8c27ceff36
The previous patch renamed several files that are cross-referenced along the Kernel documentation. Adjust the links to point to the right places. Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
468 lines
18 KiB
Text
468 lines
18 KiB
Text
==============
|
|
Memory Hotplug
|
|
==============
|
|
|
|
Created: Jul 28 2007
|
|
Add description of notifier of memory hotplug Oct 11 2007
|
|
|
|
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
|
|
8. Memory hotplug event notifier
|
|
9. Future Work List
|
|
|
|
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
|
|
available/unavailable for users. Amount of memory from user's view is
|
|
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.
|
|
|
|
Logical Memory Hotplug phase is triggered by write of sysfs file by system
|
|
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
|
|
------------
|
|
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.
|
|
|
|
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:
|
|
|
|
/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.
|
|
|
|
|
|
--------------------------------
|
|
3 sysfs files for memory hotplug
|
|
--------------------------------
|
|
All memory blocks have their device information in sysfs. Each memory block
|
|
is described under /sys/devices/system/memory as
|
|
|
|
/sys/devices/system/memory/memoryXXX
|
|
(XXX is the memory block id.)
|
|
|
|
For the memory block covered by the sysfs directory. It is expected that all
|
|
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.
|
|
|
|
For example, assume 1GiB memory block size. A device for a memory starting at
|
|
0x100000000 is /sys/device/system/memory/memory4
|
|
(0x100000000 / 1Gib = 4)
|
|
This device covers address range [0x100000000 ... 0x140000000)
|
|
|
|
Under each memory block, you can see 5 files:
|
|
|
|
/sys/devices/system/memory/memoryXXX/phys_index
|
|
/sys/devices/system/memory/memoryXXX/phys_device
|
|
/sys/devices/system/memory/memoryXXX/state
|
|
/sys/devices/system/memory/memoryXXX/removable
|
|
/sys/devices/system/memory/memoryXXX/valid_zones
|
|
|
|
'phys_index' : read-only and contains memory block id, same as XXX.
|
|
'state' : read-write
|
|
at read: contains online/offline state of memory.
|
|
at write: user can specify "online_kernel",
|
|
"online_movable", "online", "offline" command
|
|
which will be performed on all sections in the block.
|
|
'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.
|
|
'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.
|
|
|
|
NOTE:
|
|
These directories/files appear after physical memory hotplug phase.
|
|
|
|
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:
|
|
/sys/devices/system/node/node0/memory9 -> ../../memory/memory9
|
|
|
|
A backlink will also be created:
|
|
/sys/devices/system/memory/memory9/node0 -> ../../node/node0
|
|
|
|
|
|
--------------------------------
|
|
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
|
|
------------
|
|
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.
|
|
|
|
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
|
|
|
|
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.
|
|
|
|
|
|
------------------------------
|
|
5. Logical Memory hot-add phase
|
|
------------------------------
|
|
|
|
5.1. State of memory
|
|
------------
|
|
To see (online/offline) state of a memory block, read 'state' file.
|
|
|
|
% cat /sys/device/system/memory/memoryXXX/state
|
|
|
|
|
|
If the memory block is online, you'll read "online".
|
|
If the memory block is offline, you'll read "offline".
|
|
|
|
|
|
5.2. How to online memory
|
|
------------
|
|
When the memory is hot-added, the kernel decides whether or not to "online"
|
|
it according to the policy which can be read from "auto_online_blocks" file:
|
|
|
|
% cat /sys/devices/system/memory/auto_online_blocks
|
|
|
|
The default depends on the CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE kernel config
|
|
option. If it is disabled the default is "offline" which means the newly added
|
|
memory is not in a ready-to-use state and you have to "online" the newly added
|
|
memory blocks manually. Automatic onlining can be requested by writing "online"
|
|
to "auto_online_blocks" file:
|
|
|
|
% echo online > /sys/devices/system/memory/auto_online_blocks
|
|
|
|
This sets a global policy and impacts all memory blocks that will subsequently
|
|
be hotplugged. Currently offline blocks keep their state. It is possible, under
|
|
certain circumstances, that some memory blocks will be added but will fail to
|
|
online. User space tools can check their "state" files
|
|
(/sys/devices/system/memory/memoryXXX/state) and try to online them manually.
|
|
|
|
If the automatic onlining wasn't requested, failed, or some memory block was
|
|
offlined it is possible to change the individual block's state by writing to the
|
|
"state" file:
|
|
|
|
% echo online > /sys/devices/system/memory/memoryXXX/state
|
|
|
|
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:
|
|
|
|
% echo online_movable > /sys/devices/system/memory/memoryXXX/state
|
|
(NOTE: current limit: this memory block must be adjacent to ZONE_MOVABLE)
|
|
|
|
And if the memory block is in ZONE_MOVABLE, you can change it to ZONE_NORMAL:
|
|
|
|
% echo online_kernel > /sys/devices/system/memory/memoryXXX/state
|
|
(NOTE: current limit: this memory block must be adjacent to ZONE_NORMAL)
|
|
|
|
After this, memory block XXX's state will be 'online' and the amount of
|
|
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
|
|
has to make the whole memory block be unused, memory offline can fail if
|
|
the memory block includes memory which cannot be freed.
|
|
|
|
In general, memory offline can use 2 techniques.
|
|
|
|
(1) reclaim and free all memory in the memory block.
|
|
(2) migrate all pages in the memory block.
|
|
|
|
In the current implementation, Linux's memory offline uses method (2), freeing
|
|
all pages in the memory block by page migration. But not all pages are
|
|
migratable. Under current Linux, migratable pages are anonymous pages and
|
|
page caches. For offlining a memory block by migration, the kernel has to
|
|
guarantee that the memory block contains only migratable pages.
|
|
|
|
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
|
|
create ZONE_MOVABLE...a zone which is just used for movable pages.
|
|
(See also Documentation/admin-guide/kernel-parameters.rst)
|
|
|
|
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.
|
|
|
|
|
|
Note: Unfortunately, there is no information to show which memory block belongs
|
|
to ZONE_MOVABLE. This is TBD.
|
|
|
|
|
|
6.2. How to offline memory
|
|
------------
|
|
You can offline a memory block by using the same sysfs interface that was used
|
|
in memory onlining.
|
|
|
|
% echo offline > /sys/devices/system/memory/memoryXXX/state
|
|
|
|
If offline succeeds, the state of the memory block is changed to be "offline".
|
|
If it fails, some error core (like -EBUSY) will be returned by the kernel.
|
|
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.
|
|
|
|
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.)
|
|
|
|
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.
|
|
|
|
--------------------------------
|
|
8. Memory hotplug event notifier
|
|
--------------------------------
|
|
Hotplugging events are sent to a notification queue.
|
|
|
|
There are six types of notification defined in include/linux/memory.h:
|
|
|
|
MEM_GOING_ONLINE
|
|
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.
|
|
|
|
MEM_CANCEL_ONLINE
|
|
Generated if MEMORY_GOING_ONLINE fails.
|
|
|
|
MEM_ONLINE
|
|
Generated when memory has successfully brought online. The callback may
|
|
allocate pages from the new memory.
|
|
|
|
MEM_GOING_OFFLINE
|
|
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
|
|
subsystem from the indicated memory block.
|
|
|
|
MEM_CANCEL_OFFLINE
|
|
Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from
|
|
the memory block that we attempted to offline.
|
|
|
|
MEM_OFFLINE
|
|
Generated after offlining memory is complete.
|
|
|
|
A callback routine can be registered by calling
|
|
|
|
hotplug_memory_notifier(callback_func, priority)
|
|
|
|
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.
|
|
|
|
struct memory_notify {
|
|
unsigned long start_pfn;
|
|
unsigned long nr_pages;
|
|
int status_change_nid_normal;
|
|
int status_change_nid_high;
|
|
int status_change_nid;
|
|
}
|
|
|
|
start_pfn is start_pfn of online/offline memory.
|
|
nr_pages is # of pages of online/offline memory.
|
|
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.
|
|
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)
|
|
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.
|
|
If status_changed_nid* >= 0, callback should create/discard structures for the
|
|
node if necessary.
|
|
|
|
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.
|
|
|
|
--------------
|
|
9. Future Work
|
|
--------------
|
|
- allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like
|
|
sysctl or new control file.
|
|
- showing memory block and physical device relationship.
|
|
- test and make it better memory offlining.
|
|
- support HugeTLB page migration and offlining.
|
|
- memmap removing at memory offline.
|
|
- physical remove memory.
|
|
|