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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: How do i find out if a particular CPU is not removable? |
211 | A: Depending on the implementation, some architectures may show this by the |
212 | absence of the "online" file. This is done if it can be determined ahead of |
213 | time that this CPU cannot be removed. |
214 | |
215 | In some situations, this can be a run time check, i.e if you try to remove the |
216 | last CPU, this will not be permitted. You can find such failures by |
217 | investigating the return value of the "echo" command. |
218 | |
219 | Q: What happens when a CPU is being logically offlined? |
220 | A: The following happen, listed in no particular order :-) |
221 | |
222 | - A notification is sent to in-kernel registered modules by sending an event |
223 | CPU_DOWN_PREPARE or CPU_DOWN_PREPARE_FROZEN, depending on whether or not the |
224 | CPU is being offlined while tasks are frozen due to a suspend operation in |
225 | progress |
226 | - All processes are migrated away from this outgoing CPU to new CPUs. |
227 | The new CPU is chosen from each process' current cpuset, which may be |
228 | a subset of all online CPUs. |
229 | - All interrupts targeted to this CPU is migrated to a new CPU |
230 | - timers/bottom half/task lets are also migrated to a new CPU |
231 | - Once all services are migrated, kernel calls an arch specific routine |
232 | __cpu_disable() to perform arch specific cleanup. |
233 | - Once this is successful, an event for successful cleanup is sent by an event |
234 | CPU_DEAD (or CPU_DEAD_FROZEN if tasks are frozen due to a suspend while the |
235 | CPU is being offlined). |
236 | |
237 | "It is expected that each service cleans up when the CPU_DOWN_PREPARE |
238 | notifier is called, when CPU_DEAD is called its expected there is nothing |
239 | running on behalf of this CPU that was offlined" |
240 | |
241 | Q: If i have some kernel code that needs to be aware of CPU arrival and |
242 | departure, how to i arrange for proper notification? |
243 | A: This is what you would need in your kernel code to receive notifications. |
244 | |
245 | #include <linux/cpu.h> |
246 | static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb, |
247 | unsigned long action, void *hcpu) |
248 | { |
249 | unsigned int cpu = (unsigned long)hcpu; |
250 | |
251 | switch (action) { |
252 | case CPU_ONLINE: |
253 | case CPU_ONLINE_FROZEN: |
254 | foobar_online_action(cpu); |
255 | break; |
256 | case CPU_DEAD: |
257 | case CPU_DEAD_FROZEN: |
258 | foobar_dead_action(cpu); |
259 | break; |
260 | } |
261 | return NOTIFY_OK; |
262 | } |
263 | |
264 | static struct notifier_block __cpuinitdata foobar_cpu_notifer = |
265 | { |
266 | .notifier_call = foobar_cpu_callback, |
267 | }; |
268 | |
269 | You need to call register_cpu_notifier() from your init function. |
270 | Init functions could be of two types: |
271 | 1. early init (init function called when only the boot processor is online). |
272 | 2. late init (init function called _after_ all the CPUs are online). |
273 | |
274 | For the first case, you should add the following to your init function |
275 | |
276 | register_cpu_notifier(&foobar_cpu_notifier); |
277 | |
278 | For the second case, you should add the following to your init function |
279 | |
280 | register_hotcpu_notifier(&foobar_cpu_notifier); |
281 | |
282 | You can fail PREPARE notifiers if something doesn't work to prepare resources. |
283 | This will stop the activity and send a following CANCELED event back. |
284 | |
285 | CPU_DEAD should not be failed, its just a goodness indication, but bad |
286 | things will happen if a notifier in path sent a BAD notify code. |
287 | |
288 | Q: I don't see my action being called for all CPUs already up and running? |
289 | A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined. |
290 | If you need to perform some action for each cpu already in the system, then |
291 | |
292 | for_each_online_cpu(i) { |
293 | foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i); |
294 | foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, i); |
295 | } |
296 | |
297 | Q: If i would like to develop cpu hotplug support for a new architecture, |
298 | what do i need at a minimum? |
299 | A: The following are what is required for CPU hotplug infrastructure to work |
300 | correctly. |
301 | |
302 | - Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU |
303 | - __cpu_up() - Arch interface to bring up a CPU |
304 | - __cpu_disable() - Arch interface to shutdown a CPU, no more interrupts |
305 | can be handled by the kernel after the routine |
306 | returns. Including local APIC timers etc are |
307 | shutdown. |
308 | - __cpu_die() - This actually supposed to ensure death of the CPU. |
309 | Actually look at some example code in other arch |
310 | that implement CPU hotplug. The processor is taken |
311 | down from the idle() loop for that specific |
312 | architecture. __cpu_die() typically waits for some |
313 | per_cpu state to be set, to ensure the processor |
314 | dead routine is called to be sure positively. |
315 | |
316 | Q: I need to ensure that a particular cpu is not removed when there is some |
317 | work specific to this cpu is in progress. |
318 | A: There are two ways. If your code can be run in interrupt context, use |
319 | smp_call_function_single(), otherwise use work_on_cpu(). Note that |
320 | work_on_cpu() is slow, and can fail due to out of memory: |
321 | |
322 | int my_func_on_cpu(int cpu) |
323 | { |
324 | int err; |
325 | get_online_cpus(); |
326 | if (!cpu_online(cpu)) |
327 | err = -EINVAL; |
328 | else |
329 | #if NEEDS_BLOCKING |
330 | err = work_on_cpu(cpu, __my_func_on_cpu, NULL); |
331 | #else |
332 | smp_call_function_single(cpu, __my_func_on_cpu, &err, |
333 | true); |
334 | #endif |
335 | put_online_cpus(); |
336 | return err; |
337 | } |
338 | |
339 | Q: How do we determine how many CPUs are available for hotplug. |
340 | A: There is no clear spec defined way from ACPI that can give us that |
341 | information today. Based on some input from Natalie of Unisys, |
342 | that the ACPI MADT (Multiple APIC Description Tables) marks those possible |
343 | CPUs in a system with disabled status. |
344 | |
345 | Andi implemented some simple heuristics that count the number of disabled |
346 | CPUs in MADT as hotpluggable CPUS. In the case there are no disabled CPUS |
347 | we assume 1/2 the number of CPUs currently present can be hotplugged. |
348 | |
349 | Caveat: Today's ACPI MADT can only provide 256 entries since the apicid field |
350 | in MADT is only 8 bits. |
351 | |
352 | User Space Notification |
353 | |
354 | Hotplug support for devices is common in Linux today. Its being used today to |
355 | support automatic configuration of network, usb and pci devices. A hotplug |
356 | event can be used to invoke an agent script to perform the configuration task. |
357 | |
358 | You can add /etc/hotplug/cpu.agent to handle hotplug notification user space |
359 | scripts. |
360 | |
361 | #!/bin/bash |
362 | # $Id: cpu.agent |
363 | # Kernel hotplug params include: |
364 | #ACTION=%s [online or offline] |
365 | #DEVPATH=%s |
366 | # |
367 | cd /etc/hotplug |
368 | . ./hotplug.functions |
369 | |
370 | case $ACTION in |
371 | online) |
372 | echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt |
373 | ;; |
374 | offline) |
375 | echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt |
376 | ;; |
377 | *) |
378 | debug_mesg CPU $ACTION event not supported |
379 | exit 1 |
380 | ;; |
381 | esac |
382 |
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