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1 | ============== |
2 | Memory Hotplug | |
3 | ============== | |
4 | ||
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5 | Created: Jul 28 2007 |
6 | Add description of notifier of memory hotplug Oct 11 2007 | |
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7 | |
8 | This document is about memory hotplug including how-to-use and current status. | |
9 | Because Memory Hotplug is still under development, contents of this text will | |
10 | be changed often. | |
11 | ||
12 | 1. Introduction | |
13 | 1.1 purpose of memory hotplug | |
14 | 1.2. Phases of memory hotplug | |
15 | 1.3. Unit of Memory online/offline operation | |
16 | 2. Kernel Configuration | |
17 | 3. sysfs files for memory hotplug | |
18 | 4. Physical memory hot-add phase | |
19 | 4.1 Hardware(Firmware) Support | |
20 | 4.2 Notify memory hot-add event by hand | |
21 | 5. Logical Memory hot-add phase | |
22 | 5.1. State of memory | |
23 | 5.2. How to online memory | |
24 | 6. Logical memory remove | |
25 | 6.1 Memory offline and ZONE_MOVABLE | |
26 | 6.2. How to offline memory | |
27 | 7. Physical memory remove | |
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28 | 8. Memory hotplug event notifier |
29 | 9. Future Work List | |
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30 | |
31 | Note(1): x86_64's has special implementation for memory hotplug. | |
32 | This text does not describe it. | |
33 | Note(2): This text assumes that sysfs is mounted at /sys. | |
34 | ||
35 | ||
36 | --------------- | |
37 | 1. Introduction | |
38 | --------------- | |
39 | ||
40 | 1.1 purpose of memory hotplug | |
41 | ------------ | |
42 | Memory Hotplug allows users to increase/decrease the amount of memory. | |
43 | Generally, there are two purposes. | |
44 | ||
45 | (A) For changing the amount of memory. | |
46 | This is to allow a feature like capacity on demand. | |
47 | (B) For installing/removing DIMMs or NUMA-nodes physically. | |
48 | This is to exchange DIMMs/NUMA-nodes, reduce power consumption, etc. | |
49 | ||
50 | (A) is required by highly virtualized environments and (B) is required by | |
51 | hardware which supports memory power management. | |
52 | ||
53 | Linux memory hotplug is designed for both purpose. | |
54 | ||
55 | ||
56 | 1.2. Phases of memory hotplug | |
57 | --------------- | |
58 | There are 2 phases in Memory Hotplug. | |
59 | 1) Physical Memory Hotplug phase | |
60 | 2) Logical Memory Hotplug phase. | |
61 | ||
62 | The First phase is to communicate hardware/firmware and make/erase | |
63 | environment for hotplugged memory. Basically, this phase is necessary | |
64 | for the purpose (B), but this is good phase for communication between | |
65 | highly virtualized environments too. | |
66 | ||
67 | When memory is hotplugged, the kernel recognizes new memory, makes new memory | |
68 | management tables, and makes sysfs files for new memory's operation. | |
69 | ||
70 | If firmware supports notification of connection of new memory to OS, | |
71 | this phase is triggered automatically. ACPI can notify this event. If not, | |
72 | "probe" operation by system administration is used instead. | |
73 | (see Section 4.). | |
74 | ||
75 | Logical Memory Hotplug phase is to change memory state into | |
19f59460 | 76 | available/unavailable for users. Amount of memory from user's view is |
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77 | changed by this phase. The kernel makes all memory in it as free pages |
78 | when a memory range is available. | |
79 | ||
80 | In this document, this phase is described as online/offline. | |
81 | ||
19f59460 | 82 | Logical Memory Hotplug phase is triggered by write of sysfs file by system |
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83 | administrator. For the hot-add case, it must be executed after Physical Hotplug |
84 | phase by hand. | |
85 | (However, if you writes udev's hotplug scripts for memory hotplug, these | |
86 | phases can be execute in seamless way.) | |
87 | ||
88 | ||
89 | 1.3. Unit of Memory online/offline operation | |
90 | ------------ | |
91 | Memory hotplug uses SPARSEMEM memory model. SPARSEMEM divides the whole memory | |
92 | into chunks of the same size. The chunk is called a "section". The size of | |
93 | a section is architecture dependent. For example, power uses 16MiB, ia64 uses | |
94 | 1GiB. The unit of online/offline operation is "one section". (see Section 3.) | |
95 | ||
96 | To determine the size of sections, please read this file: | |
97 | ||
98 | /sys/devices/system/memory/block_size_bytes | |
99 | ||
100 | This file shows the size of sections in byte. | |
101 | ||
102 | ----------------------- | |
103 | 2. Kernel Configuration | |
104 | ----------------------- | |
105 | To use memory hotplug feature, kernel must be compiled with following | |
106 | config options. | |
107 | ||
108 | - For all memory hotplug | |
109 | Memory model -> Sparse Memory (CONFIG_SPARSEMEM) | |
110 | Allow for memory hot-add (CONFIG_MEMORY_HOTPLUG) | |
111 | ||
112 | - To enable memory removal, the followings are also necessary | |
113 | Allow for memory hot remove (CONFIG_MEMORY_HOTREMOVE) | |
114 | Page Migration (CONFIG_MIGRATION) | |
115 | ||
116 | - For ACPI memory hotplug, the followings are also necessary | |
117 | Memory hotplug (under ACPI Support menu) (CONFIG_ACPI_HOTPLUG_MEMORY) | |
118 | This option can be kernel module. | |
119 | ||
120 | - As a related configuration, if your box has a feature of NUMA-node hotplug | |
121 | via ACPI, then this option is necessary too. | |
122 | ACPI0004,PNP0A05 and PNP0A06 Container Driver (under ACPI Support menu) | |
123 | (CONFIG_ACPI_CONTAINER). | |
124 | This option can be kernel module too. | |
125 | ||
126 | -------------------------------- | |
c04fc586 | 127 | 4 sysfs files for memory hotplug |
6867c931 | 128 | -------------------------------- |
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129 | All sections have their device information in sysfs. Each section is part of |
130 | a memory block under /sys/devices/system/memory as | |
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131 | |
132 | /sys/devices/system/memory/memoryXXX | |
0c2c99b1 | 133 | (XXX is the section id.) |
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135 | Now, XXX is defined as (start_address_of_section / section_size) of the first |
136 | section contained in the memory block. The files 'phys_index' and | |
137 | 'end_phys_index' under each directory report the beginning and end section id's | |
138 | for the memory block covered by the sysfs directory. It is expected that all | |
139 | memory sections in this range are present and no memory holes exist in the | |
140 | range. Currently there is no way to determine if there is a memory hole, but | |
141 | the existence of one should not affect the hotplug capabilities of the memory | |
142 | block. | |
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143 | |
144 | For example, assume 1GiB section size. A device for a memory starting at | |
145 | 0x100000000 is /sys/device/system/memory/memory4 | |
146 | (0x100000000 / 1Gib = 4) | |
147 | This device covers address range [0x100000000 ... 0x140000000) | |
148 | ||
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149 | Under each section, you can see 4 or 5 files, the end_phys_index file being |
150 | a recent addition and not present on older kernels. | |
6867c931 | 151 | |
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152 | /sys/devices/system/memory/memoryXXX/start_phys_index |
153 | /sys/devices/system/memory/memoryXXX/end_phys_index | |
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154 | /sys/devices/system/memory/memoryXXX/phys_device |
155 | /sys/devices/system/memory/memoryXXX/state | |
c04fc586 | 156 | /sys/devices/system/memory/memoryXXX/removable |
6867c931 | 157 | |
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158 | 'phys_index' : read-only and contains section id of the first section |
159 | in the memory block, same as XXX. | |
160 | 'end_phys_index' : read-only and contains section id of the last section | |
161 | in the memory block. | |
162 | 'state' : read-write | |
163 | at read: contains online/offline state of memory. | |
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164 | at write: user can specify "online_kernel", |
165 | "online_movable", "online", "offline" command | |
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166 | which will be performed on al sections in the block. |
167 | 'phys_device' : read-only: designed to show the name of physical memory | |
168 | device. This is not well implemented now. | |
169 | 'removable' : read-only: contains an integer value indicating | |
170 | whether the memory block is removable or not | |
171 | removable. A value of 1 indicates that the memory | |
172 | block is removable and a value of 0 indicates that | |
173 | it is not removable. A memory block is removable only if | |
174 | every section in the block is removable. | |
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175 | |
176 | NOTE: | |
177 | These directories/files appear after physical memory hotplug phase. | |
178 | ||
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179 | If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed |
180 | via symbolic links located in the /sys/devices/system/node/node* directories. | |
181 | ||
182 | For example: | |
c04fc586 | 183 | /sys/devices/system/node/node0/memory9 -> ../../memory/memory9 |
6867c931 | 184 | |
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185 | A backlink will also be created: |
186 | /sys/devices/system/memory/memory9/node0 -> ../../node/node0 | |
187 | ||
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188 | -------------------------------- |
189 | 4. Physical memory hot-add phase | |
190 | -------------------------------- | |
191 | ||
192 | 4.1 Hardware(Firmware) Support | |
193 | ------------ | |
194 | On x86_64/ia64 platform, memory hotplug by ACPI is supported. | |
195 | ||
196 | In general, the firmware (ACPI) which supports memory hotplug defines | |
197 | memory class object of _HID "PNP0C80". When a notify is asserted to PNP0C80, | |
198 | Linux's ACPI handler does hot-add memory to the system and calls a hotplug udev | |
199 | script. This will be done automatically. | |
200 | ||
201 | But scripts for memory hotplug are not contained in generic udev package(now). | |
202 | You may have to write it by yourself or online/offline memory by hand. | |
203 | Please see "How to online memory", "How to offline memory" in this text. | |
204 | ||
205 | If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004", | |
206 | "PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handler | |
207 | calls hotplug code for all of objects which are defined in it. | |
208 | If memory device is found, memory hotplug code will be called. | |
209 | ||
210 | ||
211 | 4.2 Notify memory hot-add event by hand | |
212 | ------------ | |
213 | In some environments, especially virtualized environment, firmware will not | |
214 | notify memory hotplug event to the kernel. For such environment, "probe" | |
215 | interface is supported. This interface depends on CONFIG_ARCH_MEMORY_PROBE. | |
216 | ||
217 | Now, CONFIG_ARCH_MEMORY_PROBE is supported only by powerpc but it does not | |
218 | contain highly architecture codes. Please add config if you need "probe" | |
219 | interface. | |
220 | ||
221 | Probe interface is located at | |
222 | /sys/devices/system/memory/probe | |
223 | ||
224 | You can tell the physical address of new memory to the kernel by | |
225 | ||
226 | % echo start_address_of_new_memory > /sys/devices/system/memory/probe | |
227 | ||
228 | Then, [start_address_of_new_memory, start_address_of_new_memory + section_size) | |
229 | memory range is hot-added. In this case, hotplug script is not called (in | |
230 | current implementation). You'll have to online memory by yourself. | |
231 | Please see "How to online memory" in this text. | |
232 | ||
233 | ||
234 | ||
235 | ------------------------------ | |
236 | 5. Logical Memory hot-add phase | |
237 | ------------------------------ | |
238 | ||
239 | 5.1. State of memory | |
240 | ------------ | |
241 | To see (online/offline) state of memory section, read 'state' file. | |
242 | ||
243 | % cat /sys/device/system/memory/memoryXXX/state | |
244 | ||
245 | ||
246 | If the memory section is online, you'll read "online". | |
247 | If the memory section is offline, you'll read "offline". | |
248 | ||
249 | ||
250 | 5.2. How to online memory | |
251 | ------------ | |
252 | Even if the memory is hot-added, it is not at ready-to-use state. | |
253 | For using newly added memory, you have to "online" the memory section. | |
254 | ||
255 | For onlining, you have to write "online" to the section's state file as: | |
256 | ||
257 | % echo online > /sys/devices/system/memory/memoryXXX/state | |
258 | ||
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259 | This onlining will not change the ZONE type of the target memory section, |
260 | If the memory section is in ZONE_NORMAL, you can change it to ZONE_MOVABLE: | |
261 | ||
262 | % echo online_movable > /sys/devices/system/memory/memoryXXX/state | |
263 | (NOTE: current limit: this memory section must be adjacent to ZONE_MOVABLE) | |
264 | ||
265 | And if the memory section is in ZONE_MOVABLE, you can change it to ZONE_NORMAL: | |
266 | ||
267 | % echo online_kernel > /sys/devices/system/memory/memoryXXX/state | |
268 | (NOTE: current limit: this memory section must be adjacent to ZONE_NORMAL) | |
269 | ||
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270 | After this, section memoryXXX's state will be 'online' and the amount of |
271 | available memory will be increased. | |
272 | ||
273 | Currently, newly added memory is added as ZONE_NORMAL (for powerpc, ZONE_DMA). | |
274 | This may be changed in future. | |
275 | ||
276 | ||
277 | ||
278 | ------------------------ | |
279 | 6. Logical memory remove | |
280 | ------------------------ | |
281 | ||
282 | 6.1 Memory offline and ZONE_MOVABLE | |
283 | ------------ | |
284 | Memory offlining is more complicated than memory online. Because memory offline | |
285 | has to make the whole memory section be unused, memory offline can fail if | |
286 | the section includes memory which cannot be freed. | |
287 | ||
288 | In general, memory offline can use 2 techniques. | |
289 | ||
290 | (1) reclaim and free all memory in the section. | |
291 | (2) migrate all pages in the section. | |
292 | ||
293 | In the current implementation, Linux's memory offline uses method (2), freeing | |
294 | all pages in the section by page migration. But not all pages are | |
295 | migratable. Under current Linux, migratable pages are anonymous pages and | |
296 | page caches. For offlining a section by migration, the kernel has to guarantee | |
297 | that the section contains only migratable pages. | |
298 | ||
299 | Now, a boot option for making a section which consists of migratable pages is | |
300 | supported. By specifying "kernelcore=" or "movablecore=" boot option, you can | |
301 | create ZONE_MOVABLE...a zone which is just used for movable pages. | |
302 | (See also Documentation/kernel-parameters.txt) | |
303 | ||
304 | Assume the system has "TOTAL" amount of memory at boot time, this boot option | |
305 | creates ZONE_MOVABLE as following. | |
306 | ||
307 | 1) When kernelcore=YYYY boot option is used, | |
308 | Size of memory not for movable pages (not for offline) is YYYY. | |
309 | Size of memory for movable pages (for offline) is TOTAL-YYYY. | |
310 | ||
311 | 2) When movablecore=ZZZZ boot option is used, | |
312 | Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ. | |
313 | Size of memory for movable pages (for offline) is ZZZZ. | |
314 | ||
315 | ||
316 | Note) Unfortunately, there is no information to show which section belongs | |
317 | to ZONE_MOVABLE. This is TBD. | |
318 | ||
319 | ||
320 | 6.2. How to offline memory | |
321 | ------------ | |
322 | You can offline a section by using the same sysfs interface that was used in | |
323 | memory onlining. | |
324 | ||
325 | % echo offline > /sys/devices/system/memory/memoryXXX/state | |
326 | ||
327 | If offline succeeds, the state of the memory section is changed to be "offline". | |
328 | If it fails, some error core (like -EBUSY) will be returned by the kernel. | |
329 | Even if a section does not belong to ZONE_MOVABLE, you can try to offline it. | |
330 | If it doesn't contain 'unmovable' memory, you'll get success. | |
331 | ||
332 | A section under ZONE_MOVABLE is considered to be able to be offlined easily. | |
333 | But under some busy state, it may return -EBUSY. Even if a memory section | |
334 | cannot be offlined due to -EBUSY, you can retry offlining it and may be able to | |
335 | offline it (or not). | |
336 | (For example, a page is referred to by some kernel internal call and released | |
337 | soon.) | |
338 | ||
339 | Consideration: | |
340 | Memory hotplug's design direction is to make the possibility of memory offlining | |
341 | higher and to guarantee unplugging memory under any situation. But it needs | |
342 | more work. Returning -EBUSY under some situation may be good because the user | |
343 | can decide to retry more or not by himself. Currently, memory offlining code | |
344 | does some amount of retry with 120 seconds timeout. | |
345 | ||
346 | ------------------------- | |
347 | 7. Physical memory remove | |
348 | ------------------------- | |
349 | Need more implementation yet.... | |
350 | - Notification completion of remove works by OS to firmware. | |
351 | - Guard from remove if not yet. | |
352 | ||
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353 | -------------------------------- |
354 | 8. Memory hotplug event notifier | |
355 | -------------------------------- | |
c94bed8e | 356 | Memory hotplug has event notifier. There are 6 types of notification. |
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357 | |
358 | MEMORY_GOING_ONLINE | |
359 | Generated before new memory becomes available in order to be able to | |
360 | prepare subsystems to handle memory. The page allocator is still unable | |
361 | to allocate from the new memory. | |
362 | ||
363 | MEMORY_CANCEL_ONLINE | |
364 | Generated if MEMORY_GOING_ONLINE fails. | |
365 | ||
366 | MEMORY_ONLINE | |
19f59460 | 367 | Generated when memory has successfully brought online. The callback may |
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368 | allocate pages from the new memory. |
369 | ||
370 | MEMORY_GOING_OFFLINE | |
371 | Generated to begin the process of offlining memory. Allocations are no | |
372 | longer possible from the memory but some of the memory to be offlined | |
373 | is still in use. The callback can be used to free memory known to a | |
374 | subsystem from the indicated memory section. | |
375 | ||
376 | MEMORY_CANCEL_OFFLINE | |
377 | Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from | |
378 | the section that we attempted to offline. | |
379 | ||
380 | MEMORY_OFFLINE | |
381 | Generated after offlining memory is complete. | |
382 | ||
383 | A callback routine can be registered by | |
384 | hotplug_memory_notifier(callback_func, priority) | |
385 | ||
386 | The second argument of callback function (action) is event types of above. | |
387 | The third argument is passed by pointer of struct memory_notify. | |
388 | ||
389 | struct memory_notify { | |
390 | unsigned long start_pfn; | |
391 | unsigned long nr_pages; | |
d9713679 | 392 | int status_change_nid_normal; |
6715ddf9 | 393 | int status_change_nid_high; |
19f59460 | 394 | int status_change_nid; |
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395 | } |
396 | ||
397 | start_pfn is start_pfn of online/offline memory. | |
398 | nr_pages is # of pages of online/offline memory. | |
d9713679 LJ |
399 | status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask |
400 | is (will be) set/clear, if this is -1, then nodemask status is not changed. | |
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401 | status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask |
402 | is (will be) set/clear, if this is -1, then nodemask status is not changed. | |
403 | status_change_nid is set node id when N_MEMORY of nodemask is (will be) | |
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404 | set/clear. It means a new(memoryless) node gets new memory by online and a |
405 | node loses all memory. If this is -1, then nodemask status is not changed. | |
d9713679 | 406 | If status_changed_nid* >= 0, callback should create/discard structures for the |
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407 | node if necessary. |
408 | ||
6867c931 | 409 | -------------- |
10020ca2 | 410 | 9. Future Work |
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411 | -------------- |
412 | - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like | |
413 | sysctl or new control file. | |
414 | - showing memory section and physical device relationship. | |
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415 | - showing memory section is under ZONE_MOVABLE or not |
416 | - test and make it better memory offlining. | |
417 | - support HugeTLB page migration and offlining. | |
418 | - memmap removing at memory offline. | |
419 | - physical remove memory. | |
420 |