Version:  2.0.40 2.2.26 2.4.37 3.13 3.14 3.15 3.16 3.17 3.18 3.19 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10

Linux/Documentation/cpu-hotplug.txt

  1                 CPU hotplug Support in Linux(tm) Kernel
  2 
  3                 Maintainers:
  4                 CPU Hotplug Core:
  5                         Rusty Russell <rusty@rustcorp.com.au>
  6                         Srivatsa Vaddagiri <vatsa@in.ibm.com>
  7                 i386:
  8                         Zwane Mwaikambo <zwanem@gmail.com>
  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_mask = cpu_present_mask + additional_cpus
 51 
 52 cede_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".
 57 
 58 (*) Option valid only for following architectures
 59 - ia64
 60 
 61 ia64 uses the number of disabled local apics in ACPI tables MADT to
 62 determine the number of potentially hot-pluggable cpus. The implementation
 63 should only rely on this to count the # of cpus, but *MUST* not rely
 64 on the apicid values in those tables for disabled apics. In the event
 65 BIOS doesn't mark such hot-pluggable cpus as disabled entries, one could
 66 use this parameter "additional_cpus=x" to represent those cpus in the
 67 cpu_possible_mask.
 68 
 69 possible_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.
 73 
 74 CPU maps and such
 75 -----------------
 76 [More on cpumaps and primitive to manipulate, please check
 77 include/linux/cpumask.h that has more descriptive text.]
 78 
 79 cpu_possible_mask: Bitmap of possible CPUs that can ever be available in the
 80 system. This is used to allocate some boot time memory for per_cpu variables
 81 that aren't designed to grow/shrink as CPUs are made available or removed.
 82 Once set during boot time discovery phase, the map is static, i.e no bits
 83 are added or removed anytime.  Trimming it accurately for your system needs
 84 upfront can save some boot time memory. See below for how we use heuristics
 85 in x86_64 case to keep this under check.
 86 
 87 cpu_online_mask: Bitmap of all CPUs currently online. It's set in __cpu_up()
 88 after a CPU is available for kernel scheduling and ready to receive
 89 interrupts from devices. It's cleared when a CPU is brought down using
 90 __cpu_disable(), before which all OS services including interrupts are
 91 migrated to another target CPU.
 92 
 93 cpu_present_mask: Bitmap of CPUs currently present in the system. Not all
 94 of them may be online. When physical hotplug is processed by the relevant
 95 subsystem (e.g ACPI) can change and new bit either be added or removed
 96 from the map depending on the event is hot-add/hot-remove. There are currently
 97 no locking rules as of now. Typical usage is to init topology during boot,
 98 at which time hotplug is disabled.
 99 
100 You really dont need to manipulate any of the system cpu maps. They should
101 be read-only for most use. When setting up per-cpu resources almost always use
102 cpu_possible_mask/for_each_possible_cpu() to iterate.
103 
104 Never use anything other than cpumask_t to represent bitmap of CPUs.
105 
106         #include <linux/cpumask.h>
107 
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(x,mask)      - Iterate over some random collection of cpu mask.
112 
113         #include <linux/cpu.h>
114         get_online_cpus() and put_online_cpus():
115 
116 The above calls are used to inhibit cpu hotplug operations. While the
117 cpu_hotplug.refcount is non zero, the cpu_online_mask will not change.
118 If you merely need to avoid cpus going away, you could also use
119 preempt_disable() and preempt_enable() for those sections.
120 Just remember the critical section cannot call any
121 function that can sleep or schedule this process away. The preempt_disable()
122 will work as long as stop_machine_run() is used to take a cpu down.
123 
124 CPU Hotplug - Frequently Asked Questions.
125 
126 Q: How to enable my kernel to support CPU hotplug?
127 A: When doing make defconfig, Enable CPU hotplug support
128 
129    "Processor type and Features" -> Support for Hotpluggable CPUs
130 
131 Make sure that you have CONFIG_SMP turned on as well.
132 
133 You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
134 as well.
135 
136 Q: What architectures support CPU hotplug?
137 A: As of 2.6.14, the following architectures support CPU hotplug.
138 
139 i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
140 
141 Q: How to test if hotplug is supported on the newly built kernel?
142 A: You should now notice an entry in sysfs.
143 
144 Check if sysfs is mounted, using the "mount" command. You should notice
145 an entry as shown below in the output.
146 
147         ....
148         none on /sys type sysfs (rw)
149         ....
150 
151 If this is not mounted, do the following.
152 
153         #mkdir /sys
154         #mount -t sysfs sys /sys
155 
156 Now you should see entries for all present cpu, the following is an example
157 in a 8-way system.
158 
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
173 
174 Under each directory you would find an "online" file which is the control
175 file to logically online/offline a processor.
176 
177 Q: Does hot-add/hot-remove refer to physical add/remove of cpus?
178 A: The usage of hot-add/remove may not be very consistently used in the code.
179 CONFIG_HOTPLUG_CPU enables logical online/offline capability in the kernel.
180 To support physical addition/removal, one would need some BIOS hooks and
181 the platform should have something like an attention button in PCI hotplug.
182 CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.
183 
184 Q: How do I logically offline a CPU?
185 A: Do the following.
186 
187         #echo 0 > /sys/devices/system/cpu/cpuX/online
188 
189 Once the logical offline is successful, check
190 
191         #cat /proc/interrupts
192 
193 You should now not see the CPU that you removed. Also online file will report
194 the state as 0 when a CPU is offline and 1 when it's online.
195 
196         #To display the current cpu state.
197         #cat /sys/devices/system/cpu/cpuX/online
198 
199 Q: Why can't I remove CPU0 on some systems?
200 A: Some architectures may have some special dependency on a certain CPU.
201 
202 For e.g in IA64 platforms we have ability to send platform interrupts to the
203 OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
204 specifications, we didn't have a way to change the target CPU. Hence if the
205 current ACPI version doesn't support such re-direction, we disable that CPU
206 by making it not-removable.
207 
208 In such cases you will also notice that the online file is missing under cpu0.
209 
210 Q: Is CPU0 removable on X86?
211 A: Yes. If kernel is compiled with CONFIG_BOOTPARAM_HOTPLUG_CPU0=y, CPU0 is
212 removable by default. Otherwise, CPU0 is also removable by kernel option
213 cpu0_hotplug.
214 
215 But some features depend on CPU0. Two known dependencies are:
216 
217 1. Resume from hibernate/suspend depends on CPU0. Hibernate/suspend will fail if
218 CPU0 is offline and you need to online CPU0 before hibernate/suspend can
219 continue.
220 2. PIC interrupts also depend on CPU0. CPU0 can't be removed if a PIC interrupt
221 is detected.
222 
223 It's said poweroff/reboot may depend on CPU0 on some machines although I haven't
224 seen any poweroff/reboot failure so far after CPU0 is offline on a few tested
225 machines.
226 
227 Please let me know if you know or see any other dependencies of CPU0.
228 
229 If the dependencies are under your control, you can turn on CPU0 hotplug feature
230 either by CONFIG_BOOTPARAM_HOTPLUG_CPU0 or by kernel parameter cpu0_hotplug.
231 
232 --Fenghua Yu <fenghua.yu@intel.com>
233 
234 Q: How do I find out if a particular CPU is not removable?
235 A: Depending on the implementation, some architectures may show this by the
236 absence of the "online" file. This is done if it can be determined ahead of
237 time that this CPU cannot be removed.
238 
239 In some situations, this can be a run time check, i.e if you try to remove the
240 last CPU, this will not be permitted. You can find such failures by
241 investigating the return value of the "echo" command.
242 
243 Q: What happens when a CPU is being logically offlined?
244 A: The following happen, listed in no particular order :-)
245 
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 are 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).
260 
261   "It is expected that each service cleans up when the CPU_DOWN_PREPARE
262   notifier is called, when CPU_DEAD is called it's expected there is nothing
263   running on behalf of this CPU that was offlined"
264 
265 Q: If I have some kernel code that needs to be aware of CPU arrival and
266    departure, how to i arrange for proper notification?
267 A: This is what you would need in your kernel code to receive notifications.
268 
269         #include <linux/cpu.h>
270         static int foobar_cpu_callback(struct notifier_block *nfb,
271                                        unsigned long action, void *hcpu)
272         {
273                 unsigned int cpu = (unsigned long)hcpu;
274 
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         }
287 
288         static struct notifier_block foobar_cpu_notifier =
289         {
290            .notifier_call = foobar_cpu_callback,
291         };
292 
293 You need to call register_cpu_notifier() from your init function.
294 Init functions could be of two types:
295 1. early init (init function called when only the boot processor is online).
296 2. late init (init function called _after_ all the CPUs are online).
297 
298 For the first case, you should add the following to your init function
299 
300         register_cpu_notifier(&foobar_cpu_notifier);
301 
302 For the second case, you should add the following to your init function
303 
304         register_hotcpu_notifier(&foobar_cpu_notifier);
305 
306 You can fail PREPARE notifiers if something doesn't work to prepare resources.
307 This will stop the activity and send a following CANCELED event back.
308 
309 CPU_DEAD should not be failed, its just a goodness indication, but bad
310 things will happen if a notifier in path sent a BAD notify code.
311 
312 Q: I don't see my action being called for all CPUs already up and running?
313 A: 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
315    do this:
316 
317         for_each_online_cpu(i) {
318                 foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
319                 foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, i);
320         }
321 
322    However, if you want to register a hotplug callback, as well as perform
323    some initialization for CPUs that are already online, then do this:
324 
325    Version 1: (Correct)
326    ---------
327 
328         cpu_notifier_register_begin();
329 
330                 for_each_online_cpu(i) {
331                         foobar_cpu_callback(&foobar_cpu_notifier,
332                                             CPU_UP_PREPARE, i);
333                         foobar_cpu_callback(&foobar_cpu_notifier,
334                                             CPU_ONLINE, i);
335                 }
336 
337         /* Note the use of the double underscored version of the API */
338         __register_cpu_notifier(&foobar_cpu_notifier);
339 
340         cpu_notifier_register_done();
341 
342    Note that the following code is *NOT* the right way to achieve this,
343    because it is prone to an ABBA deadlock between the cpu_add_remove_lock
344    and the cpu_hotplug.lock.
345 
346    Version 2: (Wrong!)
347    ---------
348 
349         get_online_cpus();
350 
351                 for_each_online_cpu(i) {
352                         foobar_cpu_callback(&foobar_cpu_notifier,
353                                             CPU_UP_PREPARE, i);
354                         foobar_cpu_callback(&foobar_cpu_notifier,
355                                             CPU_ONLINE, i);
356                 }
357 
358         register_cpu_notifier(&foobar_cpu_notifier);
359 
360         put_online_cpus();
361 
362     So always use the first version shown above when you want to register
363     callbacks as well as initialize the already online CPUs.
364 
365 
366 Q: If I would like to develop CPU hotplug support for a new architecture,
367    what do I need at a minimum?
368 A: The following are what is required for CPU hotplug infrastructure to work
369    correctly.
370 
371     - Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU
372     - __cpu_up()        - Arch interface to bring up a CPU
373     - __cpu_disable()   - Arch interface to shutdown a CPU, no more interrupts
374                           can be handled by the kernel after the routine
375                           returns. Including local APIC timers etc are
376                           shutdown.
377      - __cpu_die()      - This actually supposed to ensure death of the CPU.
378                           Actually look at some example code in other arch
379                           that implement CPU hotplug. The processor is taken
380                           down from the idle() loop for that specific
381                           architecture. __cpu_die() typically waits for some
382                           per_cpu state to be set, to ensure the processor
383                           dead routine is called to be sure positively.
384 
385 Q: I need to ensure that a particular CPU is not removed when there is some
386    work specific to this CPU in progress.
387 A: There are two ways.  If your code can be run in interrupt context, use
388    smp_call_function_single(), otherwise use work_on_cpu().  Note that
389    work_on_cpu() is slow, and can fail due to out of memory:
390 
391         int my_func_on_cpu(int cpu)
392         {
393                 int err;
394                 get_online_cpus();
395                 if (!cpu_online(cpu))
396                         err = -EINVAL;
397                 else
398 #if NEEDS_BLOCKING
399                         err = work_on_cpu(cpu, __my_func_on_cpu, NULL);
400 #else
401                         smp_call_function_single(cpu, __my_func_on_cpu, &err,
402                                                  true);
403 #endif
404                 put_online_cpus();
405                 return err;
406         }
407 
408 Q: How do we determine how many CPUs are available for hotplug.
409 A: There is no clear spec defined way from ACPI that can give us that
410    information today. Based on some input from Natalie of Unisys,
411    that the ACPI MADT (Multiple APIC Description Tables) marks those possible
412    CPUs in a system with disabled status.
413 
414    Andi implemented some simple heuristics that count the number of disabled
415    CPUs in MADT as hotpluggable CPUS.  In the case there are no disabled CPUS
416    we assume 1/2 the number of CPUs currently present can be hotplugged.
417 
418    Caveat: ACPI MADT can only provide 256 entries in systems with only ACPI 2.0c
419    or earlier ACPI version supported, because the apicid field in MADT is only
420    8 bits. From ACPI 3.0, this limitation was removed since the apicid field
421    was extended to 32 bits with x2APIC introduced.
422 
423 User Space Notification
424 
425 Hotplug support for devices is common in Linux today. Its being used today to
426 support automatic configuration of network, usb and pci devices. A hotplug
427 event can be used to invoke an agent script to perform the configuration task.
428 
429 You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
430 scripts.
431 
432         #!/bin/bash
433         # $Id: cpu.agent
434         # Kernel hotplug params include:
435         #ACTION=%s [online or offline]
436         #DEVPATH=%s
437         #
438         cd /etc/hotplug
439         . ./hotplug.functions
440 
441         case $ACTION in
442                 online)
443                         echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt
444                         ;;
445                 offline)
446                         echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt
447                         ;;
448                 *)
449                         debug_mesg CPU $ACTION event not supported
450         exit 1
451         ;;
452         esac

This page was automatically generated by LXR 0.3.1 (source).  •  Linux is a registered trademark of Linus Torvalds  •  Contact us