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-rw-r--r--Documentation/cpu-hotplug.txt152
1 files changed, 76 insertions, 76 deletions
diff --git a/Documentation/cpu-hotplug.txt b/Documentation/cpu-hotplug.txt
index bc107cb157a8..cc60d29b954c 100644
--- a/Documentation/cpu-hotplug.txt
+++ b/Documentation/cpu-hotplug.txt
@@ -46,7 +46,7 @@ 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 46 maxcpus=2 will only boot 2. You can choose to bring the
47 other cpus later online, read FAQ's for more info. 47 other cpus later online, read FAQ's for more info.
48 48
49additional_cpus*=n Use this to limit hotpluggable cpus. This option sets 49additional_cpus=n (*) Use this to limit hotpluggable cpus. This option sets
50 cpu_possible_map = cpu_present_map + additional_cpus 50 cpu_possible_map = cpu_present_map + additional_cpus
51 51
52(*) Option valid only for following architectures 52(*) Option valid only for following architectures
@@ -54,8 +54,8 @@ additional_cpus*=n Use this to limit hotpluggable cpus. This option sets
54 54
55ia64 and x86_64 use the number of disabled local apics in ACPI tables MADT 55ia64 and x86_64 use the number of disabled local apics in ACPI tables MADT
56to determine the number of potentially hot-pluggable cpus. The implementation 56to determine the number of potentially hot-pluggable cpus. The implementation
57should only rely on this to count the #of cpus, but *MUST* not rely on the 57should only rely on this to count the # of cpus, but *MUST* not rely on the
58apicid values in those tables for disabled apics. In the event BIOS doesnt 58apicid values in those tables for disabled apics. In the event BIOS doesn't
59mark such hot-pluggable cpus as disabled entries, one could use this 59mark such hot-pluggable cpus as disabled entries, one could use this
60parameter "additional_cpus=x" to represent those cpus in the cpu_possible_map. 60parameter "additional_cpus=x" to represent those cpus in the cpu_possible_map.
61 61
@@ -101,15 +101,15 @@ cpu_possible_map/for_each_possible_cpu() to iterate.
101 101
102Never use anything other than cpumask_t to represent bitmap of CPUs. 102Never use anything other than cpumask_t to represent bitmap of CPUs.
103 103
104#include <linux/cpumask.h> 104 #include <linux/cpumask.h>
105 105
106for_each_possible_cpu - Iterate over cpu_possible_map 106 for_each_possible_cpu - Iterate over cpu_possible_map
107for_each_online_cpu - Iterate over cpu_online_map 107 for_each_online_cpu - Iterate over cpu_online_map
108for_each_present_cpu - Iterate over cpu_present_map 108 for_each_present_cpu - Iterate over cpu_present_map
109for_each_cpu_mask(x,mask) - Iterate over some random collection of cpu mask. 109 for_each_cpu_mask(x,mask) - Iterate over some random collection of cpu mask.
110 110
111#include <linux/cpu.h> 111 #include <linux/cpu.h>
112lock_cpu_hotplug() and unlock_cpu_hotplug(): 112 lock_cpu_hotplug() and unlock_cpu_hotplug():
113 113
114The above calls are used to inhibit cpu hotplug operations. While holding the 114The above calls are used to inhibit cpu hotplug operations. While holding the
115cpucontrol mutex, cpu_online_map will not change. If you merely need to avoid 115cpucontrol mutex, cpu_online_map will not change. If you merely need to avoid
@@ -120,7 +120,7 @@ will work as long as stop_machine_run() is used to take a cpu down.
120 120
121CPU Hotplug - Frequently Asked Questions. 121CPU Hotplug - Frequently Asked Questions.
122 122
123Q: How to i enable my kernel to support CPU hotplug? 123Q: How to enable my kernel to support CPU hotplug?
124A: When doing make defconfig, Enable CPU hotplug support 124A: When doing make defconfig, Enable CPU hotplug support
125 125
126 "Processor type and Features" -> Support for Hotpluggable CPUs 126 "Processor type and Features" -> Support for Hotpluggable CPUs
@@ -141,39 +141,39 @@ A: You should now notice an entry in sysfs.
141Check if sysfs is mounted, using the "mount" command. You should notice 141Check if sysfs is mounted, using the "mount" command. You should notice
142an entry as shown below in the output. 142an entry as shown below in the output.
143 143
144.... 144 ....
145none on /sys type sysfs (rw) 145 none on /sys type sysfs (rw)
146.... 146 ....
147 147
148if this is not mounted, do the following. 148If this is not mounted, do the following.
149 149
150#mkdir /sysfs 150 #mkdir /sysfs
151#mount -t sysfs sys /sys 151 #mount -t sysfs sys /sys
152 152
153now you should see entries for all present cpu, the following is an example 153Now you should see entries for all present cpu, the following is an example
154in a 8-way system. 154in a 8-way system.
155 155
156#pwd 156 #pwd
157#/sys/devices/system/cpu 157 #/sys/devices/system/cpu
158#ls -l 158 #ls -l
159total 0 159 total 0
160drwxr-xr-x 10 root root 0 Sep 19 07:44 . 160 drwxr-xr-x 10 root root 0 Sep 19 07:44 .
161drwxr-xr-x 13 root root 0 Sep 19 07:45 .. 161 drwxr-xr-x 13 root root 0 Sep 19 07:45 ..
162drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu0 162 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu0
163drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu1 163 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu1
164drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu2 164 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu2
165drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu3 165 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu3
166drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu4 166 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu4
167drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu5 167 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu5
168drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu6 168 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu6
169drwxr-xr-x 3 root root 0 Sep 19 07:48 cpu7 169 drwxr-xr-x 3 root root 0 Sep 19 07:48 cpu7
170 170
171Under each directory you would find an "online" file which is the control 171Under each directory you would find an "online" file which is the control
172file to logically online/offline a processor. 172file to logically online/offline a processor.
173 173
174Q: Does hot-add/hot-remove refer to physical add/remove of cpus? 174Q: Does hot-add/hot-remove refer to physical add/remove of cpus?
175A: The usage of hot-add/remove may not be very consistently used in the code. 175A: The usage of hot-add/remove may not be very consistently used in the code.
176CONFIG_CPU_HOTPLUG enables logical online/offline capability in the kernel. 176CONFIG_HOTPLUG_CPU enables logical online/offline capability in the kernel.
177To support physical addition/removal, one would need some BIOS hooks and 177To support physical addition/removal, one would need some BIOS hooks and
178the platform should have something like an attention button in PCI hotplug. 178the platform should have something like an attention button in PCI hotplug.
179CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs. 179CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.
@@ -181,17 +181,17 @@ CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs.
181Q: How do i logically offline a CPU? 181Q: How do i logically offline a CPU?
182A: Do the following. 182A: Do the following.
183 183
184#echo 0 > /sys/devices/system/cpu/cpuX/online 184 #echo 0 > /sys/devices/system/cpu/cpuX/online
185 185
186once the logical offline is successful, check 186Once the logical offline is successful, check
187 187
188#cat /proc/interrupts 188 #cat /proc/interrupts
189 189
190you should now not see the CPU that you removed. Also online file will report 190You should now not see the CPU that you removed. Also online file will report
191the state as 0 when a cpu if offline and 1 when its online. 191the state as 0 when a cpu if offline and 1 when its online.
192 192
193#To display the current cpu state. 193 #To display the current cpu state.
194#cat /sys/devices/system/cpu/cpuX/online 194 #cat /sys/devices/system/cpu/cpuX/online
195 195
196Q: Why cant i remove CPU0 on some systems? 196Q: Why cant i remove CPU0 on some systems?
197A: Some architectures may have some special dependency on a certain CPU. 197A: Some architectures may have some special dependency on a certain CPU.
@@ -234,8 +234,8 @@ Q: If i have some kernel code that needs to be aware of CPU arrival and
234 departure, how to i arrange for proper notification? 234 departure, how to i arrange for proper notification?
235A: This is what you would need in your kernel code to receive notifications. 235A: This is what you would need in your kernel code to receive notifications.
236 236
237 #include <linux/cpu.h> 237 #include <linux/cpu.h>
238 static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb, 238 static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb,
239 unsigned long action, void *hcpu) 239 unsigned long action, void *hcpu)
240 { 240 {
241 unsigned int cpu = (unsigned long)hcpu; 241 unsigned int cpu = (unsigned long)hcpu;
@@ -279,10 +279,10 @@ Q: I don't see my action being called for all CPUs already up and running?
279A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined. 279A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined.
280 If you need to perform some action for each cpu already in the system, then 280 If you need to perform some action for each cpu already in the system, then
281 281
282 for_each_online_cpu(i) { 282 for_each_online_cpu(i) {
283 foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i); 283 foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
284 foobar_cpu_callback(&foobar-cpu_notifier, CPU_ONLINE, i); 284 foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, i);
285 } 285 }
286 286
287Q: If i would like to develop cpu hotplug support for a new architecture, 287Q: If i would like to develop cpu hotplug support for a new architecture,
288 what do i need at a minimum? 288 what do i need at a minimum?
@@ -307,38 +307,38 @@ Q: I need to ensure that a particular cpu is not removed when there is some
307 work specific to this cpu is in progress. 307 work specific to this cpu is in progress.
308A: First switch the current thread context to preferred cpu 308A: First switch the current thread context to preferred cpu
309 309
310 int my_func_on_cpu(int cpu) 310 int my_func_on_cpu(int cpu)
311 { 311 {
312 cpumask_t saved_mask, new_mask = CPU_MASK_NONE; 312 cpumask_t saved_mask, new_mask = CPU_MASK_NONE;
313 int curr_cpu, err = 0; 313 int curr_cpu, err = 0;
314 314
315 saved_mask = current->cpus_allowed; 315 saved_mask = current->cpus_allowed;
316 cpu_set(cpu, new_mask); 316 cpu_set(cpu, new_mask);
317 err = set_cpus_allowed(current, new_mask); 317 err = set_cpus_allowed(current, new_mask);
318 318
319 if (err) 319 if (err)
320 return err; 320 return err;
321 321
322 /* 322 /*
323 * If we got scheduled out just after the return from 323 * If we got scheduled out just after the return from
324 * set_cpus_allowed() before running the work, this ensures 324 * set_cpus_allowed() before running the work, this ensures
325 * we stay locked. 325 * we stay locked.
326 */ 326 */
327 curr_cpu = get_cpu(); 327 curr_cpu = get_cpu();
328 328
329 if (curr_cpu != cpu) { 329 if (curr_cpu != cpu) {
330 err = -EAGAIN; 330 err = -EAGAIN;
331 goto ret; 331 goto ret;
332 } else { 332 } else {
333 /* 333 /*
334 * Do work : But cant sleep, since get_cpu() disables preempt 334 * Do work : But cant sleep, since get_cpu() disables preempt
335 */ 335 */
336 } 336 }
337 ret: 337 ret:
338 put_cpu(); 338 put_cpu();
339 set_cpus_allowed(current, saved_mask); 339 set_cpus_allowed(current, saved_mask);
340 return err; 340 return err;
341 } 341 }
342 342
343 343
344Q: How do we determine how many CPUs are available for hotplug. 344Q: How do we determine how many CPUs are available for hotplug.