]>
Commit | Line | Data |
---|---|---|
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 <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_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. Its set in __cpu_up() | |
88 | after a cpu is available for kernel scheduling and ready to receive | |
89 | interrupts from devices. Its 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_mask(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_HOTPLUG, and 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 /sysfs | |
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 if offline and 1 when its 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 sent 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 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). | |
260 | ||
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" | |
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 __cpuinit 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 __cpuinitdata foobar_cpu_notifer = | |
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 | ||
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 | } | |
320 | ||
321 | Q: If i would like to develop cpu hotplug support for a new architecture, | |
322 | what do i need at a minimum? | |
323 | A: The following are what is required for CPU hotplug infrastructure to work | |
324 | correctly. | |
325 | ||
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. | |
339 | ||
340 | Q: I need to ensure that a particular cpu is not removed when there is some | |
341 | work specific to this cpu is in progress. | |
342 | A: 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: | |
345 | ||
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 | |
353 | #if NEEDS_BLOCKING | |
354 | err = work_on_cpu(cpu, __my_func_on_cpu, NULL); | |
355 | #else | |
356 | smp_call_function_single(cpu, __my_func_on_cpu, &err, | |
357 | true); | |
358 | #endif | |
359 | put_online_cpus(); | |
360 | return err; | |
361 | } | |
362 | ||
363 | Q: How do we determine how many CPUs are available for hotplug. | |
364 | A: 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. | |
368 | ||
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. | |
372 | ||
373 | Caveat: Today's ACPI MADT can only provide 256 entries since the apicid field | |
374 | in MADT is only 8 bits. | |
375 | ||
376 | User Space Notification | |
377 | ||
378 | Hotplug support for devices is common in Linux today. Its being used today to | |
379 | support automatic configuration of network, usb and pci devices. A hotplug | |
380 | event can be used to invoke an agent script to perform the configuration task. | |
381 | ||
382 | You can add /etc/hotplug/cpu.agent to handle hotplug notification user space | |
383 | scripts. | |
384 | ||
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 | |
393 | ||
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 |