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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 |
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128 | -------------------------------- |
129 | All sections have their device information under /sys/devices/system/memory as | |
130 | ||
131 | /sys/devices/system/memory/memoryXXX | |
132 | (XXX is section id.) | |
133 | ||
134 | Now, XXX is defined as start_address_of_section / section_size. | |
135 | ||
136 | For example, assume 1GiB section size. A device for a memory starting at | |
137 | 0x100000000 is /sys/device/system/memory/memory4 | |
138 | (0x100000000 / 1Gib = 4) | |
139 | This device covers address range [0x100000000 ... 0x140000000) | |
140 | ||
c04fc586 | 141 | Under each section, you can see 4 files. |
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142 | |
143 | /sys/devices/system/memory/memoryXXX/phys_index | |
144 | /sys/devices/system/memory/memoryXXX/phys_device | |
145 | /sys/devices/system/memory/memoryXXX/state | |
c04fc586 | 146 | /sys/devices/system/memory/memoryXXX/removable |
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147 | |
148 | 'phys_index' : read-only and contains section id, same as XXX. | |
149 | 'state' : read-write | |
150 | at read: contains online/offline state of memory. | |
151 | at write: user can specify "online", "offline" command | |
152 | 'phys_device': read-only: designed to show the name of physical memory device. | |
153 | This is not well implemented now. | |
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154 | 'removable' : read-only: contains an integer value indicating |
155 | whether the memory section is removable or not | |
156 | removable. A value of 1 indicates that the memory | |
157 | section is removable and a value of 0 indicates that | |
158 | it is not removable. | |
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159 | |
160 | NOTE: | |
161 | These directories/files appear after physical memory hotplug phase. | |
162 | ||
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163 | If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed |
164 | via symbolic links located in the /sys/devices/system/node/node* directories. | |
165 | ||
166 | For example: | |
c04fc586 | 167 | /sys/devices/system/node/node0/memory9 -> ../../memory/memory9 |
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169 | A backlink will also be created: |
170 | /sys/devices/system/memory/memory9/node0 -> ../../node/node0 | |
171 | ||
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172 | -------------------------------- |
173 | 4. Physical memory hot-add phase | |
174 | -------------------------------- | |
175 | ||
176 | 4.1 Hardware(Firmware) Support | |
177 | ------------ | |
178 | On x86_64/ia64 platform, memory hotplug by ACPI is supported. | |
179 | ||
180 | In general, the firmware (ACPI) which supports memory hotplug defines | |
181 | memory class object of _HID "PNP0C80". When a notify is asserted to PNP0C80, | |
182 | Linux's ACPI handler does hot-add memory to the system and calls a hotplug udev | |
183 | script. This will be done automatically. | |
184 | ||
185 | But scripts for memory hotplug are not contained in generic udev package(now). | |
186 | You may have to write it by yourself or online/offline memory by hand. | |
187 | Please see "How to online memory", "How to offline memory" in this text. | |
188 | ||
189 | If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004", | |
190 | "PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handler | |
191 | calls hotplug code for all of objects which are defined in it. | |
192 | If memory device is found, memory hotplug code will be called. | |
193 | ||
194 | ||
195 | 4.2 Notify memory hot-add event by hand | |
196 | ------------ | |
197 | In some environments, especially virtualized environment, firmware will not | |
198 | notify memory hotplug event to the kernel. For such environment, "probe" | |
199 | interface is supported. This interface depends on CONFIG_ARCH_MEMORY_PROBE. | |
200 | ||
201 | Now, CONFIG_ARCH_MEMORY_PROBE is supported only by powerpc but it does not | |
202 | contain highly architecture codes. Please add config if you need "probe" | |
203 | interface. | |
204 | ||
205 | Probe interface is located at | |
206 | /sys/devices/system/memory/probe | |
207 | ||
208 | You can tell the physical address of new memory to the kernel by | |
209 | ||
210 | % echo start_address_of_new_memory > /sys/devices/system/memory/probe | |
211 | ||
212 | Then, [start_address_of_new_memory, start_address_of_new_memory + section_size) | |
213 | memory range is hot-added. In this case, hotplug script is not called (in | |
214 | current implementation). You'll have to online memory by yourself. | |
215 | Please see "How to online memory" in this text. | |
216 | ||
217 | ||
218 | ||
219 | ------------------------------ | |
220 | 5. Logical Memory hot-add phase | |
221 | ------------------------------ | |
222 | ||
223 | 5.1. State of memory | |
224 | ------------ | |
225 | To see (online/offline) state of memory section, read 'state' file. | |
226 | ||
227 | % cat /sys/device/system/memory/memoryXXX/state | |
228 | ||
229 | ||
230 | If the memory section is online, you'll read "online". | |
231 | If the memory section is offline, you'll read "offline". | |
232 | ||
233 | ||
234 | 5.2. How to online memory | |
235 | ------------ | |
236 | Even if the memory is hot-added, it is not at ready-to-use state. | |
237 | For using newly added memory, you have to "online" the memory section. | |
238 | ||
239 | For onlining, you have to write "online" to the section's state file as: | |
240 | ||
241 | % echo online > /sys/devices/system/memory/memoryXXX/state | |
242 | ||
243 | After this, section memoryXXX's state will be 'online' and the amount of | |
244 | available memory will be increased. | |
245 | ||
246 | Currently, newly added memory is added as ZONE_NORMAL (for powerpc, ZONE_DMA). | |
247 | This may be changed in future. | |
248 | ||
249 | ||
250 | ||
251 | ------------------------ | |
252 | 6. Logical memory remove | |
253 | ------------------------ | |
254 | ||
255 | 6.1 Memory offline and ZONE_MOVABLE | |
256 | ------------ | |
257 | Memory offlining is more complicated than memory online. Because memory offline | |
258 | has to make the whole memory section be unused, memory offline can fail if | |
259 | the section includes memory which cannot be freed. | |
260 | ||
261 | In general, memory offline can use 2 techniques. | |
262 | ||
263 | (1) reclaim and free all memory in the section. | |
264 | (2) migrate all pages in the section. | |
265 | ||
266 | In the current implementation, Linux's memory offline uses method (2), freeing | |
267 | all pages in the section by page migration. But not all pages are | |
268 | migratable. Under current Linux, migratable pages are anonymous pages and | |
269 | page caches. For offlining a section by migration, the kernel has to guarantee | |
270 | that the section contains only migratable pages. | |
271 | ||
272 | Now, a boot option for making a section which consists of migratable pages is | |
273 | supported. By specifying "kernelcore=" or "movablecore=" boot option, you can | |
274 | create ZONE_MOVABLE...a zone which is just used for movable pages. | |
275 | (See also Documentation/kernel-parameters.txt) | |
276 | ||
277 | Assume the system has "TOTAL" amount of memory at boot time, this boot option | |
278 | creates ZONE_MOVABLE as following. | |
279 | ||
280 | 1) When kernelcore=YYYY boot option is used, | |
281 | Size of memory not for movable pages (not for offline) is YYYY. | |
282 | Size of memory for movable pages (for offline) is TOTAL-YYYY. | |
283 | ||
284 | 2) When movablecore=ZZZZ boot option is used, | |
285 | Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ. | |
286 | Size of memory for movable pages (for offline) is ZZZZ. | |
287 | ||
288 | ||
289 | Note) Unfortunately, there is no information to show which section belongs | |
290 | to ZONE_MOVABLE. This is TBD. | |
291 | ||
292 | ||
293 | 6.2. How to offline memory | |
294 | ------------ | |
295 | You can offline a section by using the same sysfs interface that was used in | |
296 | memory onlining. | |
297 | ||
298 | % echo offline > /sys/devices/system/memory/memoryXXX/state | |
299 | ||
300 | If offline succeeds, the state of the memory section is changed to be "offline". | |
301 | If it fails, some error core (like -EBUSY) will be returned by the kernel. | |
302 | Even if a section does not belong to ZONE_MOVABLE, you can try to offline it. | |
303 | If it doesn't contain 'unmovable' memory, you'll get success. | |
304 | ||
305 | A section under ZONE_MOVABLE is considered to be able to be offlined easily. | |
306 | But under some busy state, it may return -EBUSY. Even if a memory section | |
307 | cannot be offlined due to -EBUSY, you can retry offlining it and may be able to | |
308 | offline it (or not). | |
309 | (For example, a page is referred to by some kernel internal call and released | |
310 | soon.) | |
311 | ||
312 | Consideration: | |
313 | Memory hotplug's design direction is to make the possibility of memory offlining | |
314 | higher and to guarantee unplugging memory under any situation. But it needs | |
315 | more work. Returning -EBUSY under some situation may be good because the user | |
316 | can decide to retry more or not by himself. Currently, memory offlining code | |
317 | does some amount of retry with 120 seconds timeout. | |
318 | ||
319 | ------------------------- | |
320 | 7. Physical memory remove | |
321 | ------------------------- | |
322 | Need more implementation yet.... | |
323 | - Notification completion of remove works by OS to firmware. | |
324 | - Guard from remove if not yet. | |
325 | ||
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326 | -------------------------------- |
327 | 8. Memory hotplug event notifier | |
328 | -------------------------------- | |
329 | Memory hotplug has event notifer. There are 6 types of notification. | |
330 | ||
331 | MEMORY_GOING_ONLINE | |
332 | Generated before new memory becomes available in order to be able to | |
333 | prepare subsystems to handle memory. The page allocator is still unable | |
334 | to allocate from the new memory. | |
335 | ||
336 | MEMORY_CANCEL_ONLINE | |
337 | Generated if MEMORY_GOING_ONLINE fails. | |
338 | ||
339 | MEMORY_ONLINE | |
19f59460 | 340 | Generated when memory has successfully brought online. The callback may |
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341 | allocate pages from the new memory. |
342 | ||
343 | MEMORY_GOING_OFFLINE | |
344 | Generated to begin the process of offlining memory. Allocations are no | |
345 | longer possible from the memory but some of the memory to be offlined | |
346 | is still in use. The callback can be used to free memory known to a | |
347 | subsystem from the indicated memory section. | |
348 | ||
349 | MEMORY_CANCEL_OFFLINE | |
350 | Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from | |
351 | the section that we attempted to offline. | |
352 | ||
353 | MEMORY_OFFLINE | |
354 | Generated after offlining memory is complete. | |
355 | ||
356 | A callback routine can be registered by | |
357 | hotplug_memory_notifier(callback_func, priority) | |
358 | ||
359 | The second argument of callback function (action) is event types of above. | |
360 | The third argument is passed by pointer of struct memory_notify. | |
361 | ||
362 | struct memory_notify { | |
363 | unsigned long start_pfn; | |
364 | unsigned long nr_pages; | |
19f59460 | 365 | int status_change_nid; |
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366 | } |
367 | ||
368 | start_pfn is start_pfn of online/offline memory. | |
369 | nr_pages is # of pages of online/offline memory. | |
370 | status_change_nid is set node id when N_HIGH_MEMORY of nodemask is (will be) | |
371 | set/clear. It means a new(memoryless) node gets new memory by online and a | |
372 | node loses all memory. If this is -1, then nodemask status is not changed. | |
373 | If status_changed_nid >= 0, callback should create/discard structures for the | |
374 | node if necessary. | |
375 | ||
6867c931 | 376 | -------------- |
10020ca2 | 377 | 9. Future Work |
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378 | -------------- |
379 | - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like | |
380 | sysctl or new control file. | |
381 | - showing memory section and physical device relationship. | |
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382 | - showing memory section is under ZONE_MOVABLE or not |
383 | - test and make it better memory offlining. | |
384 | - support HugeTLB page migration and offlining. | |
385 | - memmap removing at memory offline. | |
386 | - physical remove memory. | |
387 |