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Documentation / cpu-hotplug.txt

Based on kernel version 2.6.26. Page generated on 2008-07-16 21:12 EST.

1			CPU hotplug Support in Linux(tm) Kernel
2	
3			Maintainers:
4			CPU Hotplug Core:
5				Rusty Russell <rusty[AT]rustycorp.com[DOT]au>
6				Srivatsa Vaddagiri <vatsa[AT]in.ibm[DOT]com>
7			i386:
8				Zwane Mwaikambo <zwane[AT]arm.linux.org[DOT]uk>
9			ppc64:
10				Nathan Lynch <nathanl[AT]austin.ibm[DOT]com>
11				Joel Schopp <jschopp[AT]austin.ibm[DOT]com>
12			ia64/x86_64:
13				Ashok Raj <ashok.raj[AT]intel[DOT]com>
14			s390:
15				Heiko Carstens <heiko.carstens[AT]de.ibm[DOT]com>
16	
17	Authors: Ashok Raj <ashok.raj[AT]intel[DOT]com>
18	Lots of feedback: Nathan Lynch <nathanl[AT]austin.ibm[DOT]com>,
19		     Joel Schopp <jschopp[AT]austin.ibm[DOT]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_map = cpu_present_map + additional_cpus
51	
52	(*) Option valid only for following architectures
53	- x86_64, ia64
54	
55	ia64 and x86_64 use the number of disabled local apics in ACPI tables MADT
56	to determine the number of potentially hot-pluggable cpus. The implementation
57	should only rely on this to count the # of cpus, but *MUST* not rely on the
58	apicid values in those tables for disabled apics. In the event BIOS doesn't
59	mark such hot-pluggable cpus as disabled entries, one could use this
60	parameter "additional_cpus=x" to represent those cpus in the cpu_possible_map.
61	
62	s390 uses the number of cpus it detects at IPL time to also the number of bits
63	in cpu_possible_map. If it is desired to add additional cpus at a later time
64	the number should be specified using this option or the possible_cpus option.
65	
66	possible_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	
72	CPU maps and such
73	-----------------
74	[More on cpumaps and primitive to manipulate, please check
75	include/linux/cpumask.h that has more descriptive text.]
76	
77	cpu_possible_map: Bitmap of possible CPUs that can ever be available in the
78	system. This is used to allocate some boot time memory for per_cpu variables
79	that aren't designed to grow/shrink as CPUs are made available or removed.
80	Once set during boot time discovery phase, the map is static, i.e no bits
81	are added or removed anytime.  Trimming it accurately for your system needs
82	upfront can save some boot time memory. See below for how we use heuristics
83	in x86_64 case to keep this under check.
84	
85	cpu_online_map: Bitmap of all CPUs currently online. Its set in __cpu_up()
86	after a cpu is available for kernel scheduling and ready to receive
87	interrupts from devices. Its cleared when a cpu is brought down using
88	__cpu_disable(), before which all OS services including interrupts are
89	migrated to another target CPU.
90	
91	cpu_present_map: Bitmap of CPUs currently present in the system. Not all
92	of them may be online. When physical hotplug is processed by the relevant
93	subsystem (e.g ACPI) can change and new bit either be added or removed
94	from the map depending on the event is hot-add/hot-remove. There are currently
95	no locking rules as of now. Typical usage is to init topology during boot,
96	at which time hotplug is disabled.
97	
98	You really dont need to manipulate any of the system cpu maps. They should
99	be read-only for most use. When setting up per-cpu resources almost always use
100	cpu_possible_map/for_each_possible_cpu() to iterate.
101	
102	Never use anything other than cpumask_t to represent bitmap of CPUs.
103	
104		#include <linux/cpumask.h>
105	
106		for_each_possible_cpu     - Iterate over cpu_possible_map
107		for_each_online_cpu       - Iterate over cpu_online_map
108		for_each_present_cpu      - Iterate over cpu_present_map
109		for_each_cpu_mask(x,mask) - Iterate over some random collection of cpu mask.
110	
111		#include <linux/cpu.h>
112		get_online_cpus() and put_online_cpus():
113	
114	The above calls are used to inhibit cpu hotplug operations. While the
115	cpu_hotplug.refcount is non zero, the cpu_online_map will not change.
116	If you merely need to avoid cpus going away, you could also use
117	preempt_disable() and preempt_enable() for those sections.
118	Just remember the critical section cannot call any
119	function that can sleep or schedule this process away. The preempt_disable()
120	will work as long as stop_machine_run() is used to take a cpu down.
121	
122	CPU Hotplug - Frequently Asked Questions.
123	
124	Q: How to enable my kernel to support CPU hotplug?
125	A: When doing make defconfig, Enable CPU hotplug support
126	
127	   "Processor type and Features" -> Support for Hotpluggable CPUs
128	
129	Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.
130	
131	You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
132	as well.
133	
134	Q: What architectures support CPU hotplug?
135	A: As of 2.6.14, the following architectures support CPU hotplug.
136	
137	i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
138	
139	Q: How to test if hotplug is supported on the newly built kernel?
140	A: You should now notice an entry in sysfs.
141	
142	Check if sysfs is mounted, using the "mount" command. You should notice
143	an entry as shown below in the output.
144	
145		....
146		none on /sys type sysfs (rw)
147		....
148	
149	If this is not mounted, do the following.
150	
151		 #mkdir /sysfs
152		#mount -t sysfs sys /sys
153	
154	Now you should see entries for all present cpu, the following is an example
155	in a 8-way system.
156	
157		#pwd
158		#/sys/devices/system/cpu
159		#ls -l
160		total 0
161		drwxr-xr-x  10 root root 0 Sep 19 07:44 .
162		drwxr-xr-x  13 root root 0 Sep 19 07:45 ..
163		drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu0
164		drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu1
165		drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu2
166		drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu3
167		drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu4
168		drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu5
169		drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu6
170		drwxr-xr-x   3 root root 0 Sep 19 07:48 cpu7
171	
172	Under each directory you would find an "online" file which is the control
173	file to logically online/offline a processor.
174	
175	Q: Does hot-add/hot-remove refer to physical add/remove of cpus?
176	A: The usage of hot-add/remove may not be very consistently used in the code.
177	CONFIG_HOTPLUG_CPU enables logical online/offline capability in the kernel.
178	To support physical addition/removal, one would need some BIOS hooks and
179	the platform should have something like an attention button in PCI hotplug.
180	CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.
181	
182	Q: How do i logically offline a CPU?
183	A: Do the following.
184	
185		#echo 0 > /sys/devices/system/cpu/cpuX/online
186	
187	Once the logical offline is successful, check
188	
189		#cat /proc/interrupts
190	
191	You should now not see the CPU that you removed. Also online file will report
192	the state as 0 when a cpu if offline and 1 when its online.
193	
194		#To display the current cpu state.
195		#cat /sys/devices/system/cpu/cpuX/online
196	
197	Q: Why cant i remove CPU0 on some systems?
198	A: Some architectures may have some special dependency on a certain CPU.
199	
200	For e.g in IA64 platforms we have ability to sent platform interrupts to the
201	OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
202	specifications, we didn't have a way to change the target CPU. Hence if the
203	current ACPI version doesn't support such re-direction, we disable that CPU
204	by making it not-removable.
205	
206	In such cases you will also notice that the online file is missing under cpu0.
207	
208	Q: How do i find out if a particular CPU is not removable?
209	A: Depending on the implementation, some architectures may show this by the
210	absence of the "online" file. This is done if it can be determined ahead of
211	time that this CPU cannot be removed.
212	
213	In some situations, this can be a run time check, i.e if you try to remove the
214	last CPU, this will not be permitted. You can find such failures by
215	investigating the return value of the "echo" command.
216	
217	Q: What happens when a CPU is being logically offlined?
218	A: The following happen, listed in no particular order :-)
219	
220	- A notification is sent to in-kernel registered modules by sending an event
221	  CPU_DOWN_PREPARE or CPU_DOWN_PREPARE_FROZEN, depending on whether or not the
222	  CPU is being offlined while tasks are frozen due to a suspend operation in
223	  progress
224	- All processes are migrated away from this outgoing CPU to new CPUs.
225	  The new CPU is chosen from each process' current cpuset, which may be
226	  a subset of all online CPUs.
227	- All interrupts targeted to this CPU is migrated to a new CPU
228	- timers/bottom half/task lets are also migrated to a new CPU
229	- Once all services are migrated, kernel calls an arch specific routine
230	  __cpu_disable() to perform arch specific cleanup.
231	- Once this is successful, an event for successful cleanup is sent by an event
232	  CPU_DEAD (or CPU_DEAD_FROZEN if tasks are frozen due to a suspend while the
233	  CPU is being offlined).
234	
235	  "It is expected that each service cleans up when the CPU_DOWN_PREPARE
236	  notifier is called, when CPU_DEAD is called its expected there is nothing
237	  running on behalf of this CPU that was offlined"
238	
239	Q: If i have some kernel code that needs to be aware of CPU arrival and
240	   departure, how to i arrange for proper notification?
241	A: This is what you would need in your kernel code to receive notifications.
242	
243		#include <linux/cpu.h>
244		static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb,
245						    unsigned long action, void *hcpu)
246		{
247			unsigned int cpu = (unsigned long)hcpu;
248	
249			switch (action) {
250			case CPU_ONLINE:
251			case CPU_ONLINE_FROZEN:
252				foobar_online_action(cpu);
253				break;
254			case CPU_DEAD:
255			case CPU_DEAD_FROZEN:
256				foobar_dead_action(cpu);
257				break;
258			}
259			return NOTIFY_OK;
260		}
261	
262		static struct notifier_block __cpuinitdata foobar_cpu_notifer =
263		{
264		   .notifier_call = foobar_cpu_callback,
265		};
266	
267	You need to call register_cpu_notifier() from your init function.
268	Init functions could be of two types:
269	1. early init (init function called when only the boot processor is online).
270	2. late init (init function called _after_ all the CPUs are online).
271	
272	For the first case, you should add the following to your init function
273	
274		register_cpu_notifier(&foobar_cpu_notifier);
275	
276	For the second case, you should add the following to your init function
277	
278		register_hotcpu_notifier(&foobar_cpu_notifier);
279	
280	You can fail PREPARE notifiers if something doesn't work to prepare resources.
281	This will stop the activity and send a following CANCELED event back.
282	
283	CPU_DEAD should not be failed, its just a goodness indication, but bad
284	things will happen if a notifier in path sent a BAD notify code.
285	
286	Q: I don't see my action being called for all CPUs already up and running?
287	A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined.
288	   If you need to perform some action for each cpu already in the system, then
289	
290		for_each_online_cpu(i) {
291			foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
292			foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, i);
293		}
294	
295	Q: If i would like to develop cpu hotplug support for a new architecture,
296	   what do i need at a minimum?
297	A: The following are what is required for CPU hotplug infrastructure to work
298	   correctly.
299	
300	    - Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU
301	    - __cpu_up()        - Arch interface to bring up a CPU
302	    - __cpu_disable()   - Arch interface to shutdown a CPU, no more interrupts
303	                          can be handled by the kernel after the routine
304	                          returns. Including local APIC timers etc are
305	                          shutdown.
306	     - __cpu_die()      - This actually supposed to ensure death of the CPU.
307	                          Actually look at some example code in other arch
308	                          that implement CPU hotplug. The processor is taken
309	                          down from the idle() loop for that specific
310	                          architecture. __cpu_die() typically waits for some
311	                          per_cpu state to be set, to ensure the processor
312	                          dead routine is called to be sure positively.
313	
314	Q: I need to ensure that a particular cpu is not removed when there is some
315	   work specific to this cpu is in progress.
316	A: First switch the current thread context to preferred cpu
317	
318		int my_func_on_cpu(int cpu)
319		{
320			cpumask_t saved_mask, new_mask = CPU_MASK_NONE;
321			int curr_cpu, err = 0;
322	
323			saved_mask = current->cpus_allowed;
324			cpu_set(cpu, new_mask);
325			err = set_cpus_allowed(current, new_mask);
326	
327			if (err)
328				return err;
329	
330			/*
331			 * If we got scheduled out just after the return from
332			 * set_cpus_allowed() before running the work, this ensures
333			 * we stay locked.
334			 */
335			curr_cpu = get_cpu();
336	
337			if (curr_cpu != cpu) {
338				err = -EAGAIN;
339				goto ret;
340			} else {
341				/*
342				 * Do work : But cant sleep, since get_cpu() disables preempt
343				 */
344			}
345			ret:
346				put_cpu();
347				set_cpus_allowed(current, saved_mask);
348				return err;
349			}
350	
351	
352	Q: How do we determine how many CPUs are available for hotplug.
353	A: There is no clear spec defined way from ACPI that can give us that
354	   information today. Based on some input from Natalie of Unisys,
355	   that the ACPI MADT (Multiple APIC Description Tables) marks those possible
356	   CPUs in a system with disabled status.
357	
358	   Andi implemented some simple heuristics that count the number of disabled
359	   CPUs in MADT as hotpluggable CPUS.  In the case there are no disabled CPUS
360	   we assume 1/2 the number of CPUs currently present can be hotplugged.
361	
362	   Caveat: Today's ACPI MADT can only provide 256 entries since the apicid field
363	   in MADT is only 8 bits.
364	
365	User Space Notification
366	
367	Hotplug support for devices is common in Linux today. Its being used today to
368	support automatic configuration of network, usb and pci devices. A hotplug
369	event can be used to invoke an agent script to perform the configuration task.
370	
371	You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
372	scripts.
373	
374		#!/bin/bash
375		# $Id: cpu.agent
376		# Kernel hotplug params include:
377		#ACTION=%s [online or offline]
378		#DEVPATH=%s
379		#
380		cd /etc/hotplug
381		. ./hotplug.functions
382	
383		case $ACTION in
384			online)
385				echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt
386				;;
387			offline)
388				echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt
389				;;
390			*)
391				debug_mesg CPU $ACTION event not supported
392	        exit 1
393	        ;;
394		esac
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