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Based on kernel version 3.13. Page generated on 2014-01-20 22:00 EST.

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 <zwanem@gmail.com>
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_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 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 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: ACPI MADT can only provide 256 entries in systems with only ACPI 2.0c
374	   or earlier ACPI version supported, because the apicid field in MADT is only
375	   8 bits. From ACPI 3.0, this limitation was removed since the apicid field
376	   was extended to 32 bits with x2APIC introduced.
377	
378	User Space Notification
379	
380	Hotplug support for devices is common in Linux today. Its being used today to
381	support automatic configuration of network, usb and pci devices. A hotplug
382	event can be used to invoke an agent script to perform the configuration task.
383	
384	You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
385	scripts.
386	
387		#!/bin/bash
388		# $Id: cpu.agent
389		# Kernel hotplug params include:
390		#ACTION=%s [online or offline]
391		#DEVPATH=%s
392		#
393		cd /etc/hotplug
394		. ./hotplug.functions
395	
396		case $ACTION in
397			online)
398				echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt
399				;;
400			offline)
401				echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt
402				;;
403			*)
404				debug_mesg CPU $ACTION event not supported
405	        exit 1
406	        ;;
407		esac
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