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Based on kernel version 4.10.8. Page generated on 2017-04-01 14:42 EST.

1			CPU hotplug Support in Linux(tm) Kernel
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>
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>
21	Introduction
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.
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.
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.
40	General Stuff about CPU Hotplug
41	--------------------------------
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.
49	additional_cpus=n (*)	Use this to limit hotpluggable cpus. This option sets
50	  			cpu_possible_mask = cpu_present_mask + additional_cpus
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".
58	(*) Option valid only for following architectures
59	- ia64
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.
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.
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.]
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.
87	cpu_online_mask: Bitmap of all CPUs currently online. It's set in __cpu_up()
88	after a CPU is available for kernel scheduling and ready to receive
89	interrupts from devices. It's 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.
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.
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.
104	Never use anything other than cpumask_t to represent bitmap of CPUs.
106		#include <linux/cpumask.h>
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(x,mask)      - Iterate over some random collection of cpu mask.
113		#include <linux/cpu.h>
114		get_online_cpus() and put_online_cpus():
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.
124	CPU Hotplug - Frequently Asked Questions.
126	Q: How to enable my kernel to support CPU hotplug?
127	A: When doing make defconfig, Enable CPU hotplug support
129	   "Processor type and Features" -> Support for Hotpluggable CPUs
131	Make sure that you have CONFIG_SMP turned on as well.
133	You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
134	as well.
136	Q: What architectures support CPU hotplug?
137	A: As of 2.6.14, the following architectures support CPU hotplug.
139	i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
141	Q: How to test if hotplug is supported on the newly built kernel?
142	A: You should now notice an entry in sysfs.
144	Check if sysfs is mounted, using the "mount" command. You should notice
145	an entry as shown below in the output.
147		....
148		none on /sys type sysfs (rw)
149		....
151	If this is not mounted, do the following.
153		#mkdir /sys
154		#mount -t sysfs sys /sys
156	Now you should see entries for all present cpu, the following is an example
157	in a 8-way system.
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
174	Under each directory you would find an "online" file which is the control
175	file to logically online/offline a processor.
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.
184	Q: How do I logically offline a CPU?
185	A: Do the following.
187		#echo 0 > /sys/devices/system/cpu/cpuX/online
189	Once the logical offline is successful, check
191		#cat /proc/interrupts
193	You should now not see the CPU that you removed. Also online file will report
194	the state as 0 when a CPU is offline and 1 when it's online.
196		#To display the current cpu state.
197		#cat /sys/devices/system/cpu/cpuX/online
199	Q: Why can't I remove CPU0 on some systems?
200	A: Some architectures may have some special dependency on a certain CPU.
202	For e.g in IA64 platforms we have ability to send 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.
208	In such cases you will also notice that the online file is missing under cpu0.
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.
215	But some features depend on CPU0. Two known dependencies are:
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.
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.
227	Please let me know if you know or see any other dependencies of CPU0.
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.
232	--Fenghua Yu <fenghua.yu@intel.com>
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.
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.
243	Q: What happens when a CPU is being logically offlined?
244	A: The following happen, listed in no particular order :-)
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 are 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).
261	  "It is expected that each service cleans up when the CPU_DOWN_PREPARE
262	  notifier is called, when CPU_DEAD is called it's expected there is nothing
263	  running on behalf of this CPU that was offlined"
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.
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;
275			switch (action) {
276			case CPU_ONLINE:
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		}
288		static struct notifier_block foobar_cpu_notifier =
289		{
290		   .notifier_call = foobar_cpu_callback,
291		};
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).
298	For the first case, you should add the following to your init function
300		register_cpu_notifier(&foobar_cpu_notifier);
302	For the second case, you should add the following to your init function
304		register_hotcpu_notifier(&foobar_cpu_notifier);
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.
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.
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	   do this:
317		for_each_online_cpu(i) {
318			foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
319			foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, i);
320		}
322	   However, if you want to register a hotplug callback, as well as perform
323	   some initialization for CPUs that are already online, then do this:
325	   Version 1: (Correct)
326	   ---------
328	   	cpu_notifier_register_begin();
330			for_each_online_cpu(i) {
331				foobar_cpu_callback(&foobar_cpu_notifier,
332						    CPU_UP_PREPARE, i);
333				foobar_cpu_callback(&foobar_cpu_notifier,
334						    CPU_ONLINE, i);
335			}
337		/* Note the use of the double underscored version of the API */
338		__register_cpu_notifier(&foobar_cpu_notifier);
340		cpu_notifier_register_done();
342	   Note that the following code is *NOT* the right way to achieve this,
343	   because it is prone to an ABBA deadlock between the cpu_add_remove_lock
344	   and the cpu_hotplug.lock.
346	   Version 2: (Wrong!)
347	   ---------
349		get_online_cpus();
351			for_each_online_cpu(i) {
352				foobar_cpu_callback(&foobar_cpu_notifier,
353						    CPU_UP_PREPARE, i);
354				foobar_cpu_callback(&foobar_cpu_notifier,
355						    CPU_ONLINE, i);
356			}
358		register_cpu_notifier(&foobar_cpu_notifier);
360		put_online_cpus();
362	    So always use the first version shown above when you want to register
363	    callbacks as well as initialize the already online CPUs.
366	Q: If I would like to develop CPU hotplug support for a new architecture,
367	   what do I need at a minimum?
368	A: The following are what is required for CPU hotplug infrastructure to work
369	   correctly.
371	    - Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU
372	    - __cpu_up()        - Arch interface to bring up a CPU
373	    - __cpu_disable()   - Arch interface to shutdown a CPU, no more interrupts
374	                          can be handled by the kernel after the routine
375	                          returns. Including local APIC timers etc are
376	                          shutdown.
377	     - __cpu_die()      - This actually supposed to ensure death of the CPU.
378	                          Actually look at some example code in other arch
379	                          that implement CPU hotplug. The processor is taken
380	                          down from the idle() loop for that specific
381	                          architecture. __cpu_die() typically waits for some
382	                          per_cpu state to be set, to ensure the processor
383	                          dead routine is called to be sure positively.
385	Q: I need to ensure that a particular CPU is not removed when there is some
386	   work specific to this CPU in progress.
387	A: There are two ways.  If your code can be run in interrupt context, use
388	   smp_call_function_single(), otherwise use work_on_cpu().  Note that
389	   work_on_cpu() is slow, and can fail due to out of memory:
391		int my_func_on_cpu(int cpu)
392		{
393			int err;
394			get_online_cpus();
395			if (!cpu_online(cpu))
396				err = -EINVAL;
397			else
399				err = work_on_cpu(cpu, __my_func_on_cpu, NULL);
400	#else
401				smp_call_function_single(cpu, __my_func_on_cpu, &err,
402							 true);
403	#endif
404			put_online_cpus();
405			return err;
406		}
408	Q: How do we determine how many CPUs are available for hotplug.
409	A: There is no clear spec defined way from ACPI that can give us that
410	   information today. Based on some input from Natalie of Unisys,
411	   that the ACPI MADT (Multiple APIC Description Tables) marks those possible
412	   CPUs in a system with disabled status.
414	   Andi implemented some simple heuristics that count the number of disabled
415	   CPUs in MADT as hotpluggable CPUS.  In the case there are no disabled CPUS
416	   we assume 1/2 the number of CPUs currently present can be hotplugged.
418	   Caveat: ACPI MADT can only provide 256 entries in systems with only ACPI 2.0c
419	   or earlier ACPI version supported, because the apicid field in MADT is only
420	   8 bits. From ACPI 3.0, this limitation was removed since the apicid field
421	   was extended to 32 bits with x2APIC introduced.
423	User Space Notification
425	Hotplug support for devices is common in Linux today. Its being used today to
426	support automatic configuration of network, usb and pci devices. A hotplug
427	event can be used to invoke an agent script to perform the configuration task.
429	You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
430	scripts.
432		#!/bin/bash
433		# $Id: cpu.agent
434		# Kernel hotplug params include:
435		#ACTION=%s [online or offline]
436		#DEVPATH=%s
437		#
438		cd /etc/hotplug
439		. ./hotplug.functions
441		case $ACTION in
442			online)
443				echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt
444				;;
445			offline)
446				echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt
447				;;
448			*)
449				debug_mesg CPU $ACTION event not supported
450	        exit 1
451	        ;;
452		esac
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