Based on kernel version 2.6.26. Page generated on 2008-07-16 21:12 EST.
1 CPU hotplug Support in Linux(tm) Kernel 2 3 Maintainers: 4 CPU Hotplug Core: 5 Rusty Russell <rusty[AT]rustycorp.com[DOT]au> 6 Srivatsa Vaddagiri <vatsa[AT]in.ibm[DOT]com> 7 i386: 8 Zwane Mwaikambo <zwane[AT]arm.linux.org[DOT]uk> 9 ppc64: 10 Nathan Lynch <nathanl[AT]austin.ibm[DOT]com> 11 Joel Schopp <jschopp[AT]austin.ibm[DOT]com> 12 ia64/x86_64: 13 Ashok Raj <ashok.raj[AT]intel[DOT]com> 14 s390: 15 Heiko Carstens <heiko.carstens[AT]de.ibm[DOT]com> 16 17 Authors: Ashok Raj <ashok.raj[AT]intel[DOT]com> 18 Lots of feedback: Nathan Lynch <nathanl[AT]austin.ibm[DOT]com>, 19 Joel Schopp <jschopp[AT]austin.ibm[DOT]com> 20 21 Introduction 22 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. 29 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. 34 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. 39 40 General Stuff about CPU Hotplug 41 -------------------------------- 42 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. 48 49 additional_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 - x86_64, ia64 54 55 ia64 and x86_64 use the number of disabled local apics in ACPI tables MADT 56 to determine the number of potentially hot-pluggable cpus. The implementation 57 should only rely on this to count the # of cpus, but *MUST* not rely on the 58 apicid values in those tables for disabled apics. In the event BIOS doesn't 59 mark such hot-pluggable cpus as disabled entries, one could use this 60 parameter "additional_cpus=x" to represent those cpus in the cpu_possible_map. 61 62 s390 uses the number of cpus it detects at IPL time to also the number of bits 63 in cpu_possible_map. If it is desired to add additional cpus at a later time 64 the number should be specified using this option or the possible_cpus option. 65 66 possible_cpus=n [s390 only] use this to set hotpluggable cpus. 67 This option sets possible_cpus bits in 68 cpu_possible_map. Thus keeping the numbers of bits set 69 constant even if the machine gets rebooted. 70 This option overrides additional_cpus. 71 72 CPU maps and such 73 ----------------- 74 [More on cpumaps and primitive to manipulate, please check 75 include/linux/cpumask.h that has more descriptive text.] 76 77 cpu_possible_map: Bitmap of possible CPUs that can ever be available in the 78 system. This is used to allocate some boot time memory for per_cpu variables 79 that aren't designed to grow/shrink as CPUs are made available or removed. 80 Once set during boot time discovery phase, the map is static, i.e no bits 81 are added or removed anytime. Trimming it accurately for your system needs 82 upfront can save some boot time memory. See below for how we use heuristics 83 in x86_64 case to keep this under check. 84 85 cpu_online_map: Bitmap of all CPUs currently online. Its set in __cpu_up() 86 after a cpu is available for kernel scheduling and ready to receive 87 interrupts from devices. Its cleared when a cpu is brought down using 88 __cpu_disable(), before which all OS services including interrupts are 89 migrated to another target CPU. 90 91 cpu_present_map: Bitmap of CPUs currently present in the system. Not all 92 of them may be online. When physical hotplug is processed by the relevant 93 subsystem (e.g ACPI) can change and new bit either be added or removed 94 from the map depending on the event is hot-add/hot-remove. There are currently 95 no locking rules as of now. Typical usage is to init topology during boot, 96 at which time hotplug is disabled. 97 98 You really dont need to manipulate any of the system cpu maps. They should 99 be read-only for most use. When setting up per-cpu resources almost always use 100 cpu_possible_map/for_each_possible_cpu() to iterate. 101 102 Never use anything other than cpumask_t to represent bitmap of CPUs. 103 104 #include <linux/cpumask.h> 105 106 for_each_possible_cpu - Iterate over cpu_possible_map 107 for_each_online_cpu - Iterate over cpu_online_map 108 for_each_present_cpu - Iterate over cpu_present_map 109 for_each_cpu_mask(x,mask) - Iterate over some random collection of cpu mask. 110 111 #include <linux/cpu.h> 112 get_online_cpus() and put_online_cpus(): 113 114 The above calls are used to inhibit cpu hotplug operations. While the 115 cpu_hotplug.refcount is non zero, the cpu_online_map will not change. 116 If you merely need to avoid cpus going away, you could also use 117 preempt_disable() and preempt_enable() for those sections. 118 Just remember the critical section cannot call any 119 function that can sleep or schedule this process away. The preempt_disable() 120 will work as long as stop_machine_run() is used to take a cpu down. 121 122 CPU Hotplug - Frequently Asked Questions. 123 124 Q: How to enable my kernel to support CPU hotplug? 125 A: When doing make defconfig, Enable CPU hotplug support 126 127 "Processor type and Features" -> Support for Hotpluggable CPUs 128 129 Make sure that you have CONFIG_HOTPLUG, and CONFIG_SMP turned on as well. 130 131 You would need to enable CONFIG_HOTPLUG_CPU for SMP suspend/resume support 132 as well. 133 134 Q: What architectures support CPU hotplug? 135 A: As of 2.6.14, the following architectures support CPU hotplug. 136 137 i386 (Intel), ppc, ppc64, parisc, s390, ia64 and x86_64 138 139 Q: How to test if hotplug is supported on the newly built kernel? 140 A: You should now notice an entry in sysfs. 141 142 Check if sysfs is mounted, using the "mount" command. You should notice 143 an entry as shown below in the output. 144 145 .... 146 none on /sys type sysfs (rw) 147 .... 148 149 If this is not mounted, do the following. 150 151 #mkdir /sysfs 152 #mount -t sysfs sys /sys 153 154 Now you should see entries for all present cpu, the following is an example 155 in a 8-way system. 156 157 #pwd 158 #/sys/devices/system/cpu 159 #ls -l 160 total 0 161 drwxr-xr-x 10 root root 0 Sep 19 07:44 . 162 drwxr-xr-x 13 root root 0 Sep 19 07:45 .. 163 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu0 164 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu1 165 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu2 166 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu3 167 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu4 168 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu5 169 drwxr-xr-x 3 root root 0 Sep 19 07:44 cpu6 170 drwxr-xr-x 3 root root 0 Sep 19 07:48 cpu7 171 172 Under each directory you would find an "online" file which is the control 173 file to logically online/offline a processor. 174 175 Q: Does hot-add/hot-remove refer to physical add/remove of cpus? 176 A: The usage of hot-add/remove may not be very consistently used in the code. 177 CONFIG_HOTPLUG_CPU enables logical online/offline capability in the kernel. 178 To support physical addition/removal, one would need some BIOS hooks and 179 the platform should have something like an attention button in PCI hotplug. 180 CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of CPUs. 181 182 Q: How do i logically offline a CPU? 183 A: Do the following. 184 185 #echo 0 > /sys/devices/system/cpu/cpuX/online 186 187 Once the logical offline is successful, check 188 189 #cat /proc/interrupts 190 191 You should now not see the CPU that you removed. Also online file will report 192 the state as 0 when a cpu if offline and 1 when its online. 193 194 #To display the current cpu state. 195 #cat /sys/devices/system/cpu/cpuX/online 196 197 Q: Why cant i remove CPU0 on some systems? 198 A: Some architectures may have some special dependency on a certain CPU. 199 200 For e.g in IA64 platforms we have ability to sent platform interrupts to the 201 OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI 202 specifications, we didn't have a way to change the target CPU. Hence if the 203 current ACPI version doesn't support such re-direction, we disable that CPU 204 by making it not-removable. 205 206 In such cases you will also notice that the online file is missing under cpu0. 207 208 Q: How do i find out if a particular CPU is not removable? 209 A: Depending on the implementation, some architectures may show this by the 210 absence of the "online" file. This is done if it can be determined ahead of 211 time that this CPU cannot be removed. 212 213 In some situations, this can be a run time check, i.e if you try to remove the 214 last CPU, this will not be permitted. You can find such failures by 215 investigating the return value of the "echo" command. 216 217 Q: What happens when a CPU is being logically offlined? 218 A: The following happen, listed in no particular order :-) 219 220 - A notification is sent to in-kernel registered modules by sending an event 221 CPU_DOWN_PREPARE or CPU_DOWN_PREPARE_FROZEN, depending on whether or not the 222 CPU is being offlined while tasks are frozen due to a suspend operation in 223 progress 224 - All processes are migrated away from this outgoing CPU to new CPUs. 225 The new CPU is chosen from each process' current cpuset, which may be 226 a subset of all online CPUs. 227 - All interrupts targeted to this CPU is migrated to a new CPU 228 - timers/bottom half/task lets are also migrated to a new CPU 229 - Once all services are migrated, kernel calls an arch specific routine 230 __cpu_disable() to perform arch specific cleanup. 231 - Once this is successful, an event for successful cleanup is sent by an event 232 CPU_DEAD (or CPU_DEAD_FROZEN if tasks are frozen due to a suspend while the 233 CPU is being offlined). 234 235 "It is expected that each service cleans up when the CPU_DOWN_PREPARE 236 notifier is called, when CPU_DEAD is called its expected there is nothing 237 running on behalf of this CPU that was offlined" 238 239 Q: If i have some kernel code that needs to be aware of CPU arrival and 240 departure, how to i arrange for proper notification? 241 A: This is what you would need in your kernel code to receive notifications. 242 243 #include <linux/cpu.h> 244 static int __cpuinit foobar_cpu_callback(struct notifier_block *nfb, 245 unsigned long action, void *hcpu) 246 { 247 unsigned int cpu = (unsigned long)hcpu; 248 249 switch (action) { 250 case CPU_ONLINE: 251 case CPU_ONLINE_FROZEN: 252 foobar_online_action(cpu); 253 break; 254 case CPU_DEAD: 255 case CPU_DEAD_FROZEN: 256 foobar_dead_action(cpu); 257 break; 258 } 259 return NOTIFY_OK; 260 } 261 262 static struct notifier_block __cpuinitdata foobar_cpu_notifer = 263 { 264 .notifier_call = foobar_cpu_callback, 265 }; 266 267 You need to call register_cpu_notifier() from your init function. 268 Init functions could be of two types: 269 1. early init (init function called when only the boot processor is online). 270 2. late init (init function called _after_ all the CPUs are online). 271 272 For the first case, you should add the following to your init function 273 274 register_cpu_notifier(&foobar_cpu_notifier); 275 276 For the second case, you should add the following to your init function 277 278 register_hotcpu_notifier(&foobar_cpu_notifier); 279 280 You can fail PREPARE notifiers if something doesn't work to prepare resources. 281 This will stop the activity and send a following CANCELED event back. 282 283 CPU_DEAD should not be failed, its just a goodness indication, but bad 284 things will happen if a notifier in path sent a BAD notify code. 285 286 Q: I don't see my action being called for all CPUs already up and running? 287 A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined. 288 If you need to perform some action for each cpu already in the system, then 289 290 for_each_online_cpu(i) { 291 foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i); 292 foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, i); 293 } 294 295 Q: If i would like to develop cpu hotplug support for a new architecture, 296 what do i need at a minimum? 297 A: The following are what is required for CPU hotplug infrastructure to work 298 correctly. 299 300 - Make sure you have an entry in Kconfig to enable CONFIG_HOTPLUG_CPU 301 - __cpu_up() - Arch interface to bring up a CPU 302 - __cpu_disable() - Arch interface to shutdown a CPU, no more interrupts 303 can be handled by the kernel after the routine 304 returns. Including local APIC timers etc are 305 shutdown. 306 - __cpu_die() - This actually supposed to ensure death of the CPU. 307 Actually look at some example code in other arch 308 that implement CPU hotplug. The processor is taken 309 down from the idle() loop for that specific 310 architecture. __cpu_die() typically waits for some 311 per_cpu state to be set, to ensure the processor 312 dead routine is called to be sure positively. 313 314 Q: I need to ensure that a particular cpu is not removed when there is some 315 work specific to this cpu is in progress. 316 A: First switch the current thread context to preferred cpu 317 318 int my_func_on_cpu(int cpu) 319 { 320 cpumask_t saved_mask, new_mask = CPU_MASK_NONE; 321 int curr_cpu, err = 0; 322 323 saved_mask = current->cpus_allowed; 324 cpu_set(cpu, new_mask); 325 err = set_cpus_allowed(current, new_mask); 326 327 if (err) 328 return err; 329 330 /* 331 * If we got scheduled out just after the return from 332 * set_cpus_allowed() before running the work, this ensures 333 * we stay locked. 334 */ 335 curr_cpu = get_cpu(); 336 337 if (curr_cpu != cpu) { 338 err = -EAGAIN; 339 goto ret; 340 } else { 341 /* 342 * Do work : But cant sleep, since get_cpu() disables preempt 343 */ 344 } 345 ret: 346 put_cpu(); 347 set_cpus_allowed(current, saved_mask); 348 return err; 349 } 350 351 352 Q: How do we determine how many CPUs are available for hotplug. 353 A: There is no clear spec defined way from ACPI that can give us that 354 information today. Based on some input from Natalie of Unisys, 355 that the ACPI MADT (Multiple APIC Description Tables) marks those possible 356 CPUs in a system with disabled status. 357 358 Andi implemented some simple heuristics that count the number of disabled 359 CPUs in MADT as hotpluggable CPUS. In the case there are no disabled CPUS 360 we assume 1/2 the number of CPUs currently present can be hotplugged. 361 362 Caveat: Today's ACPI MADT can only provide 256 entries since the apicid field 363 in MADT is only 8 bits. 364 365 User Space Notification 366 367 Hotplug support for devices is common in Linux today. Its being used today to 368 support automatic configuration of network, usb and pci devices. A hotplug 369 event can be used to invoke an agent script to perform the configuration task. 370 371 You can add /etc/hotplug/cpu.agent to handle hotplug notification user space 372 scripts. 373 374 #!/bin/bash 375 # $Id: cpu.agent 376 # Kernel hotplug params include: 377 #ACTION=%s [online or offline] 378 #DEVPATH=%s 379 # 380 cd /etc/hotplug 381 . ./hotplug.functions 382 383 case $ACTION in 384 online) 385 echo `date` ":cpu.agent" add cpu >> /tmp/hotplug.txt 386 ;; 387 offline) 388 echo `date` ":cpu.agent" remove cpu >>/tmp/hotplug.txt 389 ;; 390 *) 391 debug_mesg CPU $ACTION event not supported 392 exit 1 393 ;; 394 esac
