Documentation/cpu-hotplug.txt

   1                 CPU hotplug Support in Linux(tm) Kernel
   2
   3                 Maintainers:
   4                 CPU Hotplug Core:
   5                         Rusty Russell <rusty@rustycorp.com.au>
   6                         Srivatsa Vaddagiri <vatsa@in.ibm.com>
   7                 i386:
   8                         Zwane Mwaikambo <zwane@arm.linux.org.uk>
   9                 ppc64:
  10                         Nathan Lynch <nathanl@austin.ibm.com>
  11                         Joel Schopp <jschopp@austin.ibm.com>
  12                 ia64/x86_64:
  13                         Ashok Raj <ashok.raj@intel.com>
  14                 s390:
  15                         Heiko Carstens <heiko.carstens@de.ibm.com>
  16
  17 Authors: Ashok Raj <ashok.raj@intel.com>
  18 Lots of feedback: Nathan Lynch <nathanl@austin.ibm.com>,
  19              Joel Schopp <jschopp@austin.ibm.com>
  20
  21 Introduction
  22
  23 Modern advances in system architectures have introduced advanced error
  24 reporting and correction capabilities in processors. CPU architectures permit
  25 partitioning support, where compute resources of a single CPU could be made
  26 available to virtual machine environments. There are couple OEMS that
  27 support NUMA hardware which are hot pluggable as well, where physical
  28 node insertion and removal require support for CPU hotplug.
  29
  30 Such advances require CPUs available to a kernel to be removed either for
  31 provisioning reasons, or for RAS purposes to keep an offending CPU off
  32 system execution path. Hence the need for CPU hotplug support in the
  33 Linux kernel.
  34
  35 A more novel use of CPU-hotplug support is its use today in suspend
  36 resume support for SMP. Dual-core and HT support makes even
  37 a laptop run SMP kernels which didn't support these methods. SMP support
  38 for suspend/resume is a work in progress.
  39
  40 General Stuff about CPU Hotplug
  41 --------------------------------
  42
  43 Command Line Switches
  44 ---------------------
  45 maxcpus=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.
  48
  49 additional_cpus=n (*)   Use this to limit hotpluggable cpus. This option sets
  50                         cpu_possible_map = cpu_present_map + additional_cpus
  51
  52 (*) Option valid only for following architectures
  53 - ia64
  54
  55 ia64 uses the number of disabled local apics in ACPI tables MADT to
  56 determine the number of potentially hot-pluggable cpus. The implementation
  57 should only rely on this to count the # of cpus, but *MUST* not rely
  58 on the apicid values in those tables for disabled apics. In the event
  59 BIOS doesn't mark such hot-pluggable cpus as disabled entries, one could
  60 use this parameter "additional_cpus=x" to represent those cpus in the
  61 cpu_possible_map.
  62
  63 possible_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.
  67
  68 CPU maps and such
  69 -----------------
  70 [More on cpumaps and primitive to manipulate, please check
  71 include/linux/cpumask.h that has more descriptive text.]
  72
  73 cpu_possible_map: Bitmap of possible CPUs that can ever be available in the
  74 system. This is used to allocate some boot time memory for per_cpu variables
  75 that aren't designed to grow/shrink as CPUs are made available or removed.
  76 Once set during boot time discovery phase, the map is static, i.e no bits
  77 are added or removed anytime.  Trimming it accurately for your system needs
  78 upfront can save some boot time memory. See below for how we use heuristics
  79 in x86_64 case to keep this under check.
  80
  81 cpu_online_map: Bitmap of all CPUs currently online. Its set in __cpu_up()
  82 after a cpu is available for kernel scheduling and ready to receive
  83 interrupts from devices. Its cleared when a cpu is brought down using
  84 __cpu_disable(), before which all OS services including interrupts are
  85 migrated to another target CPU.
  86
  87 cpu_present_map: Bitmap of CPUs currently present in the system. Not all
  88 of them may be online. When physical hotplug is processed by the relevant
  89 subsystem (e.g ACPI) can change and new bit either be added or removed
  90 from the map depending on the event is hot-add/hot-remove. There are currently
  91 no locking rules as of now. Typical usage is to init topology during boot,
  92 at which time hotplug is disabled.
  93
  94 You really dont need to manipulate any of the system cpu maps. They should
  95 be read-only for most use. When setting up per-cpu resources almost always use
  96 cpu_possible_map/for_each_possible_cpu() to iterate.
  97
  98 Never use anything other than cpumask_t to represent bitmap of CPUs.
  99
 100         #include <linux/cpumask.h>
 101
 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.
 106
 107         #include <linux/cpu.h>
 108         get_online_cpus() and put_online_cpus():
 109
 110 The above calls are used to inhibit cpu hotplug operations. While the
 111 cpu_hotplug.refcount is non zero, the cpu_online_map will not change.
 112 If you merely need to avoid cpus going away, you could also use
 113 preempt_disable() and preempt_enable() for those sections.
 114 Just remember the critical section cannot call any
 115 function that can sleep or schedule this process away. The preempt_disable()
 116 will work as long as stop_machine_run() is used to take a cpu down.
 117
 118 CPU Hotplug - Frequently Asked Questions.
 119
 120 Q: How to enable my kernel to support CPU hotplug?
 121 A: When doing make defconfig, Enable CPU hotplug support
 122
 123    "Processor type and Features" -> Support for Hotpluggable CPUs
 124
 125 Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.
 126
 127 You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
 128 as well.
 129
 130 Q: What architectures support CPU hotplug?
 131 A: As of 2.6.14, the following architectures support CPU hotplug.
 132
 133 i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
 134
 135 Q: How to test if hotplug is supported on the newly built kernel?
 136 A: You should now notice an entry in sysfs.
 137
 138 Check if sysfs is mounted, using the "mount" command. You should notice
 139 an entry as shown below in the output.
 140
 141         ....
 142         none on /sys type sysfs (rw)
 143         ....
 144
 145 If this is not mounted, do the following.
 146
 147          #mkdir /sysfs
 148         #mount -t sysfs sys /sys
 149
 150 Now you should see entries for all present cpu, the following is an example
 151 in a 8-way system.
 152
 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
 167
 168 Under each directory you would find an "online" file which is the control
 169 file to logically online/offline a processor.
 170
 171 Q: Does hot-add/hot-remove refer to physical add/remove of cpus?
 172 A: The usage of hot-add/remove may not be very consistently used in the code.
 173 CONFIG_HOTPLUG_CPU enables logical online/offline capability in the kernel.
 174 To support physical addition/removal, one would need some BIOS hooks and
 175 the platform should have something like an attention button in PCI hotplug.
 176 CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.
 177
 178 Q: How do i logically offline a CPU?
 179 A: Do the following.
 180
 181         #echo 0 > /sys/devices/system/cpu/cpuX/online
 182
 183 Once the logical offline is successful, check
 184
 185         #cat /proc/interrupts
 186
 187 You should now not see the CPU that you removed. Also online file will report
 188 the state as 0 when a cpu if offline and 1 when its online.
 189
 190         #To display the current cpu state.
 191         #cat /sys/devices/system/cpu/cpuX/online
 192
 193 Q: Why cant i remove CPU0 on some systems?
 194 A: Some architectures may have some special dependency on a certain CPU.
 195
 196 For e.g in IA64 platforms we have ability to sent platform interrupts to the
 197 OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
 198 specifications, we didn't have a way to change the target CPU. Hence if the
 199 current ACPI version doesn't support such re-direction, we disable that CPU
 200 by making it not-removable.
 201
 202 In such cases you will also notice that the online file is missing under cpu0.
 203
 204 Q: How do i find out if a particular CPU is not removable?
 205 A: Depending on the implementation, some architectures may show this by the
 206 absence of the "online" file. This is done if it can be determined ahead of
 207 time that this CPU cannot be removed.
 208
 209 In some situations, this can be a run time check, i.e if you try to remove the
 210 last CPU, this will not be permitted. You can find such failures by
 211 investigating the return value of the "echo" command.
 212
 213 Q: What happens when a CPU is being logically offlined?
 214 A: The following happen, listed in no particular order :-)
 215
 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).
 230
 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"
 234
 235 Q: If i have some kernel code that needs to be aware of CPU arrival and
 236    departure, how to i arrange for proper notification?
 237 A: This is what you would need in your kernel code to receive notifications.
 238
 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;
 244
 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         }
 257
 258         static struct notifier_block __cpuinitdata foobar_cpu_notifer =
 259         {
 260            .notifier_call = foobar_cpu_callback,
 261         };
 262
 263 You need to call register_cpu_notifier() from your init function.
 264 Init functions could be of two types:
 265 1. early init (init function called when only the boot processor is online).
 266 2. late init (init function called _after_ all the CPUs are online).
 267
 268 For the first case, you should add the following to your init function
 269
 270         register_cpu_notifier(&foobar_cpu_notifier);
 271
 272 For the second case, you should add the following to your init function
 273
 274         register_hotcpu_notifier(&foobar_cpu_notifier);
 275
 276 You can fail PREPARE notifiers if something doesn't work to prepare resources.
 277 This will stop the activity and send a following CANCELED event back.
 278
 279 CPU_DEAD should not be failed, its just a goodness indication, but bad
 280 things will happen if a notifier in path sent a BAD notify code.
 281
 282 Q: I don't see my action being called for all CPUs already up and running?
 283 A: 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
 285
 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         }
 290
 291 Q: If i would like to develop cpu hotplug support for a new architecture,
 292    what do i need at a minimum?
 293 A: The following are what is required for CPU hotplug infrastructure to work
 294    correctly.
 295
 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.
 309
 310 Q: I need to ensure that a particular cpu is not removed when there is some
 311    work specific to this cpu is in progress.
 312 A: First switch the current thread context to preferred cpu
 313
 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;
 318
 319                 saved_mask = current->cpus_allowed;
 320                 cpu_set(cpu, new_mask);
 321                 err = set_cpus_allowed(current, new_mask);
 322
 323                 if (err)
 324                         return err;
 325
 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();
 332
 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                 }
 346
 347
 348 Q: How do we determine how many CPUs are available for hotplug.
 349 A: 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.
 353
 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.
 357
 358    Caveat: Today's ACPI MADT can only provide 256 entries since the apicid field
 359    in MADT is only 8 bits.
 360
 361 User Space Notification
 362
 363 Hotplug support for devices is common in Linux today. Its being used today to
 364 support automatic configuration of network, usb and pci devices. A hotplug
 365 event can be used to invoke an agent script to perform the configuration task.
 366
 367 You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
 368 scripts.
 369
 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
 378
 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