memory-hotplug.txt 17.1 KB
Newer Older
Yasunori Goto's avatar
Yasunori Goto committed
1 2 3 4
==============
Memory Hotplug
==============

5 6
Created:					Jul 28 2007
Add description of notifier of memory hotplug	Oct 11 2007
Yasunori Goto's avatar
Yasunori Goto committed
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

This document is about memory hotplug including how-to-use and current status.
Because Memory Hotplug is still under development, contents of this text will
be changed often.

1. Introduction
  1.1 purpose of memory hotplug
  1.2. Phases of memory hotplug
  1.3. Unit of Memory online/offline operation
2. Kernel Configuration
3. sysfs files for memory hotplug
4. Physical memory hot-add phase
  4.1 Hardware(Firmware) Support
  4.2 Notify memory hot-add event by hand
5. Logical Memory hot-add phase
  5.1. State of memory
  5.2. How to online memory
6. Logical memory remove
  6.1 Memory offline and ZONE_MOVABLE
  6.2. How to offline memory
7. Physical memory remove
28 29
8. Memory hotplug event notifier
9. Future Work List
Yasunori Goto's avatar
Yasunori Goto committed
30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75

Note(1): x86_64's has special implementation for memory hotplug.
         This text does not describe it.
Note(2): This text assumes that sysfs is mounted at /sys.


---------------
1. Introduction
---------------

1.1 purpose of memory hotplug
------------
Memory Hotplug allows users to increase/decrease the amount of memory.
Generally, there are two purposes.

(A) For changing the amount of memory.
    This is to allow a feature like capacity on demand.
(B) For installing/removing DIMMs or NUMA-nodes physically.
    This is to exchange DIMMs/NUMA-nodes, reduce power consumption, etc.

(A) is required by highly virtualized environments and (B) is required by
hardware which supports memory power management.

Linux memory hotplug is designed for both purpose.


1.2. Phases of memory hotplug
---------------
There are 2 phases in Memory Hotplug.
  1) Physical Memory Hotplug phase
  2) Logical Memory Hotplug phase.

The First phase is to communicate hardware/firmware and make/erase
environment for hotplugged memory. Basically, this phase is necessary
for the purpose (B), but this is good phase for communication between
highly virtualized environments too.

When memory is hotplugged, the kernel recognizes new memory, makes new memory
management tables, and makes sysfs files for new memory's operation.

If firmware supports notification of connection of new memory to OS,
this phase is triggered automatically. ACPI can notify this event. If not,
"probe" operation by system administration is used instead.
(see Section 4.).

Logical Memory Hotplug phase is to change memory state into
76
available/unavailable for users. Amount of memory from user's view is
Yasunori Goto's avatar
Yasunori Goto committed
77 78 79 80 81
changed by this phase. The kernel makes all memory in it as free pages
when a memory range is available.

In this document, this phase is described as online/offline.

82
Logical Memory Hotplug phase is triggered by write of sysfs file by system
Yasunori Goto's avatar
Yasunori Goto committed
83 84 85 86 87 88 89 90
administrator. For the hot-add case, it must be executed after Physical Hotplug
phase by hand.
(However, if you writes udev's hotplug scripts for memory hotplug, these
 phases can be execute in seamless way.)


1.3. Unit of Memory online/offline operation
------------
91 92 93 94
Memory hotplug uses SPARSEMEM memory model which allows memory to be divided
into chunks of the same size. These chunks are called "sections". The size of
a memory section is architecture dependent. For example, power uses 16MiB, ia64
uses 1GiB.
Yasunori Goto's avatar
Yasunori Goto committed
95

96 97 98 99 100 101 102
Memory sections are combined into chunks referred to as "memory blocks". The
size of a memory block is architecture dependent and represents the logical
unit upon which memory online/offline operations are to be performed. The
default size of a memory block is the same as memory section size unless an
architecture specifies otherwise. (see Section 3.)

To determine the size (in bytes) of a memory block please read this file:
Yasunori Goto's avatar
Yasunori Goto committed
103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130

/sys/devices/system/memory/block_size_bytes


-----------------------
2. Kernel Configuration
-----------------------
To use memory hotplug feature, kernel must be compiled with following
config options.

- For all memory hotplug
    Memory model -> Sparse Memory  (CONFIG_SPARSEMEM)
    Allow for memory hot-add       (CONFIG_MEMORY_HOTPLUG)

- To enable memory removal, the followings are also necessary
    Allow for memory hot remove    (CONFIG_MEMORY_HOTREMOVE)
    Page Migration                 (CONFIG_MIGRATION)

- For ACPI memory hotplug, the followings are also necessary
    Memory hotplug (under ACPI Support menu) (CONFIG_ACPI_HOTPLUG_MEMORY)
    This option can be kernel module.

- As a related configuration, if your box has a feature of NUMA-node hotplug
  via ACPI, then this option is necessary too.
    ACPI0004,PNP0A05 and PNP0A06 Container Driver (under ACPI Support menu)
    (CONFIG_ACPI_CONTAINER).
    This option can be kernel module too.

131

Yasunori Goto's avatar
Yasunori Goto committed
132
--------------------------------
133
3 sysfs files for memory hotplug
Yasunori Goto's avatar
Yasunori Goto committed
134
--------------------------------
135 136
All memory blocks have their device information in sysfs.  Each memory block
is described under /sys/devices/system/memory as
Yasunori Goto's avatar
Yasunori Goto committed
137 138

/sys/devices/system/memory/memoryXXX
139
(XXX is the memory block id.)
Yasunori Goto's avatar
Yasunori Goto committed
140

141
For the memory block covered by the sysfs directory.  It is expected that all
142 143 144 145
memory sections in this range are present and no memory holes exist in the
range. Currently there is no way to determine if there is a memory hole, but
the existence of one should not affect the hotplug capabilities of the memory
block.
Yasunori Goto's avatar
Yasunori Goto committed
146

147
For example, assume 1GiB memory block size. A device for a memory starting at
Yasunori Goto's avatar
Yasunori Goto committed
148 149 150 151
0x100000000 is /sys/device/system/memory/memory4
(0x100000000 / 1Gib = 4)
This device covers address range [0x100000000 ... 0x140000000)

152
Under each memory block, you can see 5 files:
Yasunori Goto's avatar
Yasunori Goto committed
153

154
/sys/devices/system/memory/memoryXXX/phys_index
Yasunori Goto's avatar
Yasunori Goto committed
155 156
/sys/devices/system/memory/memoryXXX/phys_device
/sys/devices/system/memory/memoryXXX/state
157
/sys/devices/system/memory/memoryXXX/removable
158
/sys/devices/system/memory/memoryXXX/valid_zones
Yasunori Goto's avatar
Yasunori Goto committed
159

160
'phys_index'      : read-only and contains memory block id, same as XXX.
161 162
'state'           : read-write
                    at read:  contains online/offline state of memory.
163 164
                    at write: user can specify "online_kernel",
                    "online_movable", "online", "offline" command
165
                    which will be performed on all sections in the block.
166 167 168 169 170 171 172 173
'phys_device'     : read-only: designed to show the name of physical memory
                    device.  This is not well implemented now.
'removable'       : read-only: contains an integer value indicating
                    whether the memory block is removable or not
                    removable.  A value of 1 indicates that the memory
                    block is removable and a value of 0 indicates that
                    it is not removable. A memory block is removable only if
                    every section in the block is removable.
174 175 176 177 178 179 180 181 182
'valid_zones'     : read-only: designed to show which zones this memory block
		    can be onlined to.
		    The first column shows it's default zone.
		    "memory6/valid_zones: Normal Movable" shows this memoryblock
		    can be onlined to ZONE_NORMAL by default and to ZONE_MOVABLE
		    by online_movable.
		    "memory7/valid_zones: Movable Normal" shows this memoryblock
		    can be onlined to ZONE_MOVABLE by default and to ZONE_NORMAL
		    by online_kernel.
Yasunori Goto's avatar
Yasunori Goto committed
183 184 185 186

NOTE:
  These directories/files appear after physical memory hotplug phase.

187 188 189 190
If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed
via symbolic links located in the /sys/devices/system/node/node* directories.

For example:
191
/sys/devices/system/node/node0/memory9 -> ../../memory/memory9
Yasunori Goto's avatar
Yasunori Goto committed
192

193 194 195
A backlink will also be created:
/sys/devices/system/memory/memory9/node0 -> ../../node/node0

196

Yasunori Goto's avatar
Yasunori Goto committed
197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221
--------------------------------
4. Physical memory hot-add phase
--------------------------------

4.1 Hardware(Firmware) Support
------------
On x86_64/ia64 platform, memory hotplug by ACPI is supported.

In general, the firmware (ACPI) which supports memory hotplug defines
memory class object of _HID "PNP0C80". When a notify is asserted to PNP0C80,
Linux's ACPI handler does hot-add memory to the system and calls a hotplug udev
script. This will be done automatically.

But scripts for memory hotplug are not contained in generic udev package(now).
You may have to write it by yourself or online/offline memory by hand.
Please see "How to online memory", "How to offline memory" in this text.

If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004",
"PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handler
calls hotplug code for all of objects which are defined in it.
If memory device is found, memory hotplug code will be called.


4.2 Notify memory hot-add event by hand
------------
222 223 224 225 226 227
On some architectures, the firmware may not notify the kernel of a memory
hotplug event.  Therefore, the memory "probe" interface is supported to
explicitly notify the kernel.  This interface depends on
CONFIG_ARCH_MEMORY_PROBE and can be configured on powerpc, sh, and x86
if hotplug is supported, although for x86 this should be handled by ACPI
notification.
Yasunori Goto's avatar
Yasunori Goto committed
228 229 230 231 232 233 234 235

Probe interface is located at
/sys/devices/system/memory/probe

You can tell the physical address of new memory to the kernel by

% echo start_address_of_new_memory > /sys/devices/system/memory/probe

236 237 238 239
Then, [start_address_of_new_memory, start_address_of_new_memory +
memory_block_size] memory range is hot-added. In this case, hotplug script is
not called (in current implementation). You'll have to online memory by
yourself.  Please see "How to online memory" in this text.
Yasunori Goto's avatar
Yasunori Goto committed
240 241 242 243 244 245 246 247


------------------------------
5. Logical Memory hot-add phase
------------------------------

5.1. State of memory
------------
248
To see (online/offline) state of a memory block, read 'state' file.
Yasunori Goto's avatar
Yasunori Goto committed
249 250 251 252

% cat /sys/device/system/memory/memoryXXX/state


253 254
If the memory block is online, you'll read "online".
If the memory block is offline, you'll read "offline".
Yasunori Goto's avatar
Yasunori Goto committed
255 256 257 258 259


5.2. How to online memory
------------
Even if the memory is hot-added, it is not at ready-to-use state.
260
For using newly added memory, you have to "online" the memory block.
Yasunori Goto's avatar
Yasunori Goto committed
261

262
For onlining, you have to write "online" to the memory block's state file as:
Yasunori Goto's avatar
Yasunori Goto committed
263 264 265

% echo online > /sys/devices/system/memory/memoryXXX/state

266 267
This onlining will not change the ZONE type of the target memory block,
If the memory block is in ZONE_NORMAL, you can change it to ZONE_MOVABLE:
268 269

% echo online_movable > /sys/devices/system/memory/memoryXXX/state
270
(NOTE: current limit: this memory block must be adjacent to ZONE_MOVABLE)
271

272
And if the memory block is in ZONE_MOVABLE, you can change it to ZONE_NORMAL:
273 274

% echo online_kernel > /sys/devices/system/memory/memoryXXX/state
275
(NOTE: current limit: this memory block must be adjacent to ZONE_NORMAL)
276

277
After this, memory block XXX's state will be 'online' and the amount of
Yasunori Goto's avatar
Yasunori Goto committed
278 279 280 281 282 283 284 285 286 287 288 289 290 291
available memory will be increased.

Currently, newly added memory is added as ZONE_NORMAL (for powerpc, ZONE_DMA).
This may be changed in future.



------------------------
6. Logical memory remove
------------------------

6.1 Memory offline and ZONE_MOVABLE
------------
Memory offlining is more complicated than memory online. Because memory offline
292 293
has to make the whole memory block be unused, memory offline can fail if
the memory block includes memory which cannot be freed.
Yasunori Goto's avatar
Yasunori Goto committed
294 295 296

In general, memory offline can use 2 techniques.

297 298
(1) reclaim and free all memory in the memory block.
(2) migrate all pages in the memory block.
Yasunori Goto's avatar
Yasunori Goto committed
299 300

In the current implementation, Linux's memory offline uses method (2), freeing
301
all  pages in the memory block by page migration. But not all pages are
Yasunori Goto's avatar
Yasunori Goto committed
302
migratable. Under current Linux, migratable pages are anonymous pages and
303 304
page caches. For offlining a memory block by migration, the kernel has to
guarantee that the memory block contains only migratable pages.
Yasunori Goto's avatar
Yasunori Goto committed
305

306 307
Now, a boot option for making a memory block which consists of migratable pages
is supported. By specifying "kernelcore=" or "movablecore=" boot option, you can
Yasunori Goto's avatar
Yasunori Goto committed
308 309 310 311 312 313 314 315 316 317 318 319 320 321 322
create ZONE_MOVABLE...a zone which is just used for movable pages.
(See also Documentation/kernel-parameters.txt)

Assume the system has "TOTAL" amount of memory at boot time, this boot option
creates ZONE_MOVABLE as following.

1) When kernelcore=YYYY boot option is used,
  Size of memory not for movable pages (not for offline) is YYYY.
  Size of memory for movable pages (for offline) is TOTAL-YYYY.

2) When movablecore=ZZZZ boot option is used,
  Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ.
  Size of memory for movable pages (for offline) is ZZZZ.


323
Note: Unfortunately, there is no information to show which memory block belongs
Yasunori Goto's avatar
Yasunori Goto committed
324 325 326 327 328
to ZONE_MOVABLE. This is TBD.


6.2. How to offline memory
------------
329 330
You can offline a memory block by using the same sysfs interface that was used
in memory onlining.
Yasunori Goto's avatar
Yasunori Goto committed
331 332 333

% echo offline > /sys/devices/system/memory/memoryXXX/state

334
If offline succeeds, the state of the memory block is changed to be "offline".
Yasunori Goto's avatar
Yasunori Goto committed
335
If it fails, some error core (like -EBUSY) will be returned by the kernel.
336 337
Even if a memory block does not belong to ZONE_MOVABLE, you can try to offline
it.  If it doesn't contain 'unmovable' memory, you'll get success.
Yasunori Goto's avatar
Yasunori Goto committed
338

339 340 341 342 343
A memory block under ZONE_MOVABLE is considered to be able to be offlined
easily.  But under some busy state, it may return -EBUSY. Even if a memory
block cannot be offlined due to -EBUSY, you can retry offlining it and may be
able to offline it (or not). (For example, a page is referred to by some kernel
internal call and released soon.)
Yasunori Goto's avatar
Yasunori Goto committed
344 345 346 347 348 349 350 351 352 353 354 355 356 357 358

Consideration:
Memory hotplug's design direction is to make the possibility of memory offlining
higher and to guarantee unplugging memory under any situation. But it needs
more work. Returning -EBUSY under some situation may be good because the user
can decide to retry more or not by himself. Currently, memory offlining code
does some amount of retry with 120 seconds timeout.

-------------------------
7. Physical memory remove
-------------------------
Need more implementation yet....
 - Notification completion of remove works by OS to firmware.
 - Guard from remove if not yet.

359 360 361
--------------------------------
8. Memory hotplug event notifier
--------------------------------
362
Hotplugging events are sent to a notification queue.
363

364 365 366
There are six types of notification defined in include/linux/memory.h:

MEM_GOING_ONLINE
367 368 369 370
  Generated before new memory becomes available in order to be able to
  prepare subsystems to handle memory. The page allocator is still unable
  to allocate from the new memory.

371
MEM_CANCEL_ONLINE
372 373
  Generated if MEMORY_GOING_ONLINE fails.

374
MEM_ONLINE
375
  Generated when memory has successfully brought online. The callback may
376 377
  allocate pages from the new memory.

378
MEM_GOING_OFFLINE
379 380 381
  Generated to begin the process of offlining memory. Allocations are no
  longer possible from the memory but some of the memory to be offlined
  is still in use. The callback can be used to free memory known to a
382
  subsystem from the indicated memory block.
383

384
MEM_CANCEL_OFFLINE
385
  Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from
386
  the memory block that we attempted to offline.
387

388
MEM_OFFLINE
389 390
  Generated after offlining memory is complete.

391 392
A callback routine can be registered by calling

393 394
  hotplug_memory_notifier(callback_func, priority)

395 396 397 398 399 400 401 402 403 404 405 406
Callback functions with higher values of priority are called before callback
functions with lower values.

A callback function must have the following prototype:

  int callback_func(
    struct notifier_block *self, unsigned long action, void *arg);

The first argument of the callback function (self) is a pointer to the block
of the notifier chain that points to the callback function itself.
The second argument (action) is one of the event types described above.
The third argument (arg) passes a pointer of struct memory_notify.
407 408 409 410

struct memory_notify {
       unsigned long start_pfn;
       unsigned long nr_pages;
411
       int status_change_nid_normal;
412
       int status_change_nid_high;
413
       int status_change_nid;
414 415 416 417
}

start_pfn is start_pfn of online/offline memory.
nr_pages is # of pages of online/offline memory.
418 419
status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask
is (will be) set/clear, if this is -1, then nodemask status is not changed.
420 421 422
status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask
is (will be) set/clear, if this is -1, then nodemask status is not changed.
status_change_nid is set node id when N_MEMORY of nodemask is (will be)
423 424
set/clear. It means a new(memoryless) node gets new memory by online and a
node loses all memory. If this is -1, then nodemask status is not changed.
425
If status_changed_nid* >= 0, callback should create/discard structures for the
426 427
node if necessary.

428 429 430 431 432 433 434 435 436 437 438 439
The callback routine shall return one of the values
NOTIFY_DONE, NOTIFY_OK, NOTIFY_BAD, NOTIFY_STOP
defined in include/linux/notifier.h

NOTIFY_DONE and NOTIFY_OK have no effect on the further processing.

NOTIFY_BAD is used as response to the MEM_GOING_ONLINE, MEM_GOING_OFFLINE,
MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops
further processing of the notification queue.

NOTIFY_STOP stops further processing of the notification queue.

Yasunori Goto's avatar
Yasunori Goto committed
440
--------------
441
9. Future Work
Yasunori Goto's avatar
Yasunori Goto committed
442 443 444
--------------
  - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like
    sysctl or new control file.
445
  - showing memory block and physical device relationship.
Yasunori Goto's avatar
Yasunori Goto committed
446 447 448 449 450
  - test and make it better memory offlining.
  - support HugeTLB page migration and offlining.
  - memmap removing at memory offline.
  - physical remove memory.