mirror of
https://github.com/edk2-porting/linux-next.git
synced 2024-12-24 05:04:00 +08:00
dee6da22ef
Let's document the magic a bit, especially why device_hotplug_lock is required when adding/removing memory and how it all play together with requests to online/offline memory from user space. Link: http://lkml.kernel.org/r/20180925091457.28651-7-david@redhat.com Signed-off-by: David Hildenbrand <david@redhat.com> Reviewed-by: Pavel Tatashin <pavel.tatashin@microsoft.com> Reviewed-by: Rashmica Gupta <rashmica.g@gmail.com> Reviewed-by: Oscar Salvador <osalvador@suse.de> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Michal Hocko <mhocko@suse.com> Cc: Balbir Singh <bsingharora@gmail.com> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com> Cc: Dan Williams <dan.j.williams@intel.com> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Haiyang Zhang <haiyangz@microsoft.com> Cc: Heiko Carstens <heiko.carstens@de.ibm.com> Cc: John Allen <jallen@linux.vnet.ibm.com> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com> Cc: Juergen Gross <jgross@suse.com> Cc: Kate Stewart <kstewart@linuxfoundation.org> Cc: "K. Y. Srinivasan" <kys@microsoft.com> Cc: Len Brown <lenb@kernel.org> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Mathieu Malaterre <malat@debian.org> Cc: Michael Ellerman <mpe@ellerman.id.au> Cc: Michael Neuling <mikey@neuling.org> Cc: Nathan Fontenot <nfont@linux.vnet.ibm.com> Cc: Paul Mackerras <paulus@samba.org> Cc: Philippe Ombredanne <pombredanne@nexb.com> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com> Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net> Cc: Stephen Hemminger <sthemmin@microsoft.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: YASUAKI ISHIMATSU <yasu.isimatu@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
445 lines
17 KiB
ReStructuredText
445 lines
17 KiB
ReStructuredText
.. _admin_guide_memory_hotplug:
|
|
|
|
==============
|
|
Memory Hotplug
|
|
==============
|
|
|
|
:Created: Jul 28 2007
|
|
:Updated: Add some details about locking internals: Aug 20 2018
|
|
|
|
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.
|
|
|
|
.. contents:: :local:
|
|
|
|
.. note::
|
|
|
|
(1) x86_64's has special implementation for memory hotplug.
|
|
This text does not describe it.
|
|
(2) This text assumes that sysfs is mounted at ``/sys``.
|
|
|
|
|
|
Introduction
|
|
============
|
|
|
|
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.
|
|
|
|
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 :ref:`memory_hotplug_physical_mem`).
|
|
|
|
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.)
|
|
|
|
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 :ref:`memory_hotplug_sysfs_files`.)
|
|
|
|
To determine the size (in bytes) of a memory block please read this file::
|
|
|
|
/sys/devices/system/memory/block_size_bytes
|
|
|
|
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 following are also necessary:
|
|
- Allow for memory hot remove (``CONFIG_MEMORY_HOTREMOVE``)
|
|
- Page Migration (``CONFIG_MIGRATION``)
|
|
|
|
- For ACPI memory hotplug, the following 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.
|
|
|
|
|
|
.. _memory_hotplug_sysfs_files:
|
|
|
|
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
|
|
|
|
where 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
|
|
|
|
.. _memory_hotplug_physical_mem:
|
|
|
|
Physical memory hot-add phase
|
|
=============================
|
|
|
|
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 :ref:`memory_hotplug_how_to_online_memory` and
|
|
:ref:`memory_hotplug_how_to_offline_memory`.
|
|
|
|
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.
|
|
|
|
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 :ref:`memory_hotplug_how_to_online_memory`.
|
|
|
|
Logical Memory hot-add phase
|
|
============================
|
|
|
|
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".
|
|
|
|
|
|
.. _memory_hotplug_how_to_online_memory:
|
|
|
|
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 doesn't belong to any zone an appropriate kernel zone
|
|
(usually ZONE_NORMAL) will be used unless movable_node kernel command line
|
|
option is specified when ZONE_MOVABLE will be used.
|
|
|
|
You can explicitly request to associate it with ZONE_MOVABLE by::
|
|
|
|
% echo online_movable > /sys/devices/system/memory/memoryXXX/state
|
|
|
|
.. note:: current limit: this memory block must be adjacent to ZONE_MOVABLE
|
|
|
|
Or you can explicitly request a kernel zone (usually ZONE_NORMAL) by::
|
|
|
|
% echo online_kernel > /sys/devices/system/memory/memoryXXX/state
|
|
|
|
.. note:: current limit: this memory block must be adjacent to ZONE_NORMAL
|
|
|
|
An explicit zone onlining can fail (e.g. when the range is already within
|
|
and existing and incompatible zone already).
|
|
|
|
After this, memory block XXX's state will be 'online' and the amount of
|
|
available memory will be increased.
|
|
|
|
This may be changed in future.
|
|
|
|
Logical memory remove
|
|
=====================
|
|
|
|
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.
|
|
|
|
.. _memory_hotplug_how_to_offline_memory:
|
|
|
|
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.
|
|
|
|
Physical memory remove
|
|
======================
|
|
|
|
Need more implementation yet....
|
|
- Notification completion of remove works by OS to firmware.
|
|
- Guard from remove if not yet.
|
|
|
|
|
|
Locking Internals
|
|
=================
|
|
|
|
When adding/removing memory that uses memory block devices (i.e. ordinary RAM),
|
|
the device_hotplug_lock should be held to:
|
|
|
|
- synchronize against online/offline requests (e.g. via sysfs). This way, memory
|
|
block devices can only be accessed (.online/.state attributes) by user
|
|
space once memory has been fully added. And when removing memory, we
|
|
know nobody is in critical sections.
|
|
- synchronize against CPU hotplug and similar (e.g. relevant for ACPI and PPC)
|
|
|
|
Especially, there is a possible lock inversion that is avoided using
|
|
device_hotplug_lock when adding memory and user space tries to online that
|
|
memory faster than expected:
|
|
|
|
- device_online() will first take the device_lock(), followed by
|
|
mem_hotplug_lock
|
|
- add_memory_resource() will first take the mem_hotplug_lock, followed by
|
|
the device_lock() (while creating the devices, during bus_add_device()).
|
|
|
|
As the device is visible to user space before taking the device_lock(), this
|
|
can result in a lock inversion.
|
|
|
|
onlining/offlining of memory should be done via device_online()/
|
|
device_offline() - to make sure it is properly synchronized to actions
|
|
via sysfs. Holding device_hotplug_lock is advised (to e.g. protect online_type)
|
|
|
|
When adding/removing/onlining/offlining memory or adding/removing
|
|
heterogeneous/device memory, we should always hold the mem_hotplug_lock in
|
|
write mode to serialise memory hotplug (e.g. access to global/zone
|
|
variables).
|
|
|
|
In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) in read
|
|
mode allows for a quite efficient get_online_mems/put_online_mems
|
|
implementation, so code accessing memory can protect from that memory
|
|
vanishing.
|
|
|
|
|
|
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.
|