called for all CPUs already up and running?
 313A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined.
 314   If you need to perform some action for each cpu already in the system, then
 316        for_each_online_cpu(i) {
 317                foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
 318                foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, i);
 319        }
 321Q: If i would like to develop cpu hotplug support for a new architecture,
 322   what do i need at a minimum?
 323A: The following are what is required for CPU hotplug infrastructure to work
 324   correctly.
 326    - Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU
 327    - __cpu_up()        - Arch interface to bring up a CPU
 328    - __cpu_disable()   - Arch interface to shutdown a CPU, no more interrupts
 329                          can be handled by the kernel after the routine
 330                          returns. Including local APIC timers etc are
 331                          shutdown.
 332     - __cpu_die()      - This actually supposed to ensure death of the CPU.
 333                          Actually look at some example code in other arch
 334                          that implement CPU hotplug. The processor is taken
 335                          down from the idle() loop for that specific
 336                          architecture. __cpu_die() typically waits for some
 337                          per_cpu state to be set, to ensure the processor
 338                          dead routine is called to be sure positively.
 340Q: I need to ensure that a particular cpu is not removed when there is some
 341   work specific to this cpu is in progress.
 342A: There are two ways.  If your code can be run in interrupt context, use
 343   smp_call_function_single(), otherwise use work_on_cpu().  Note that
 344   work_on_cpu() is slow, and can fail due to out of memory:
 346        int my_func_on_cpu(int cpu)
 347        {
 348                int err;
 349                get_online_cpus();
 350                if (!cpu_online(cpu))
 351                        err = -EINVAL;
 352                else
 354                        err = work_on_cpu(cpu, __my_func_on_cpu, NULL);
 356                        smp_call_function_single(cpu, __my_func_on_cpu, &err,
 357                                                 true);
 359                put_online_cpus();
 360                return err;
 361        }
 363Q: How do we determine how many CPUs are available for hotplug.
 364A: There is no clear spec defined way from ACPI that can give us that
 365   information today. Based on some input from Natalie of Unisys,
 366   that the ACPI MADT (Multiple APIC Description Tables) marks those possible
 367   CPUs in a system with disabled status.
 369   Andi implemented some simple heuristics that count the number of disabled
 370   CPUs in MADT as hotpluggable CPUS.  In the case there are no disabled CPUS
 371   we assume 1/2 the number of CPUs currently present can be hotplugged.
 373   Caveat: Today's ACPI MADT can only provide 256 entries since the apicid field
 374   in MADT is only 8 bits.
 376User Space Notification
 378Hotplug support for devices is common in Linux today. Its being used today to
 379support automatic configuration of network, usb and pci devices. A hotplug
 380event can be used to invoke an agent script to perform the configuration task.
 382You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
 385        #!/bin/bash
 386        # $Id: cpu.agent
 387        # Kernel hotplug params include:
 388        #ACTION=%s [online or offline]
 389        #DEVPATH=%s
 390        #
 391        cd /etc/hotplug
 392        . ./hotplug.functions
 394        case $ACTION in
 395                online)
 396                        echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt
 397                        ;;
 398                offline)
 399                        echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt
 400                        ;;
 401                *)
 402                        debug_mesg CPU $ACTION event not supported
 403        exit 1
 404        ;;
 405        esac