Root/Documentation/cpu-hotplug.txt

1        CPU hotplug Support in Linux(tm) Kernel
2
3        Maintainers:
4        CPU Hotplug Core:
5            Rusty Russell <rusty@rustycorp.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_map = cpu_present_map + additional_cpus
51
52(*) Option valid only for following architectures
53- ia64
54
55ia64 uses the number of disabled local apics in ACPI tables MADT to
56determine the number of potentially hot-pluggable cpus. The implementation
57should only rely on this to count the # of cpus, but *MUST* not rely
58on the apicid values in those tables for disabled apics. In the event
59BIOS doesn't mark such hot-pluggable cpus as disabled entries, one could
60use this parameter "additional_cpus=x" to represent those cpus in the
61cpu_possible_map.
62
63possible_cpus=n [s390,x86_64] use this to set hotpluggable cpus.
64            This option sets possible_cpus bits in
65            cpu_possible_map. Thus keeping the numbers of bits set
66            constant even if the machine gets rebooted.
67
68CPU maps and such
69-----------------
70[More on cpumaps and primitive to manipulate, please check
71include/linux/cpumask.h that has more descriptive text.]
72
73cpu_possible_map: Bitmap of possible CPUs that can ever be available in the
74system. This is used to allocate some boot time memory for per_cpu variables
75that aren't designed to grow/shrink as CPUs are made available or removed.
76Once set during boot time discovery phase, the map is static, i.e no bits
77are added or removed anytime. Trimming it accurately for your system needs
78upfront can save some boot time memory. See below for how we use heuristics
79in x86_64 case to keep this under check.
80
81cpu_online_map: Bitmap of all CPUs currently online. Its set in __cpu_up()
82after a cpu is available for kernel scheduling and ready to receive
83interrupts from devices. Its cleared when a cpu is brought down using
84__cpu_disable(), before which all OS services including interrupts are
85migrated to another target CPU.
86
87cpu_present_map: Bitmap of CPUs currently present in the system. Not all
88of them may be online. When physical hotplug is processed by the relevant
89subsystem (e.g ACPI) can change and new bit either be added or removed
90from the map depending on the event is hot-add/hot-remove. There are currently
91no locking rules as of now. Typical usage is to init topology during boot,
92at which time hotplug is disabled.
93
94You really dont need to manipulate any of the system cpu maps. They should
95be read-only for most use. When setting up per-cpu resources almost always use
96cpu_possible_map/for_each_possible_cpu() to iterate.
97
98Never use anything other than cpumask_t to represent bitmap of CPUs.
99
100    #include <linux/cpumask.h>
101
102    for_each_possible_cpu - Iterate over cpu_possible_map
103    for_each_online_cpu - Iterate over cpu_online_map
104    for_each_present_cpu - Iterate over cpu_present_map
105    for_each_cpu_mask(x,mask) - Iterate over some random collection of cpu mask.
106
107    #include <linux/cpu.h>
108    get_online_cpus() and put_online_cpus():
109
110The above calls are used to inhibit cpu hotplug operations. While the
111cpu_hotplug.refcount is non zero, the cpu_online_map will not change.
112If you merely need to avoid cpus going away, you could also use
113preempt_disable() and preempt_enable() for those sections.
114Just remember the critical section cannot call any
115function that can sleep or schedule this process away. The preempt_disable()
116will work as long as stop_machine_run() is used to take a cpu down.
117
118CPU Hotplug - Frequently Asked Questions.
119
120Q: How to enable my kernel to support CPU hotplug?
121A: When doing make defconfig, Enable CPU hotplug support
122
123   "Processor type and Features" -> Support for Hotpluggable CPUs
124
125Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well.
126
127You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support
128as well.
129
130Q: What architectures support CPU hotplug?
131A: As of 2.6.14, the following architectures support CPU hotplug.
132
133i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64
134
135Q: How to test if hotplug is supported on the newly built kernel?
136A: You should now notice an entry in sysfs.
137
138Check if sysfs is mounted, using the "mount" command. You should notice
139an entry as shown below in the output.
140
141    ....
142    none on /sys type sysfs (rw)
143    ....
144
145If this is not mounted, do the following.
146
147     #mkdir /sysfs
148    #mount -t sysfs sys /sys
149
150Now you should see entries for all present cpu, the following is an example
151in a 8-way system.
152
153    #pwd
154    #/sys/devices/system/cpu
155    #ls -l
156    total 0
157    drwxr-xr-x 10 root root 0 Sep 19 07:44 .
158    drwxr-xr-x 13 root root 0 Sep 19 07:45 ..
159    drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu0
160    drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu1
161    drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu2
162    drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu3
163    drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu4
164    drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu5
165    drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu6
166    drwxr-xr-x 3 root root 0 Sep 19 07:48 cpu7
167
168Under each directory you would find an "online" file which is the control
169file to logically online/offline a processor.
170
171Q: Does hot-add/hot-remove refer to physical add/remove of cpus?
172A: The usage of hot-add/remove may not be very consistently used in the code.
173CONFIG_HOTPLUG_CPU enables logical online/offline capability in the kernel.
174To support physical addition/removal, one would need some BIOS hooks and
175the platform should have something like an attention button in PCI hotplug.
176CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.
177
178Q: How do i logically offline a CPU?
179A: Do the following.
180
181    #echo 0 > /sys/devices/system/cpu/cpuX/online
182
183Once the logical offline is successful, check
184
185    #cat /proc/interrupts
186
187You should now not see the CPU that you removed. Also online file will report
188the state as 0 when a cpu if offline and 1 when its online.
189
190    #To display the current cpu state.
191    #cat /sys/devices/system/cpu/cpuX/online
192
193Q: Why cant i remove CPU0 on some systems?
194A: Some architectures may have some special dependency on a certain CPU.
195
196For e.g in IA64 platforms we have ability to sent platform interrupts to the
197OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
198specifications, we didn't have a way to change the target CPU. Hence if the
199current ACPI version doesn't support such re-direction, we disable that CPU
200by making it not-removable.
201
202In such cases you will also notice that the online file is missing under cpu0.
203
204Q: How do i find out if a particular CPU is not removable?
205A: Depending on the implementation, some architectures may show this by the
206absence of the "online" file. This is done if it can be determined ahead of
207time that this CPU cannot be removed.
208
209In some situations, this can be a run time check, i.e if you try to remove the
210last CPU, this will not be permitted. You can find such failures by
211investigating the return value of the "echo" command.
212
213Q: What happens when a CPU is being logically offlined?
214A: The following happen, listed in no particular order :-)
215
216- A notification is sent to in-kernel registered modules by sending an event
217  CPU_DOWN_PREPARE or CPU_DOWN_PREPARE_FROZEN, depending on whether or not the
218  CPU is being offlined while tasks are frozen due to a suspend operation in
219  progress
220- All processes are migrated away from this outgoing CPU to new CPUs.
221  The new CPU is chosen from each process' current cpuset, which may be
222  a subset of all online CPUs.
223- All interrupts targeted to this CPU is migrated to a new CPU
224- timers/bottom half/task lets are also migrated to a new CPU
225- Once all services are migrated, kernel calls an arch specific routine
226  __cpu_disable() to perform arch specific cleanup.
227- Once this is successful, an event for successful cleanup is sent by an event
228  CPU_DEAD (or CPU_DEAD_FROZEN if tasks are frozen due to a suspend while the
229  CPU is being offlined).
230
231  "It is expected that each service cleans up when the CPU_DOWN_PREPARE
232  notifier is called, when CPU_DEAD is called its expected there is nothing
233  running on behalf of this CPU that was offlined"
234
235Q: If i have some kernel code that needs to be aware of CPU arrival and
236   departure, how to i arrange for proper notification?
237A: This is what you would need in your kernel code to receive notifications.
238
239    #include <linux/cpu.h>
240    static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb,
241                        unsigned long action, void *hcpu)
242    {
243        unsigned int cpu = (unsigned long)hcpu;
244
245        switch (action) {
246        case CPU_ONLINE:
247        case CPU_ONLINE_FROZEN:
248            foobar_online_action(cpu);
249            break;
250        case CPU_DEAD:
251        case CPU_DEAD_FROZEN:
252            foobar_dead_action(cpu);
253            break;
254        }
255        return NOTIFY_OK;
256    }
257
258    static struct notifier_block __cpuinitdata foobar_cpu_notifer =
259    {
260       .notifier_call = foobar_cpu_callback,
261    };
262
263You need to call register_cpu_notifier() from your init function.
264Init functions could be of two types:
2651. early init (init function called when only the boot processor is online).
2662. late init (init function called _after_ all the CPUs are online).
267
268For the first case, you should add the following to your init function
269
270    register_cpu_notifier(&foobar_cpu_notifier);
271
272For the second case, you should add the following to your init function
273
274    register_hotcpu_notifier(&foobar_cpu_notifier);
275
276You can fail PREPARE notifiers if something doesn't work to prepare resources.
277This will stop the activity and send a following CANCELED event back.
278
279CPU_DEAD should not be failed, its just a goodness indication, but bad
280things will happen if a notifier in path sent a BAD notify code.
281
282Q: I don't see my action being called for all CPUs already up and running?
283A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined.
284   If you need to perform some action for each cpu already in the system, then
285
286    for_each_online_cpu(i) {
287        foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
288        foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, i);
289    }
290
291Q: If i would like to develop cpu hotplug support for a new architecture,
292   what do i need at a minimum?
293A: The following are what is required for CPU hotplug infrastructure to work
294   correctly.
295
296    - Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU
297    - __cpu_up() - Arch interface to bring up a CPU
298    - __cpu_disable() - Arch interface to shutdown a CPU, no more interrupts
299                          can be handled by the kernel after the routine
300                          returns. Including local APIC timers etc are
301                          shutdown.
302     - __cpu_die() - This actually supposed to ensure death of the CPU.
303                          Actually look at some example code in other arch
304                          that implement CPU hotplug. The processor is taken
305                          down from the idle() loop for that specific
306                          architecture. __cpu_die() typically waits for some
307                          per_cpu state to be set, to ensure the processor
308                          dead routine is called to be sure positively.
309
310Q: I need to ensure that a particular cpu is not removed when there is some
311   work specific to this cpu is in progress.
312A: First switch the current thread context to preferred cpu
313
314    int my_func_on_cpu(int cpu)
315    {
316        cpumask_t saved_mask, new_mask = CPU_MASK_NONE;
317        int curr_cpu, err = 0;
318
319        saved_mask = current->cpus_allowed;
320        cpu_set(cpu, new_mask);
321        err = set_cpus_allowed(current, new_mask);
322
323        if (err)
324            return err;
325
326        /*
327         * If we got scheduled out just after the return from
328         * set_cpus_allowed() before running the work, this ensures
329         * we stay locked.
330         */
331        curr_cpu = get_cpu();
332
333        if (curr_cpu != cpu) {
334            err = -EAGAIN;
335            goto ret;
336        } else {
337            /*
338             * Do work : But cant sleep, since get_cpu() disables preempt
339             */
340        }
341        ret:
342            put_cpu();
343            set_cpus_allowed(current, saved_mask);
344            return err;
345        }
346
347
348Q: How do we determine how many CPUs are available for hotplug.
349A: There is no clear spec defined way from ACPI that can give us that
350   information today. Based on some input from Natalie of Unisys,
351   that the ACPI MADT (Multiple APIC Description Tables) marks those possible
352   CPUs in a system with disabled status.
353
354   Andi implemented some simple heuristics that count the number of disabled
355   CPUs in MADT as hotpluggable CPUS. In the case there are no disabled CPUS
356   we assume 1/2 the number of CPUs currently present can be hotplugged.
357
358   Caveat: Today's ACPI MADT can only provide 256 entries since the apicid field
359   in MADT is only 8 bits.
360
361User Space Notification
362
363Hotplug support for devices is common in Linux today. Its being used today to
364support automatic configuration of network, usb and pci devices. A hotplug
365event can be used to invoke an agent script to perform the configuration task.
366
367You can add /etc/hotplug/cpu.agent to handle hotplug notification user space
368scripts.
369
370    #!/bin/bash
371    # $Id: cpu.agent
372    # Kernel hotplug params include:
373    #ACTION=%s [online or offline]
374    #DEVPATH=%s
375    #
376    cd /etc/hotplug
377    . ./hotplug.functions
378
379    case $ACTION in
380        online)
381            echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt
382            ;;
383        offline)
384            echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt
385            ;;
386        *)
387            debug_mesg CPU $ACTION event not supported
388        exit 1
389        ;;
390    esac
391

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