1 CPU hotplug Support in Linux(tm) Kernel
5 Rusty Russell <rusty@rustycorp.com.au>
6 Srivatsa Vaddagiri <vatsa@in.ibm.com>
8 Zwane Mwaikambo <zwane@arm.linux.org.uk>
10 Nathan Lynch <nathanl@austin.ibm.com>
11 Joel Schopp <jschopp@austin.ibm.com>
13 Ashok Raj <ashok.raj@intel.com>
15 Authors: Ashok Raj <ashok.raj@intel.com>
16 Lots of feedback: Nathan Lynch <nathanl@austin.ibm.com>,
17 Joel Schopp <jschopp@austin.ibm.com>
21 Modern advances in system architectures have introduced advanced error
22 reporting and correction capabilities in processors. CPU architectures permit
23 partitioning support, where compute resources of a single CPU could be made
24 available to virtual machine environments. There are couple OEMS that
25 support NUMA hardware which are hot pluggable as well, where physical
26 node insertion and removal require support for CPU hotplug.
28 Such advances require CPUs available to a kernel to be removed either for
29 provisioning reasons, or for RAS purposes to keep an offending CPU off
30 system execution path. Hence the need for CPU hotplug support in the
33 A more novel use of CPU-hotplug support is its use today in suspend
34 resume support for SMP. Dual-core and HT support makes even
35 a laptop run SMP kernels which didn't support these methods. SMP support
36 for suspend/resume is a work in progress.
38 General Stuff about CPU Hotplug
39 --------------------------------
43 maxcpus=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.
47 additional_cpus*=n Use this to limit hotpluggable cpus. This option sets
48 cpu_possible_map = cpu_present_map + additional_cpus
50 (*) Option valid only for following architectures
53 ia64 and x86_64 use the number of disabled local apics in ACPI tables MADT
54 to determine the number of potentially hot-pluggable cpus. The implementation
55 should only rely on this to count the #of cpus, but *MUST* not rely on the
56 apicid values in those tables for disabled apics. In the event BIOS doesnt
57 mark such hot-pluggable cpus as disabled entries, one could use this
58 parameter "additional_cpus=x" to represent those cpus in the cpu_possible_map.
63 [More on cpumaps and primitive to manipulate, please check
64 include/linux/cpumask.h that has more descriptive text.]
66 cpu_possible_map: Bitmap of possible CPUs that can ever be available in the
67 system. This is used to allocate some boot time memory for per_cpu variables
68 that aren't designed to grow/shrink as CPUs are made available or removed.
69 Once set during boot time discovery phase, the map is static, i.e no bits
70 are added or removed anytime. Trimming it accurately for your system needs
71 upfront can save some boot time memory. See below for how we use heuristics
72 in x86_64 case to keep this under check.
74 cpu_online_map: Bitmap of all CPUs currently online. Its set in __cpu_up()
75 after a cpu is available for kernel scheduling and ready to receive
76 interrupts from devices. Its cleared when a cpu is brought down using
77 __cpu_disable(), before which all OS services including interrupts are
78 migrated to another target CPU.
80 cpu_present_map: Bitmap of CPUs currently present in the system. Not all
81 of them may be online. When physical hotplug is processed by the relevant
82 subsystem (e.g ACPI) can change and new bit either be added or removed
83 from the map depending on the event is hot-add/hot-remove. There are currently
84 no locking rules as of now. Typical usage is to init topology during boot,
85 at which time hotplug is disabled.
87 You really dont need to manipulate any of the system cpu maps. They should
88 be read-only for most use. When setting up per-cpu resources almost always use
89 cpu_possible_map/for_each_cpu() to iterate.
91 Never use anything other than cpumask_t to represent bitmap of CPUs.
93 #include <linux/cpumask.h>
95 for_each_cpu - Iterate over cpu_possible_map
96 for_each_online_cpu - Iterate over cpu_online_map
97 for_each_present_cpu - Iterate over cpu_present_map
98 for_each_cpu_mask(x,mask) - Iterate over some random collection of cpu mask.
100 #include <linux/cpu.h>
101 lock_cpu_hotplug() and unlock_cpu_hotplug():
103 The above calls are used to inhibit cpu hotplug operations. While holding the
104 cpucontrol mutex, cpu_online_map will not change. If you merely need to avoid
105 cpus going away, you could also use preempt_disable() and preempt_enable()
106 for those sections. Just remember the critical section cannot call any
107 function that can sleep or schedule this process away. The preempt_disable()
108 will work as long as stop_machine_run() is used to take a cpu down.
110 CPU Hotplug - Frequently Asked Questions.
112 Q: How to i enable my kernel to support CPU hotplug?
113 A: When doing make defconfig, Enable CPU hotplug support
115 "Processor type and Features" -> Support for Hotpluggable CPUs
117 Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.
119 You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
122 Q: What architectures support CPU hotplug?
123 A: As of 2.6.14, the following architectures support CPU hotplug.
125 i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
127 Q: How to test if hotplug is supported on the newly built kernel?
128 A: You should now notice an entry in sysfs.
130 Check if sysfs is mounted, using the "mount" command. You should notice
131 an entry as shown below in the output.
134 none on /sys type sysfs (rw)
137 if this is not mounted, do the following.
140 #mount -t sysfs sys /sys
142 now you should see entries for all present cpu, the following is an example
146 #/sys/devices/system/cpu
149 drwxr-xr-x 10 root root 0 Sep 19 07:44 .
150 drwxr-xr-x 13 root root 0 Sep 19 07:45 ..
151 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu0
152 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu1
153 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu2
154 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu3
155 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu4
156 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu5
157 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu6
158 drwxr-xr-x 3 root root 0 Sep 19 07:48 cpu7
160 Under each directory you would find an "online" file which is the control
161 file to logically online/offline a processor.
163 Q: Does hot-add/hot-remove refer to physical add/remove of cpus?
164 A: The usage of hot-add/remove may not be very consistently used in the code.
165 CONFIG_CPU_HOTPLUG enables logical online/offline capability in the kernel.
166 To support physical addition/removal, one would need some BIOS hooks and
167 the platform should have something like an attention button in PCI hotplug.
168 CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.
170 Q: How do i logically offline a CPU?
173 #echo 0 > /sys/devices/system/cpu/cpuX/online
175 once the logical offline is successful, check
177 #cat /proc/interrupts
179 you should now not see the CPU that you removed. Also online file will report
180 the state as 0 when a cpu if offline and 1 when its online.
182 #To display the current cpu state.
183 #cat /sys/devices/system/cpu/cpuX/online
185 Q: Why cant i remove CPU0 on some systems?
186 A: Some architectures may have some special dependency on a certain CPU.
188 For e.g in IA64 platforms we have ability to sent platform interrupts to the
189 OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
190 specifications, we didn't have a way to change the target CPU. Hence if the
191 current ACPI version doesn't support such re-direction, we disable that CPU
192 by making it not-removable.
194 In such cases you will also notice that the online file is missing under cpu0.
196 Q: How do i find out if a particular CPU is not removable?
197 A: Depending on the implementation, some architectures may show this by the
198 absence of the "online" file. This is done if it can be determined ahead of
199 time that this CPU cannot be removed.
201 In some situations, this can be a run time check, i.e if you try to remove the
202 last CPU, this will not be permitted. You can find such failures by
203 investigating the return value of the "echo" command.
205 Q: What happens when a CPU is being logically offlined?
206 A: The following happen, listed in no particular order :-)
208 - A notification is sent to in-kernel registered modules by sending an event
210 - All process is migrated away from this outgoing CPU to a new CPU
211 - All interrupts targeted to this CPU is migrated to a new CPU
212 - timers/bottom half/task lets are also migrated to a new CPU
213 - Once all services are migrated, kernel calls an arch specific routine
214 __cpu_disable() to perform arch specific cleanup.
215 - Once this is successful, an event for successful cleanup is sent by an event
218 "It is expected that each service cleans up when the CPU_DOWN_PREPARE
219 notifier is called, when CPU_DEAD is called its expected there is nothing
220 running on behalf of this CPU that was offlined"
222 Q: If i have some kernel code that needs to be aware of CPU arrival and
223 departure, how to i arrange for proper notification?
224 A: This is what you would need in your kernel code to receive notifications.
226 #include <linux/cpu.h>
227 static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb,
228 unsigned long action, void *hcpu)
230 unsigned int cpu = (unsigned long)hcpu;
234 foobar_online_action(cpu);
237 foobar_dead_action(cpu);
243 static struct notifier_block foobar_cpu_notifer =
245 .notifier_call = foobar_cpu_callback,
249 In your init function,
251 register_cpu_notifier(&foobar_cpu_notifier);
253 You can fail PREPARE notifiers if something doesn't work to prepare resources.
254 This will stop the activity and send a following CANCELED event back.
256 CPU_DEAD should not be failed, its just a goodness indication, but bad
257 things will happen if a notifier in path sent a BAD notify code.
259 Q: I don't see my action being called for all CPUs already up and running?
260 A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined.
261 If you need to perform some action for each cpu already in the system, then
263 for_each_online_cpu(i) {
264 foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
265 foobar_cpu_callback(&foobar-cpu_notifier, CPU_ONLINE, i);
268 Q: If i would like to develop cpu hotplug support for a new architecture,
269 what do i need at a minimum?
270 A: The following are what is required for CPU hotplug infrastructure to work
273 - Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU
274 - __cpu_up() - Arch interface to bring up a CPU
275 - __cpu_disable() - Arch interface to shutdown a CPU, no more interrupts
276 can be handled by the kernel after the routine
277 returns. Including local APIC timers etc are
279 - __cpu_die() - This actually supposed to ensure death of the CPU.
280 Actually look at some example code in other arch
281 that implement CPU hotplug. The processor is taken
282 down from the idle() loop for that specific
283 architecture. __cpu_die() typically waits for some
284 per_cpu state to be set, to ensure the processor
285 dead routine is called to be sure positively.
287 Q: I need to ensure that a particular cpu is not removed when there is some
288 work specific to this cpu is in progress.
289 A: First switch the current thread context to preferred cpu
291 int my_func_on_cpu(int cpu)
293 cpumask_t saved_mask, new_mask = CPU_MASK_NONE;
294 int curr_cpu, err = 0;
296 saved_mask = current->cpus_allowed;
297 cpu_set(cpu, new_mask);
298 err = set_cpus_allowed(current, new_mask);
304 * If we got scheduled out just after the return from
305 * set_cpus_allowed() before running the work, this ensures
308 curr_cpu = get_cpu();
310 if (curr_cpu != cpu) {
315 * Do work : But cant sleep, since get_cpu() disables preempt
320 set_cpus_allowed(current, saved_mask);
325 Q: How do we determine how many CPUs are available for hotplug.
326 A: There is no clear spec defined way from ACPI that can give us that
327 information today. Based on some input from Natalie of Unisys,
328 that the ACPI MADT (Multiple APIC Description Tables) marks those possible
329 CPUs in a system with disabled status.
331 Andi implemented some simple heuristics that count the number of disabled
332 CPUs in MADT as hotpluggable CPUS. In the case there are no disabled CPUS
333 we assume 1/2 the number of CPUs currently present can be hotplugged.
335 Caveat: Today's ACPI MADT can only provide 256 entries since the apicid field
336 in MADT is only 8 bits.
338 User Space Notification
340 Hotplug support for devices is common in Linux today. Its being used today to
341 support automatic configuration of network, usb and pci devices. A hotplug
342 event can be used to invoke an agent script to perform the configuration task.
344 You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
349 # Kernel hotplug params include:
350 #ACTION=%s [online or offline]
354 . ./hotplug.functions
358 echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt
361 echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt
364 debug_mesg CPU $ACTION event not supported