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authorAshok Raj <ashok.raj@intel.com>2006-01-08 04:03:17 -0500
committerLinus Torvalds <torvalds@g5.osdl.org>2006-01-08 23:13:55 -0500
commitc809406b4f2dfac9006d7eb8dca6b9f990f10b61 (patch)
tree090455ba2acfb5a6667737edab624560be17afaa /Documentation/cpu-hotplug.txt
parent55a82ab3181be039c6440d3f2f69260ad6fe2988 (diff)
[PATCH] Updated CPU hotplug documentation
Thanks to Nathan Lynch for the review and comments. Thanks to Joel Schopp for the pointer to add user space scipts. Signed-off-by: Ashok Raj <ashok.raj@intel.com> Signed-off-by: Nathan Lynch <nathanl@austin.ibm.com> Signed-off-by: Joel Schopp <jschopp@austin.ibm.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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1 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>
14
15Authors: Ashok Raj <ashok.raj@intel.com>
16Lots of feedback: Nathan Lynch <nathanl@austin.ibm.com>,
17 Joel Schopp <jschopp@austin.ibm.com>
18
19Introduction
20
21Modern advances in system architectures have introduced advanced error
22reporting and correction capabilities in processors. CPU architectures permit
23partitioning support, where compute resources of a single CPU could be made
24available to virtual machine environments. There are couple OEMS that
25support NUMA hardware which are hot pluggable as well, where physical
26node insertion and removal require support for CPU hotplug.
27
28Such advances require CPUs available to a kernel to be removed either for
29provisioning reasons, or for RAS purposes to keep an offending CPU off
30system execution path. Hence the need for CPU hotplug support in the
31Linux kernel.
32
33A more novel use of CPU-hotplug support is its use today in suspend
34resume support for SMP. Dual-core and HT support makes even
35a laptop run SMP kernels which didn't support these methods. SMP support
36for suspend/resume is a work in progress.
37
38General Stuff about CPU Hotplug
39--------------------------------
40
41Command Line Switches
42---------------------
43maxcpus=n Restrict boot time cpus to n. Say if you have 4 cpus, using
44 maxcpus=2 will only boot 2. You can choose to bring the
45 other cpus later online, read FAQ's for more info.
46
47additional_cpus=n [x86_64 only] use this to limit hotpluggable cpus.
48 This option sets
49 cpu_possible_map = cpu_present_map + additional_cpus
50
51CPU maps and such
52-----------------
53[More on cpumaps and primitive to manipulate, please check
54include/linux/cpumask.h that has more descriptive text.]
55
56cpu_possible_map: Bitmap of possible CPUs that can ever be available in the
57system. This is used to allocate some boot time memory for per_cpu variables
58that aren't designed to grow/shrink as CPUs are made available or removed.
59Once set during boot time discovery phase, the map is static, i.e no bits
60are added or removed anytime. Trimming it accurately for your system needs
61upfront can save some boot time memory. See below for how we use heuristics
62in x86_64 case to keep this under check.
63
64cpu_online_map: Bitmap of all CPUs currently online. Its set in __cpu_up()
65after a cpu is available for kernel scheduling and ready to receive
66interrupts from devices. Its cleared when a cpu is brought down using
67__cpu_disable(), before which all OS services including interrupts are
68migrated to another target CPU.
69
70cpu_present_map: Bitmap of CPUs currently present in the system. Not all
71of them may be online. When physical hotplug is processed by the relevant
72subsystem (e.g ACPI) can change and new bit either be added or removed
73from the map depending on the event is hot-add/hot-remove. There are currently
74no locking rules as of now. Typical usage is to init topology during boot,
75at which time hotplug is disabled.
76
77You really dont need to manipulate any of the system cpu maps. They should
78be read-only for most use. When setting up per-cpu resources almost always use
79cpu_possible_map/for_each_cpu() to iterate.
80
81Never use anything other than cpumask_t to represent bitmap of CPUs.
82
83#include <linux/cpumask.h>
84
85for_each_cpu - Iterate over cpu_possible_map
86for_each_online_cpu - Iterate over cpu_online_map
87for_each_present_cpu - Iterate over cpu_present_map
88for_each_cpu_mask(x,mask) - Iterate over some random collection of cpu mask.
89
90#include <linux/cpu.h>
91lock_cpu_hotplug() and unlock_cpu_hotplug():
92
93The above calls are used to inhibit cpu hotplug operations. While holding the
94cpucontrol mutex, cpu_online_map will not change. If you merely need to avoid
95cpus going away, you could also use preempt_disable() and preempt_enable()
96for those sections. Just remember the critical section cannot call any
97function that can sleep or schedule this process away. The preempt_disable()
98will work as long as stop_machine_run() is used to take a cpu down.
99
100CPU Hotplug - Frequently Asked Questions.
101
102Q: How to i enable my kernel to support CPU hotplug?
103A: When doing make defconfig, Enable CPU hotplug support
104
105 "Processor type and Features" -> Support for Hotpluggable CPUs
106
107Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.
108
109You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
110as well.
111
112Q: What architectures support CPU hotplug?
113A: As of 2.6.14, the following architectures support CPU hotplug.
114
115i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
116
117Q: How to test if hotplug is supported on the newly built kernel?
118A: You should now notice an entry in sysfs.
119
120Check if sysfs is mounted, using the "mount" command. You should notice
121an entry as shown below in the output.
122
123....
124none on /sys type sysfs (rw)
125....
126
127if this is not mounted, do the following.
128
129#mkdir /sysfs
130#mount -t sysfs sys /sys
131
132now you should see entries for all present cpu, the following is an example
133in a 8-way system.
134
135#pwd
136#/sys/devices/system/cpu
137#ls -l
138total 0
139drwxr-xr-x 10 root root 0 Sep 19 07:44 .
140drwxr-xr-x 13 root root 0 Sep 19 07:45 ..
141drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu0
142drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu1
143drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu2
144drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu3
145drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu4
146drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu5
147drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu6
148drwxr-xr-x 3 root root 0 Sep 19 07:48 cpu7
149
150Under each directory you would find an "online" file which is the control
151file to logically online/offline a processor.
152
153Q: Does hot-add/hot-remove refer to physical add/remove of cpus?
154A: The usage of hot-add/remove may not be very consistently used in the code.
155CONFIG_CPU_HOTPLUG enables logical online/offline capability in the kernel.
156To support physical addition/removal, one would need some BIOS hooks and
157the platform should have something like an attention button in PCI hotplug.
158CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.
159
160Q: How do i logically offline a CPU?
161A: Do the following.
162
163#echo 0 > /sys/devices/system/cpu/cpuX/online
164
165once the logical offline is successful, check
166
167#cat /proc/interrupts
168
169you should now not see the CPU that you removed. Also online file will report
170the state as 0 when a cpu if offline and 1 when its online.
171
172#To display the current cpu state.
173#cat /sys/devices/system/cpu/cpuX/online
174
175Q: Why cant i remove CPU0 on some systems?
176A: Some architectures may have some special dependency on a certain CPU.
177
178For e.g in IA64 platforms we have ability to sent platform interrupts to the
179OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
180specifications, we didn't have a way to change the target CPU. Hence if the
181current ACPI version doesn't support such re-direction, we disable that CPU
182by making it not-removable.
183
184In such cases you will also notice that the online file is missing under cpu0.
185
186Q: How do i find out if a particular CPU is not removable?
187A: Depending on the implementation, some architectures may show this by the
188absence of the "online" file. This is done if it can be determined ahead of
189time that this CPU cannot be removed.
190
191In some situations, this can be a run time check, i.e if you try to remove the
192last CPU, this will not be permitted. You can find such failures by
193investigating the return value of the "echo" command.
194
195Q: What happens when a CPU is being logically offlined?
196A: The following happen, listed in no particular order :-)
197
198- A notification is sent to in-kernel registered modules by sending an event
199 CPU_DOWN_PREPARE
200- All process is migrated away from this outgoing CPU to a new CPU
201- All interrupts targeted to this CPU is migrated to a new CPU
202- timers/bottom half/task lets are also migrated to a new CPU
203- Once all services are migrated, kernel calls an arch specific routine
204 __cpu_disable() to perform arch specific cleanup.
205- Once this is successful, an event for successful cleanup is sent by an event
206 CPU_DEAD.
207
208 "It is expected that each service cleans up when the CPU_DOWN_PREPARE
209 notifier is called, when CPU_DEAD is called its expected there is nothing
210 running on behalf of this CPU that was offlined"
211
212Q: If i have some kernel code that needs to be aware of CPU arrival and
213 departure, how to i arrange for proper notification?
214A: This is what you would need in your kernel code to receive notifications.
215
216 #include <linux/cpu.h>
217 static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb,
218 unsigned long action, void *hcpu)
219 {
220 unsigned int cpu = (unsigned long)hcpu;
221
222 switch (action) {
223 case CPU_ONLINE:
224 foobar_online_action(cpu);
225 break;
226 case CPU_DEAD:
227 foobar_dead_action(cpu);
228 break;
229 }
230 return NOTIFY_OK;
231 }
232
233 static struct notifier_block foobar_cpu_notifer =
234 {
235 .notifier_call = foobar_cpu_callback,
236 };
237
238
239In your init function,
240
241 register_cpu_notifier(&foobar_cpu_notifier);
242
243You can fail PREPARE notifiers if something doesn't work to prepare resources.
244This will stop the activity and send a following CANCELED event back.
245
246CPU_DEAD should not be failed, its just a goodness indication, but bad
247things will happen if a notifier in path sent a BAD notify code.
248
249Q: I don't see my action being called for all CPUs already up and running?
250A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined.
251 If you need to perform some action for each cpu already in the system, then
252
253 for_each_online_cpu(i) {
254 foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
255 foobar_cpu_callback(&foobar-cpu_notifier, CPU_ONLINE, i);
256 }
257
258Q: If i would like to develop cpu hotplug support for a new architecture,
259 what do i need at a minimum?
260A: The following are what is required for CPU hotplug infrastructure to work
261 correctly.
262
263 - Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU
264 - __cpu_up() - Arch interface to bring up a CPU
265 - __cpu_disable() - Arch interface to shutdown a CPU, no more interrupts
266 can be handled by the kernel after the routine
267 returns. Including local APIC timers etc are
268 shutdown.
269 - __cpu_die() - This actually supposed to ensure death of the CPU.
270 Actually look at some example code in other arch
271 that implement CPU hotplug. The processor is taken
272 down from the idle() loop for that specific
273 architecture. __cpu_die() typically waits for some
274 per_cpu state to be set, to ensure the processor
275 dead routine is called to be sure positively.
276
277Q: I need to ensure that a particular cpu is not removed when there is some
278 work specific to this cpu is in progress.
279A: First switch the current thread context to preferred cpu
280
281 int my_func_on_cpu(int cpu)
282 {
283 cpumask_t saved_mask, new_mask = CPU_MASK_NONE;
284 int curr_cpu, err = 0;
285
286 saved_mask = current->cpus_allowed;
287 cpu_set(cpu, new_mask);
288 err = set_cpus_allowed(current, new_mask);
289
290 if (err)
291 return err;
292
293 /*
294 * If we got scheduled out just after the return from
295 * set_cpus_allowed() before running the work, this ensures
296 * we stay locked.
297 */
298 curr_cpu = get_cpu();
299
300 if (curr_cpu != cpu) {
301 err = -EAGAIN;
302 goto ret;
303 } else {
304 /*
305 * Do work : But cant sleep, since get_cpu() disables preempt
306 */
307 }
308 ret:
309 put_cpu();
310 set_cpus_allowed(current, saved_mask);
311 return err;
312 }
313
314
315Q: How do we determine how many CPUs are available for hotplug.
316A: There is no clear spec defined way from ACPI that can give us that
317 information today. Based on some input from Natalie of Unisys,
318 that the ACPI MADT (Multiple APIC Description Tables) marks those possible
319 CPUs in a system with disabled status.
320
321 Andi implemented some simple heuristics that count the number of disabled
322 CPUs in MADT as hotpluggable CPUS. In the case there are no disabled CPUS
323 we assume 1/2 the number of CPUs currently present can be hotplugged.
324
325 Caveat: Today's ACPI MADT can only provide 256 entries since the apicid field
326 in MADT is only 8 bits.
327
328User Space Notification
329
330Hotplug support for devices is common in Linux today. Its being used today to
331support automatic configuration of network, usb and pci devices. A hotplug
332event can be used to invoke an agent script to perform the configuration task.
333
334You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
335scripts.
336
337 #!/bin/bash
338 # $Id: cpu.agent
339 # Kernel hotplug params include:
340 #ACTION=%s [online or offline]
341 #DEVPATH=%s
342 #
343 cd /etc/hotplug
344 . ./hotplug.functions
345
346 case $ACTION in
347 online)
348 echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt
349 ;;
350 offline)
351 echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt
352 ;;
353 *)
354 debug_mesg CPU $ACTION event not supported
355 exit 1
356 ;;
357 esac