Root/Documentation/cpu-hotplug.txt

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 <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        s390:
15            Heiko Carstens <heiko.carstens@de.ibm.com>
16
17Authors: Ashok Raj <ashok.raj@intel.com>
18Lots of feedback: Nathan Lynch <nathanl@austin.ibm.com>,
19         Joel Schopp <jschopp@austin.ibm.com>
20
21Introduction
22
23Modern advances in system architectures have introduced advanced error
24reporting and correction capabilities in processors. CPU architectures permit
25partitioning support, where compute resources of a single CPU could be made
26available to virtual machine environments. There are couple OEMS that
27support NUMA hardware which are hot pluggable as well, where physical
28node insertion and removal require support for CPU hotplug.
29
30Such advances require CPUs available to a kernel to be removed either for
31provisioning reasons, or for RAS purposes to keep an offending CPU off
32system execution path. Hence the need for CPU hotplug support in the
33Linux kernel.
34
35A more novel use of CPU-hotplug support is its use today in suspend
36resume support for SMP. Dual-core and HT support makes even
37a laptop run SMP kernels which didn't support these methods. SMP support
38for suspend/resume is a work in progress.
39
40General Stuff about CPU Hotplug
41--------------------------------
42
43Command Line Switches
44---------------------
45maxcpus=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
49additional_cpus=n (*) Use this to limit hotpluggable cpus. This option sets
50              cpu_possible_mask = cpu_present_mask + additional_cpus
51
52cede_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
61ia64 uses the number of disabled local apics in ACPI tables MADT to
62determine the number of potentially hot-pluggable cpus. The implementation
63should only rely on this to count the # of cpus, but *MUST* not rely
64on the apicid values in those tables for disabled apics. In the event
65BIOS doesn't mark such hot-pluggable cpus as disabled entries, one could
66use this parameter "additional_cpus=x" to represent those cpus in the
67cpu_possible_mask.
68
69possible_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
74CPU maps and such
75-----------------
76[More on cpumaps and primitive to manipulate, please check
77include/linux/cpumask.h that has more descriptive text.]
78
79cpu_possible_mask: Bitmap of possible CPUs that can ever be available in the
80system. This is used to allocate some boot time memory for per_cpu variables
81that aren't designed to grow/shrink as CPUs are made available or removed.
82Once set during boot time discovery phase, the map is static, i.e no bits
83are added or removed anytime. Trimming it accurately for your system needs
84upfront can save some boot time memory. See below for how we use heuristics
85in x86_64 case to keep this under check.
86
87cpu_online_mask: Bitmap of all CPUs currently online. Its set in __cpu_up()
88after a cpu is available for kernel scheduling and ready to receive
89interrupts from devices. Its cleared when a cpu is brought down using
90__cpu_disable(), before which all OS services including interrupts are
91migrated to another target CPU.
92
93cpu_present_mask: Bitmap of CPUs currently present in the system. Not all
94of them may be online. When physical hotplug is processed by the relevant
95subsystem (e.g ACPI) can change and new bit either be added or removed
96from the map depending on the event is hot-add/hot-remove. There are currently
97no locking rules as of now. Typical usage is to init topology during boot,
98at which time hotplug is disabled.
99
100You really dont need to manipulate any of the system cpu maps. They should
101be read-only for most use. When setting up per-cpu resources almost always use
102cpu_possible_mask/for_each_possible_cpu() to iterate.
103
104Never 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
116The above calls are used to inhibit cpu hotplug operations. While the
117cpu_hotplug.refcount is non zero, the cpu_online_mask will not change.
118If you merely need to avoid cpus going away, you could also use
119preempt_disable() and preempt_enable() for those sections.
120Just remember the critical section cannot call any
121function that can sleep or schedule this process away. The preempt_disable()
122will work as long as stop_machine_run() is used to take a cpu down.
123
124CPU Hotplug - Frequently Asked Questions.
125
126Q: How to enable my kernel to support CPU hotplug?
127A: When doing make defconfig, Enable CPU hotplug support
128
129   "Processor type and Features" -> Support for Hotpluggable CPUs
130
131Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.
132
133You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
134as well.
135
136Q: What architectures support CPU hotplug?
137A: As of 2.6.14, the following architectures support CPU hotplug.
138
139i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
140
141Q: How to test if hotplug is supported on the newly built kernel?
142A: You should now notice an entry in sysfs.
143
144Check if sysfs is mounted, using the "mount" command. You should notice
145an entry as shown below in the output.
146
147    ....
148    none on /sys type sysfs (rw)
149    ....
150
151If this is not mounted, do the following.
152
153     #mkdir /sysfs
154    #mount -t sysfs sys /sys
155
156Now you should see entries for all present cpu, the following is an example
157in 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
174Under each directory you would find an "online" file which is the control
175file to logically online/offline a processor.
176
177Q: Does hot-add/hot-remove refer to physical add/remove of cpus?
178A: The usage of hot-add/remove may not be very consistently used in the code.
179CONFIG_HOTPLUG_CPU enables logical online/offline capability in the kernel.
180To support physical addition/removal, one would need some BIOS hooks and
181the platform should have something like an attention button in PCI hotplug.
182CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.
183
184Q: How do i logically offline a CPU?
185A: Do the following.
186
187    #echo 0 > /sys/devices/system/cpu/cpuX/online
188
189Once the logical offline is successful, check
190
191    #cat /proc/interrupts
192
193You should now not see the CPU that you removed. Also online file will report
194the 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
199Q: Why can't i remove CPU0 on some systems?
200A: Some architectures may have some special dependency on a certain CPU.
201
202For e.g in IA64 platforms we have ability to sent platform interrupts to the
203OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
204specifications, we didn't have a way to change the target CPU. Hence if the
205current ACPI version doesn't support such re-direction, we disable that CPU
206by making it not-removable.
207
208In such cases you will also notice that the online file is missing under cpu0.
209
210Q: Is CPU0 removable on X86?
211A: Yes. If kernel is compiled with CONFIG_BOOTPARAM_HOTPLUG_CPU0=y, CPU0 is
212removable by default. Otherwise, CPU0 is also removable by kernel option
213cpu0_hotplug.
214
215But some features depend on CPU0. Two known dependencies are:
216
2171. Resume from hibernate/suspend depends on CPU0. Hibernate/suspend will fail if
218CPU0 is offline and you need to online CPU0 before hibernate/suspend can
219continue.
2202. PIC interrupts also depend on CPU0. CPU0 can't be removed if a PIC interrupt
221is detected.
222
223It's said poweroff/reboot may depend on CPU0 on some machines although I haven't
224seen any poweroff/reboot failure so far after CPU0 is offline on a few tested
225machines.
226
227Please let me know if you know or see any other dependencies of CPU0.
228
229If the dependencies are under your control, you can turn on CPU0 hotplug feature
230either by CONFIG_BOOTPARAM_HOTPLUG_CPU0 or by kernel parameter cpu0_hotplug.
231
232--Fenghua Yu <fenghua.yu@intel.com>
233
234Q: How do i find out if a particular CPU is not removable?
235A: Depending on the implementation, some architectures may show this by the
236absence of the "online" file. This is done if it can be determined ahead of
237time that this CPU cannot be removed.
238
239In some situations, this can be a run time check, i.e if you try to remove the
240last CPU, this will not be permitted. You can find such failures by
241investigating the return value of the "echo" command.
242
243Q: What happens when a CPU is being logically offlined?
244A: 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
265Q: If i have some kernel code that needs to be aware of CPU arrival and
266   departure, how to i arrange for proper notification?
267A: This is what you would need in your kernel code to receive notifications.
268
269    #include <linux/cpu.h>
270    static int __cpuinit 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 __cpuinitdata foobar_cpu_notifer =
289    {
290       .notifier_call = foobar_cpu_callback,
291    };
292
293You need to call register_cpu_notifier() from your init function.
294Init functions could be of two types:
2951. early init (init function called when only the boot processor is online).
2962. late init (init function called _after_ all the CPUs are online).
297
298For the first case, you should add the following to your init function
299
300    register_cpu_notifier(&foobar_cpu_notifier);
301
302For the second case, you should add the following to your init function
303
304    register_hotcpu_notifier(&foobar_cpu_notifier);
305
306You can fail PREPARE notifiers if something doesn't work to prepare resources.
307This will stop the activity and send a following CANCELED event back.
308
309CPU_DEAD should not be failed, its just a goodness indication, but bad
310things will happen if a notifier in path sent a BAD notify code.
311
312Q: I don't see my action being called for all CPUs already up and running?
313A: 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
321Q: If i would like to develop cpu hotplug support for a new architecture,
322   what do i need at a minimum?
323A: 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
340Q: I need to ensure that a particular cpu is not removed when there is some
341   work specific to this cpu is in progress.
342A: 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
363Q: How do we determine how many CPUs are available for hotplug.
364A: 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: Today's ACPI MADT can only provide 256 entries since the apicid field
374   in MADT is only 8 bits.
375
376User Space Notification
377
378Hotplug support for devices is common in Linux today. Its being used today to
379support automatic configuration of network, usb and pci devices. A hotplug
380event can be used to invoke an agent script to perform the configuration task.
381
382You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
383scripts.
384
385    #!/bin/bash
386    # $Id: cpu.agent
387    # Kernel hotplug params include:
388    #ACTION=%s [online or offline]
389    #DEVPATH=%s
390    #
391    cd /etc/hotplug
392    . ./hotplug.functions
393
394    case $ACTION in
395        online)
396            echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt
397            ;;
398        offline)
399            echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt
400            ;;
401        *)
402            debug_mesg CPU $ACTION event not supported
403        exit 1
404        ;;
405    esac
406

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