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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
Heiko Carstens6303dbf2006-02-20 18:27:58 -080049additional_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 Raj8f8b11382006-02-16 14:01:48 -080051
Heiko Carstens6303dbf2006-02-20 18:27:58 -080052(*) Option valid only for following architectures
53- x86_64, ia64, s390
54
Ashok Raj8f8b11382006-02-16 14:01:48 -080055ia64 and x86_64 use the number of disabled local apics in ACPI tables MADT
56to determine the number of potentially hot-pluggable cpus. The implementation
57should only rely on this to count the #of cpus, but *MUST* not rely on the
58apicid values in those tables for disabled apics. In the event BIOS doesnt
59mark such hot-pluggable cpus as disabled entries, one could use this
60parameter "additional_cpus=x" to represent those cpus in the cpu_possible_map.
61
Heiko Carstens6303dbf2006-02-20 18:27:58 -080062s390 uses the number of cpus it detects at IPL time to also the number of bits
63in cpu_possible_map. If it is desired to add additional cpus at a later time
64the number should be specified using this option or the possible_cpus option.
Ashok Raj8f8b11382006-02-16 14:01:48 -080065
Heiko Carstens37a33022006-02-17 13:52:47 -080066possible_cpus=n [s390 only] use this to set hotpluggable cpus.
67 This option sets possible_cpus bits in
68 cpu_possible_map. Thus keeping the numbers of bits set
69 constant even if the machine gets rebooted.
70 This option overrides additional_cpus.
71
Ashok Rajc8094062006-01-08 01:03:17 -080072CPU maps and such
73-----------------
74[More on cpumaps and primitive to manipulate, please check
75include/linux/cpumask.h that has more descriptive text.]
76
77cpu_possible_map: Bitmap of possible CPUs that can ever be available in the
78system. This is used to allocate some boot time memory for per_cpu variables
79that aren't designed to grow/shrink as CPUs are made available or removed.
80Once set during boot time discovery phase, the map is static, i.e no bits
81are added or removed anytime. Trimming it accurately for your system needs
82upfront can save some boot time memory. See below for how we use heuristics
83in x86_64 case to keep this under check.
84
85cpu_online_map: Bitmap of all CPUs currently online. Its set in __cpu_up()
86after a cpu is available for kernel scheduling and ready to receive
87interrupts from devices. Its cleared when a cpu is brought down using
88__cpu_disable(), before which all OS services including interrupts are
89migrated to another target CPU.
90
91cpu_present_map: Bitmap of CPUs currently present in the system. Not all
92of them may be online. When physical hotplug is processed by the relevant
93subsystem (e.g ACPI) can change and new bit either be added or removed
94from the map depending on the event is hot-add/hot-remove. There are currently
95no locking rules as of now. Typical usage is to init topology during boot,
96at which time hotplug is disabled.
97
98You really dont need to manipulate any of the system cpu maps. They should
99be read-only for most use. When setting up per-cpu resources almost always use
100cpu_possible_map/for_each_cpu() to iterate.
101
102Never use anything other than cpumask_t to represent bitmap of CPUs.
103
104#include <linux/cpumask.h>
105
106for_each_cpu - Iterate over cpu_possible_map
107for_each_online_cpu - Iterate over cpu_online_map
108for_each_present_cpu - Iterate over cpu_present_map
109for_each_cpu_mask(x,mask) - Iterate over some random collection of cpu mask.
110
111#include <linux/cpu.h>
112lock_cpu_hotplug() and unlock_cpu_hotplug():
113
114The above calls are used to inhibit cpu hotplug operations. While holding the
115cpucontrol mutex, cpu_online_map will not change. If you merely need to avoid
116cpus going away, you could also use preempt_disable() and preempt_enable()
117for those sections. Just remember the critical section cannot call any
118function that can sleep or schedule this process away. The preempt_disable()
119will work as long as stop_machine_run() is used to take a cpu down.
120
121CPU Hotplug - Frequently Asked Questions.
122
123Q: How to i enable my kernel to support CPU hotplug?
124A: When doing make defconfig, Enable CPU hotplug support
125
126 "Processor type and Features" -> Support for Hotpluggable CPUs
127
128Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.
129
130You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
131as well.
132
133Q: What architectures support CPU hotplug?
134A: As of 2.6.14, the following architectures support CPU hotplug.
135
136i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
137
138Q: How to test if hotplug is supported on the newly built kernel?
139A: You should now notice an entry in sysfs.
140
141Check if sysfs is mounted, using the "mount" command. You should notice
142an entry as shown below in the output.
143
144....
145none on /sys type sysfs (rw)
146....
147
148if this is not mounted, do the following.
149
150#mkdir /sysfs
151#mount -t sysfs sys /sys
152
153now you should see entries for all present cpu, the following is an example
154in a 8-way system.
155
156#pwd
157#/sys/devices/system/cpu
158#ls -l
159total 0
160drwxr-xr-x 10 root root 0 Sep 19 07:44 .
161drwxr-xr-x 13 root root 0 Sep 19 07:45 ..
162drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu0
163drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu1
164drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu2
165drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu3
166drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu4
167drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu5
168drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu6
169drwxr-xr-x 3 root root 0 Sep 19 07:48 cpu7
170
171Under each directory you would find an "online" file which is the control
172file to logically online/offline a processor.
173
174Q: Does hot-add/hot-remove refer to physical add/remove of cpus?
175A: The usage of hot-add/remove may not be very consistently used in the code.
176CONFIG_CPU_HOTPLUG enables logical online/offline capability in the kernel.
177To support physical addition/removal, one would need some BIOS hooks and
178the platform should have something like an attention button in PCI hotplug.
179CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.
180
181Q: How do i logically offline a CPU?
182A: Do the following.
183
184#echo 0 > /sys/devices/system/cpu/cpuX/online
185
186once the logical offline is successful, check
187
188#cat /proc/interrupts
189
190you should now not see the CPU that you removed. Also online file will report
191the state as 0 when a cpu if offline and 1 when its online.
192
193#To display the current cpu state.
194#cat /sys/devices/system/cpu/cpuX/online
195
196Q: Why cant i remove CPU0 on some systems?
197A: Some architectures may have some special dependency on a certain CPU.
198
199For e.g in IA64 platforms we have ability to sent platform interrupts to the
200OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
201specifications, we didn't have a way to change the target CPU. Hence if the
202current ACPI version doesn't support such re-direction, we disable that CPU
203by making it not-removable.
204
205In such cases you will also notice that the online file is missing under cpu0.
206
207Q: How do i find out if a particular CPU is not removable?
208A: Depending on the implementation, some architectures may show this by the
209absence of the "online" file. This is done if it can be determined ahead of
210time that this CPU cannot be removed.
211
212In some situations, this can be a run time check, i.e if you try to remove the
213last CPU, this will not be permitted. You can find such failures by
214investigating the return value of the "echo" command.
215
216Q: What happens when a CPU is being logically offlined?
217A: The following happen, listed in no particular order :-)
218
219- A notification is sent to in-kernel registered modules by sending an event
220 CPU_DOWN_PREPARE
221- All process is migrated away from this outgoing CPU to a new CPU
222- All interrupts targeted to this CPU is migrated to a new CPU
223- timers/bottom half/task lets are also migrated to a new CPU
224- Once all services are migrated, kernel calls an arch specific routine
225 __cpu_disable() to perform arch specific cleanup.
226- Once this is successful, an event for successful cleanup is sent by an event
227 CPU_DEAD.
228
229 "It is expected that each service cleans up when the CPU_DOWN_PREPARE
230 notifier is called, when CPU_DEAD is called its expected there is nothing
231 running on behalf of this CPU that was offlined"
232
233Q: If i have some kernel code that needs to be aware of CPU arrival and
234 departure, how to i arrange for proper notification?
235A: This is what you would need in your kernel code to receive notifications.
236
237 #include <linux/cpu.h>
238 static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb,
239 unsigned long action, void *hcpu)
240 {
241 unsigned int cpu = (unsigned long)hcpu;
242
243 switch (action) {
244 case CPU_ONLINE:
245 foobar_online_action(cpu);
246 break;
247 case CPU_DEAD:
248 foobar_dead_action(cpu);
249 break;
250 }
251 return NOTIFY_OK;
252 }
253
254 static struct notifier_block foobar_cpu_notifer =
255 {
256 .notifier_call = foobar_cpu_callback,
257 };
258
259
260In your init function,
261
262 register_cpu_notifier(&foobar_cpu_notifier);
263
264You can fail PREPARE notifiers if something doesn't work to prepare resources.
265This will stop the activity and send a following CANCELED event back.
266
267CPU_DEAD should not be failed, its just a goodness indication, but bad
268things will happen if a notifier in path sent a BAD notify code.
269
270Q: I don't see my action being called for all CPUs already up and running?
271A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined.
272 If you need to perform some action for each cpu already in the system, then
273
274 for_each_online_cpu(i) {
275 foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
276 foobar_cpu_callback(&foobar-cpu_notifier, CPU_ONLINE, i);
277 }
278
279Q: If i would like to develop cpu hotplug support for a new architecture,
280 what do i need at a minimum?
281A: The following are what is required for CPU hotplug infrastructure to work
282 correctly.
283
284 - Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU
285 - __cpu_up() - Arch interface to bring up a CPU
286 - __cpu_disable() - Arch interface to shutdown a CPU, no more interrupts
287 can be handled by the kernel after the routine
288 returns. Including local APIC timers etc are
289 shutdown.
290 - __cpu_die() - This actually supposed to ensure death of the CPU.
291 Actually look at some example code in other arch
292 that implement CPU hotplug. The processor is taken
293 down from the idle() loop for that specific
294 architecture. __cpu_die() typically waits for some
295 per_cpu state to be set, to ensure the processor
296 dead routine is called to be sure positively.
297
298Q: I need to ensure that a particular cpu is not removed when there is some
299 work specific to this cpu is in progress.
300A: First switch the current thread context to preferred cpu
301
302 int my_func_on_cpu(int cpu)
303 {
304 cpumask_t saved_mask, new_mask = CPU_MASK_NONE;
305 int curr_cpu, err = 0;
306
307 saved_mask = current->cpus_allowed;
308 cpu_set(cpu, new_mask);
309 err = set_cpus_allowed(current, new_mask);
310
311 if (err)
312 return err;
313
314 /*
315 * If we got scheduled out just after the return from
316 * set_cpus_allowed() before running the work, this ensures
317 * we stay locked.
318 */
319 curr_cpu = get_cpu();
320
321 if (curr_cpu != cpu) {
322 err = -EAGAIN;
323 goto ret;
324 } else {
325 /*
326 * Do work : But cant sleep, since get_cpu() disables preempt
327 */
328 }
329 ret:
330 put_cpu();
331 set_cpus_allowed(current, saved_mask);
332 return err;
333 }
334
335
336Q: How do we determine how many CPUs are available for hotplug.
337A: There is no clear spec defined way from ACPI that can give us that
338 information today. Based on some input from Natalie of Unisys,
339 that the ACPI MADT (Multiple APIC Description Tables) marks those possible
340 CPUs in a system with disabled status.
341
342 Andi implemented some simple heuristics that count the number of disabled
343 CPUs in MADT as hotpluggable CPUS. In the case there are no disabled CPUS
344 we assume 1/2 the number of CPUs currently present can be hotplugged.
345
346 Caveat: Today's ACPI MADT can only provide 256 entries since the apicid field
347 in MADT is only 8 bits.
348
349User Space Notification
350
351Hotplug support for devices is common in Linux today. Its being used today to
352support automatic configuration of network, usb and pci devices. A hotplug
353event can be used to invoke an agent script to perform the configuration task.
354
355You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
356scripts.
357
358 #!/bin/bash
359 # $Id: cpu.agent
360 # Kernel hotplug params include:
361 #ACTION=%s [online or offline]
362 #DEVPATH=%s
363 #
364 cd /etc/hotplug
365 . ./hotplug.functions
366
367 case $ACTION in
368 online)
369 echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt
370 ;;
371 offline)
372 echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt
373 ;;
374 *)
375 debug_mesg CPU $ACTION event not supported
376 exit 1
377 ;;
378 esac