blob: 9d620c153b04f2469bf664e6489e3af6817663e2 [file] [log] [blame]
Ashok Rajc8094062006-01-08 01:03:17 -08001 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>
Heiko Carstens255acee2006-02-17 13:52:46 -080014 s390:
15 Heiko Carstens <heiko.carstens@de.ibm.com>
Ashok Rajc8094062006-01-08 01:03:17 -080016
17Authors: Ashok Raj <ashok.raj@intel.com>
18Lots of feedback: Nathan Lynch <nathanl@austin.ibm.com>,
19 Joel Schopp <jschopp@austin.ibm.com>
20
21Introduction
22
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.
29
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.
34
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.
39
40General Stuff about CPU Hotplug
41--------------------------------
42
43Command Line Switches
44---------------------
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.
48
Satoru Takeuchica926e82006-10-19 23:29:06 -070049additional_cpus=n (*) Use this to limit hotpluggable cpus. This option sets
Heiko Carstens255acee2006-02-17 13:52:46 -080050 cpu_possible_map = cpu_present_map + additional_cpus
Ashok Raj8f8b1132006-02-16 14:01:48 -080051
Heiko Carstens6303dbf2006-02-20 18:27:58 -080052(*) Option valid only for following architectures
Mike Travis3b11ce72008-12-17 15:21:39 -080053- ia64
Heiko Carstens6303dbf2006-02-20 18:27:58 -080054
Mike Travis3b11ce72008-12-17 15:21:39 -080055ia64 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
61cpu_possible_map.
Ashok Raj8f8b1132006-02-16 14:01:48 -080062
Mike Travis3b11ce72008-12-17 15:21:39 -080063possible_cpus=n [s390,x86_64] use this to set hotpluggable cpus.
Heiko Carstens37a33022006-02-17 13:52:47 -080064 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.
Heiko Carstens37a33022006-02-17 13:52:47 -080067
Ashok Rajc8094062006-01-08 01:03:17 -080068CPU maps and such
69-----------------
70[More on cpumaps and primitive to manipulate, please check
71include/linux/cpumask.h that has more descriptive text.]
72
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.
80
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.
86
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.
93
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
KAMEZAWA Hiroyuki3c30a752006-03-28 01:56:39 -080096cpu_possible_map/for_each_possible_cpu() to iterate.
Ashok Rajc8094062006-01-08 01:03:17 -080097
98Never use anything other than cpumask_t to represent bitmap of CPUs.
99
Satoru Takeuchica926e82006-10-19 23:29:06 -0700100 #include <linux/cpumask.h>
Ashok Rajc8094062006-01-08 01:03:17 -0800101
Satoru Takeuchica926e82006-10-19 23:29:06 -0700102 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.
Ashok Rajc8094062006-01-08 01:03:17 -0800106
Satoru Takeuchica926e82006-10-19 23:29:06 -0700107 #include <linux/cpu.h>
Gautham R Shenoy86ef5c92008-01-25 21:08:02 +0100108 get_online_cpus() and put_online_cpus():
Ashok Rajc8094062006-01-08 01:03:17 -0800109
Gautham R Shenoy86ef5c92008-01-25 21:08:02 +0100110The 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
Ashok Rajc8094062006-01-08 01:03:17 -0800115function 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.
117
118CPU Hotplug - Frequently Asked Questions.
119
Satoru Takeuchica926e82006-10-19 23:29:06 -0700120Q: How to enable my kernel to support CPU hotplug?
Ashok Rajc8094062006-01-08 01:03:17 -0800121A: When doing make defconfig, Enable CPU hotplug support
122
123 "Processor type and Features" -> Support for Hotpluggable CPUs
124
125Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.
126
127You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
128as well.
129
130Q: What architectures support CPU hotplug?
131A: As of 2.6.14, the following architectures support CPU hotplug.
132
133i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
134
135Q: How to test if hotplug is supported on the newly built kernel?
136A: You should now notice an entry in sysfs.
137
138Check if sysfs is mounted, using the "mount" command. You should notice
139an entry as shown below in the output.
140
Satoru Takeuchica926e82006-10-19 23:29:06 -0700141 ....
142 none on /sys type sysfs (rw)
143 ....
Ashok Rajc8094062006-01-08 01:03:17 -0800144
Satoru Takeuchica926e82006-10-19 23:29:06 -0700145If this is not mounted, do the following.
Ashok Rajc8094062006-01-08 01:03:17 -0800146
Satoru Takeuchica926e82006-10-19 23:29:06 -0700147 #mkdir /sysfs
148 #mount -t sysfs sys /sys
Ashok Rajc8094062006-01-08 01:03:17 -0800149
Satoru Takeuchica926e82006-10-19 23:29:06 -0700150Now you should see entries for all present cpu, the following is an example
Ashok Rajc8094062006-01-08 01:03:17 -0800151in a 8-way system.
152
Satoru Takeuchica926e82006-10-19 23:29:06 -0700153 #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
Ashok Rajc8094062006-01-08 01:03:17 -0800167
168Under each directory you would find an "online" file which is the control
169file to logically online/offline a processor.
170
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.
Satoru Takeuchica926e82006-10-19 23:29:06 -0700173CONFIG_HOTPLUG_CPU enables logical online/offline capability in the kernel.
Ashok Rajc8094062006-01-08 01:03:17 -0800174To 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.
177
178Q: How do i logically offline a CPU?
179A: Do the following.
180
Satoru Takeuchica926e82006-10-19 23:29:06 -0700181 #echo 0 > /sys/devices/system/cpu/cpuX/online
Ashok Rajc8094062006-01-08 01:03:17 -0800182
Satoru Takeuchica926e82006-10-19 23:29:06 -0700183Once the logical offline is successful, check
Ashok Rajc8094062006-01-08 01:03:17 -0800184
Satoru Takeuchica926e82006-10-19 23:29:06 -0700185 #cat /proc/interrupts
Ashok Rajc8094062006-01-08 01:03:17 -0800186
Satoru Takeuchica926e82006-10-19 23:29:06 -0700187You should now not see the CPU that you removed. Also online file will report
Ashok Rajc8094062006-01-08 01:03:17 -0800188the state as 0 when a cpu if offline and 1 when its online.
189
Satoru Takeuchica926e82006-10-19 23:29:06 -0700190 #To display the current cpu state.
191 #cat /sys/devices/system/cpu/cpuX/online
Ashok Rajc8094062006-01-08 01:03:17 -0800192
193Q: Why cant i remove CPU0 on some systems?
194A: Some architectures may have some special dependency on a certain CPU.
195
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.
201
202In such cases you will also notice that the online file is missing under cpu0.
203
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.
208
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.
212
213Q: What happens when a CPU is being logically offlined?
214A: The following happen, listed in no particular order :-)
215
216- A notification is sent to in-kernel registered modules by sending an event
Rafael J. Wysocki8bb78442007-05-09 02:35:10 -0700217 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
Cliff Wickman470fd642007-10-18 23:40:46 -0700220- 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.
Ashok Rajc8094062006-01-08 01:03:17 -0800223- 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
Rafael J. Wysocki8bb78442007-05-09 02:35:10 -0700228 CPU_DEAD (or CPU_DEAD_FROZEN if tasks are frozen due to a suspend while the
229 CPU is being offlined).
Ashok Rajc8094062006-01-08 01:03:17 -0800230
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
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.
238
Satoru Takeuchica926e82006-10-19 23:29:06 -0700239 #include <linux/cpu.h>
240 static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb,
Ashok Rajc8094062006-01-08 01:03:17 -0800241 unsigned long action, void *hcpu)
242 {
243 unsigned int cpu = (unsigned long)hcpu;
244
245 switch (action) {
246 case CPU_ONLINE:
Rafael J. Wysocki8bb78442007-05-09 02:35:10 -0700247 case CPU_ONLINE_FROZEN:
Ashok Rajc8094062006-01-08 01:03:17 -0800248 foobar_online_action(cpu);
249 break;
250 case CPU_DEAD:
Rafael J. Wysocki8bb78442007-05-09 02:35:10 -0700251 case CPU_DEAD_FROZEN:
Ashok Rajc8094062006-01-08 01:03:17 -0800252 foobar_dead_action(cpu);
253 break;
254 }
255 return NOTIFY_OK;
256 }
257
Chandra Seetharaman7c7165c2006-07-30 03:03:36 -0700258 static struct notifier_block __cpuinitdata foobar_cpu_notifer =
Ashok Rajc8094062006-01-08 01:03:17 -0800259 {
260 .notifier_call = foobar_cpu_callback,
261 };
262
Chandra Seetharaman7c7165c2006-07-30 03:03:36 -0700263You 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).
Ashok Rajc8094062006-01-08 01:03:17 -0800267
Chandra Seetharaman7c7165c2006-07-30 03:03:36 -0700268For the first case, you should add the following to your init function
Ashok Rajc8094062006-01-08 01:03:17 -0800269
270 register_cpu_notifier(&foobar_cpu_notifier);
271
Chandra Seetharaman7c7165c2006-07-30 03:03:36 -0700272For the second case, you should add the following to your init function
273
274 register_hotcpu_notifier(&foobar_cpu_notifier);
275
Ashok Rajc8094062006-01-08 01:03:17 -0800276You can fail PREPARE notifiers if something doesn't work to prepare resources.
277This will stop the activity and send a following CANCELED event back.
278
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.
281
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
285
Satoru Takeuchica926e82006-10-19 23:29:06 -0700286 for_each_online_cpu(i) {
Ashok Rajc8094062006-01-08 01:03:17 -0800287 foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
Satoru Takeuchica926e82006-10-19 23:29:06 -0700288 foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, i);
289 }
Ashok Rajc8094062006-01-08 01:03:17 -0800290
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.
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
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
313
Satoru Takeuchica926e82006-10-19 23:29:06 -0700314 int my_func_on_cpu(int cpu)
315 {
316 cpumask_t saved_mask, new_mask = CPU_MASK_NONE;
317 int curr_cpu, err = 0;
Ashok Rajc8094062006-01-08 01:03:17 -0800318
Satoru Takeuchica926e82006-10-19 23:29:06 -0700319 saved_mask = current->cpus_allowed;
320 cpu_set(cpu, new_mask);
321 err = set_cpus_allowed(current, new_mask);
Ashok Rajc8094062006-01-08 01:03:17 -0800322
Satoru Takeuchica926e82006-10-19 23:29:06 -0700323 if (err)
324 return err;
Ashok Rajc8094062006-01-08 01:03:17 -0800325
Satoru Takeuchica926e82006-10-19 23:29:06 -0700326 /*
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();
Ashok Rajc8094062006-01-08 01:03:17 -0800332
Satoru Takeuchica926e82006-10-19 23:29:06 -0700333 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 }
Ashok Rajc8094062006-01-08 01:03:17 -0800346
347
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.
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
361User Space Notification
362
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.
366
367You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
368scripts.
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