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_map = cpu_present_map + additional_cpus
  52(*) Option valid only for following architectures
  53- ia64
  55ia64 uses the number of disabled local apics in ACPI tables MADT to
  56determine the number of potentially hot-pluggable cpus. The implementation
  57should only rely on this to count the # of cpus, but *MUST* not rely
  58on the apicid values in those tables for disabled apics. In the event
  59BIOS doesn't mark such hot-pluggable cpus as disabled entries, one could
  60use this parameter "additional_cpus=x" to represent those cpus in the
  63possible_cpus=n         [s390,x86_64] use this to set hotpluggable cpus.
  64                        This option sets possible_cpus bits in
  65                        cpu_possible_map. Thus keeping the numbers of bits set
  66                        constant even if the machine gets rebooted.
  68CPU maps and such
  70[More on cpumaps and primitive to manipulate, please check
  71include/linux/cpumask.h that has more descriptive text.]
  73cpu_possible_map: Bitmap of possible CPUs that can ever be available in the
  74system. This is used to allocate some boot time memory for per_cpu variables
  75that aren't designed to grow/shrink as CPUs are made available or removed.
  76Once set during boot time discovery phase, the map is static, i.e no bits
  77are added or removed anytime.  Trimming it accurately for your system needs
  78upfront can save some boot time memory. See below for how we use heuristics
  79in x86_64 case to keep this under check.
  81cpu_online_map: Bitmap of all CPUs currently online. Its set in __cpu_up()
  82after a cpu is available for kernel scheduling and ready to receive
  83interrupts from devices. Its cleared when a cpu is brought down using
  84__cpu_disable(), before which all OS services including interrupts are
  85migrated to another target CPU.
  87cpu_present_map: Bitmap of CPUs currently present in the system. Not all
  88of them may be online. When physical hotplug is processed by the relevant
  89subsystem (e.g ACPI) can change and new bit either be added or removed
  90from the map depending on the event is hot-add/hot-remove. There are currently
  91no locking rules as of now. Typical usage is to init topology during boot,
  92at which time hotplug is disabled.
  94You really dont need to manipulate any of the system cpu maps. They should
  95be read-only for most use. When setting up per-cpu resources almost always use
  96cpu_possible_map/for_each_possible_cpu() to iterate.
  98Never use anything other than cpumask_t to represent bitmap of CPUs.
 100        #include <linux/cpumask.h>
 102        for_each_possible_cpu     - Iterate over cpu_possible_map
 103        for_each_online_cpu       - Iterate over cpu_online_map
 104        for_each_present_cpu      - Iterate over cpu_present_map
 105        for_each_cpu_mask(x,mask) - Iterate over some random collection of cpu mask.
 107        #include <linux/cpu.h>
 108        get_online_cpus() and put_online_cpus():
 110The above calls are used to inhibit cpu hotplug operations. While the
 111cpu_hotplug.refcount is non zero, the cpu_online_map will not change.
 112If you merely need to avoid cpus going away, you could also use
 113preempt_disable() and preempt_enable() for those sections.
 114Just remember the critical section cannot call any
 115function that can sleep or schedule this process away. The preempt_disable()
 116will work as long as stop_machine_run() is used to take a cpu down.
 118CPU Hotplug - Frequently Asked Questions.
 120Q: How to enable my kernel to support CPU hotplug?
 121A: When doing make defconfig, Enable CPU hotplug support
 123   "Processor type and Features" -> Support for Hotpluggable CPUs
 125Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.
 127You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
 128as well.
 130Q: What architectures support CPU hotplug?
 131A: As of 2.6.14, the following architectures support CPU hotplug.
 133i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
 135Q: How to test if hotplug is supported on the newly built kernel?
 136A: You should now notice an entry in sysfs.
 138Check if sysfs is mounted, using the "mount" command. You should notice
 139an entry as shown below in the output.
 141        ....
 142        none on /sys type sysfs (rw)
 143        ....
 145If this is not mounted, do the following.
 147         #mkdir /sysfs
 148        #mount -t sysfs sys /sys
 150Now you should see entries for all present cpu, the following is an example
 151in a 8-way system.
 153        #pwd
 154        #/sys/devices/system/cpu
 155        #ls -l
 156        total 0
 157        drwxr-xr-x  10 root root 0 Sep 19 07:44 .
 158        drwxr-xr-x  13 root root 0 Sep 19 07:45 ..
 159        drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu0
 160        drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu1
 161        drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu2
 162        drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu3
 163        drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu4
 164        drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu5
 165        drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu6
 166        drwxr-xr-x   3 root root 0 Sep 19 07:48 cpu7
 168Under each directory you would find an "online" file which is the control
 169file to logically online/offline a processor.
 171Q: Does hot-add/hot-remove refer to physical add/remove of cpus?
 172A: The usage of hot-add/remove may not be very consistently used in the code.
 173CONFIG_HOTPLUG_CPU enables logical online/offline capability in the kernel.
 174To support physical addition/removal, one would need some BIOS hooks and
 175the platform should have something like an attention button in PCI hotplug.
 176CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.
 178Q: How do i logically offline a CPU?
 179A: Do the following.
 181        #echo 0 > /sys/devices/system/cpu/cpuX/online
 183Once the logical offline is successful, check
 185        #cat /proc/interrupts
 187You should now not see the CPU that you removed. Also online file will report
 188the state as 0 when a cpu if offline and 1 when its online.
 190        #To display the current cpu state.
 191        #cat /sys/devices/system/cpu/cpuX/online
 193Q: Why cant i remove CPU0 on some systems?
 194A: Some architectures may have some special dependency on a certain CPU.
 196For e.g in IA64 platforms we have ability to sent platform interrupts to the
 197OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
 198specifications, we didn't have a way to change the target CPU. Hence if the
 199current ACPI version doesn't support such re-direction, we disable that CPU
 200by making it not-removable.
 202In such cases you will also notice that the online file is missing under cpu0.
 204Q: How do i find out if a particular CPU is not removable?
 205A: Depending on the implementation, some architectures may show this by the
 206absence of the "online" file. This is done if it can be determined ahead of
 207time that this CPU cannot be removed.
 209In some situations, this can be a run time check, i.e if you try to remove the
 210last CPU, this will not be permitted. You can find such failures by
 211investigating the return value of the "echo" command.
 213Q: What happens when a CPU is being logically offlined?
 214A: The following happen, listed in no particular order :-)
 216- A notification is sent to in-kernel registered modules by sending an event
 217  CPU_DOWN_PREPARE or CPU_DOWN_PREPARE_FROZEN, depending on whether or not the
 218  CPU is being offlined while tasks are frozen due to a suspend operation in
 219  progress
 220- All processes are migrated away from this outgoing CPU to new CPUs.
 221  The new CPU is chosen from each process' current cpuset, which may be
 222  a subset of all online CPUs.
 223- All interrupts targeted to this CPU is migrated to a new CPU
 224- timers/bottom half/task lets are also migrated to a new CPU
 225- Once all services are migrated, kernel calls an arch specific routine
 226  __cpu_disable() to perform arch specific cleanup.
 227- Once this is successful, an event for successful cleanup is sent by an event
 228  CPU_DEAD (or CPU_DEAD_FROZEN if tasks are frozen due to a suspend while the
 229  CPU is being offlined).
 231  "It is expected that each service cleans up when the CPU_DOWN_PREPARE
 232  notifier is called, when CPU_DEAD is called its expected there is nothing
 233  running on behalf of this CPU that was offlined"
 235Q: If i have some kernel code that needs to be aware of CPU arrival and
 236   departure, how to i arrange for proper notification?
 237A: This is what you would need in your kernel code to receive notifications.
 239        #include <linux/cpu.h>
 240        static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb,
 241                                            unsigned long action, void *hcpu)
 242        {
 243                unsigned int cpu = (unsigned long)hcpu;
 245                switch (action) {
 246                case CPU_ONLINE:
 247                case CPU_ONLINE_FROZEN:
 248                        foobar_online_action(cpu);
 249                        break;
 250                case CPU_DEAD:
 251                case CPU_DEAD_FROZEN:
 252                        foobar_dead_action(cpu);
 253                        break;
 254                }
 255                return NOTIFY_OK;
 256        }
 258        static struct notifier_block __cpuinitdata foobar_cpu_notifer =
 259        {
 260           .notifier_call = foobar_cpu_callback,
 261        };
 263You need to call register_cpu_notifier() from your init function.
 264Init functions could be of two types:
 2651. early init (init function called when only the boot processor is online).
 2662. late init (init function called _after_ all the CPUs are online).
 268For the first case, you should add the following to your init function
 270        register_cpu_notifier(&foobar_cpu_notifier);
 272For the second case, you should add the following to your init function
 274        register_hotcpu_notifier(&foobar_cpu_notifier);
 276You can fail PREPARE notifiers if something doesn't work to prepare resources.
 277This will stop the activity and send a following CANCELED event back.
 279CPU_DEAD should not be failed, its just a goodness indication, but bad
 280things will happen if a notifier in path sent a BAD notify code.
 282Q: I don't see my action being called for all CPUs already up and running?
 283A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined.
 284   If you need to perform some action for each cpu already in the system, then
 286        for_each_online_cpu(i) {
 287                foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
 288                foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, i);
 289        }
 291Q: If i would like to develop cpu hotplug support for a new architecture,
 292   what do i need at a minimum?
 293A: The following are what is required for CPU hotplug infrastructure to work
 294   correctly.
 296    - Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU
 297    - __cpu_up()        - Arch interface to bring up a CPU
 298    - __cpu_disable()   - Arch interface to shutdown a CPU, no more interrupts
 299                          can be handled by the kernel after the routine
 300                          returns. Including local APIC timers etc are
 301                          shutdown.
 302     - __cpu_die()      - This actually supposed to ensure death of the CPU.
 303                          Actually look at some example code in other arch
 304                          that implement CPU hotplug. The processor is taken
 305                          down from the idle() loop for that specific
 306                          architecture. __cpu_die() typically waits for some
 307                          per_cpu state to be set, to ensure the processor
 308                          dead routine is called to be sure positively.
 310Q: I need to ensure that a particular cpu is not removed when there is some
 311   work specific to this cpu is in progress.
 312A: First switch the current thread context to preferred cpu
 314        int my_func_on_cpu(int cpu)
 315        {
 316                cpumask_t saved_mask, new_mask = CPU_MASK_NONE;
 317                int curr_cpu, err = 0;
 319                saved_mask = current->cpus_allowed;
 320                cpu_set(cpu, new_mask);
 321                err = set_cpus_allowed(current, new_mask);
 323                if (err)
 324                        return err;
 326                /*
 327                 * If we got scheduled out just after the return from
 328                 * set_cpus_allowed() before running the work, this ensures
 329                 * we stay locked.
 330                 */
 331                curr_cpu = get_cpu();
 333                if (curr_cpu != cpu) {
 334                        err = -EAGAIN;
 335                        goto ret;
 336                } else {
 337                        /*
 338                         * Do work : But cant sleep, since get_cpu() disables preempt
 339                         */
 340                }
 341                ret:
 342                        put_cpu();
 343                        set_cpus_allowed(current, saved_mask);
 344                        return err;
 345                }
 348Q: How do we determine how many CPUs are available for hotplug.
 349A: There is no clear spec defined way from ACPI that can give us that
 350   information today. Based on some input from Natalie of Unisys,
 351   that the ACPI MADT (Multiple APIC Description Tables) marks those possible
 352   CPUs in a system with disabled status.
 354   Andi implemented some simple heuristics that count the number of disabled
 355   CPUs in MADT as hotpluggable CPUS.  In the case there are no disabled CPUS
 356   we assume 1/2 the number of CPUs currently present can be hotplugged.
 358   Caveat: Today's ACPI MADT can only provide 256 entries since the apicid field
 359   in MADT is only 8 bits.
 361User Space Notification
 363Hotplug support for devices is common in Linux today. Its being used today to
 364support automatic configuration of network, usb and pci devices. A hotplug
 365event can be used to invoke an agent script to perform the configuration task.
 367You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
 370        #!/bin/bash
 371        # $Id: cpu.agent
 372        # Kernel hotplug params include:
 373        #ACTION=%s [online or offline]
 374        #DEVPATH=%s
 375        #
 376        cd /etc/hotplug
 377        . ./hotplug.functions
 379        case $ACTION in
 380                online)
 381                        echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt
 382                        ;;
 383                offline)
 384                        echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt
 385                        ;;
 386                *)
 387                        debug_mesg CPU $ACTION event not supported
 388        exit 1
 389        ;;
 390        esac