508 lines
19 KiB
Plaintext
508 lines
19 KiB
Plaintext
==============
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Memory Hotplug
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==============
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:Created: Jul 28 2007
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:Updated: Add description of notifier of memory hotplug: Oct 11 2007
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This document is about memory hotplug including how-to-use and current status.
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Because Memory Hotplug is still under development, contents of this text will
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be changed often.
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.. CONTENTS
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1. Introduction
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1.1 purpose of memory hotplug
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1.2. Phases of memory hotplug
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1.3. Unit of Memory online/offline operation
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2. Kernel Configuration
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3. sysfs files for memory hotplug
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4. Physical memory hot-add phase
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4.1 Hardware(Firmware) Support
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4.2 Notify memory hot-add event by hand
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5. Logical Memory hot-add phase
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5.1. State of memory
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5.2. How to online memory
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6. Logical memory remove
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6.1 Memory offline and ZONE_MOVABLE
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6.2. How to offline memory
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7. Physical memory remove
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8. Memory hotplug event notifier
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9. Future Work List
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.. note::
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(1) x86_64's has special implementation for memory hotplug.
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This text does not describe it.
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(2) This text assumes that sysfs is mounted at /sys.
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Introduction
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============
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purpose of memory hotplug
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-------------------------
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Memory Hotplug allows users to increase/decrease the amount of memory.
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Generally, there are two purposes.
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(A) For changing the amount of memory.
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This is to allow a feature like capacity on demand.
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(B) For installing/removing DIMMs or NUMA-nodes physically.
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This is to exchange DIMMs/NUMA-nodes, reduce power consumption, etc.
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(A) is required by highly virtualized environments and (B) is required by
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hardware which supports memory power management.
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Linux memory hotplug is designed for both purpose.
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Phases of memory hotplug
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------------------------
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There are 2 phases in Memory Hotplug:
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1) Physical Memory Hotplug phase
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2) Logical Memory Hotplug phase.
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The First phase is to communicate hardware/firmware and make/erase
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environment for hotplugged memory. Basically, this phase is necessary
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for the purpose (B), but this is good phase for communication between
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highly virtualized environments too.
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When memory is hotplugged, the kernel recognizes new memory, makes new memory
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management tables, and makes sysfs files for new memory's operation.
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If firmware supports notification of connection of new memory to OS,
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this phase is triggered automatically. ACPI can notify this event. If not,
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"probe" operation by system administration is used instead.
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(see :ref:`memory_hotplug_physical_mem`).
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Logical Memory Hotplug phase is to change memory state into
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available/unavailable for users. Amount of memory from user's view is
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changed by this phase. The kernel makes all memory in it as free pages
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when a memory range is available.
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In this document, this phase is described as online/offline.
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Logical Memory Hotplug phase is triggered by write of sysfs file by system
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administrator. For the hot-add case, it must be executed after Physical Hotplug
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phase by hand.
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(However, if you writes udev's hotplug scripts for memory hotplug, these
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phases can be execute in seamless way.)
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Unit of Memory online/offline operation
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---------------------------------------
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Memory hotplug uses SPARSEMEM memory model which allows memory to be divided
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into chunks of the same size. These chunks are called "sections". The size of
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a memory section is architecture dependent. For example, power uses 16MiB, ia64
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uses 1GiB.
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Memory sections are combined into chunks referred to as "memory blocks". The
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size of a memory block is architecture dependent and represents the logical
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unit upon which memory online/offline operations are to be performed. The
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default size of a memory block is the same as memory section size unless an
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architecture specifies otherwise. (see :ref:`memory_hotplug_sysfs_files`.)
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To determine the size (in bytes) of a memory block please read this file:
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/sys/devices/system/memory/block_size_bytes
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Kernel Configuration
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====================
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To use memory hotplug feature, kernel must be compiled with following
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config options.
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- For all memory hotplug:
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- Memory model -> Sparse Memory (CONFIG_SPARSEMEM)
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- Allow for memory hot-add (CONFIG_MEMORY_HOTPLUG)
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- To enable memory removal, the following are also necessary:
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- Allow for memory hot remove (CONFIG_MEMORY_HOTREMOVE)
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- Page Migration (CONFIG_MIGRATION)
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- For ACPI memory hotplug, the following are also necessary:
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- Memory hotplug (under ACPI Support menu) (CONFIG_ACPI_HOTPLUG_MEMORY)
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- This option can be kernel module.
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- As a related configuration, if your box has a feature of NUMA-node hotplug
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via ACPI, then this option is necessary too.
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- ACPI0004,PNP0A05 and PNP0A06 Container Driver (under ACPI Support menu)
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(CONFIG_ACPI_CONTAINER).
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This option can be kernel module too.
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.. _memory_hotplug_sysfs_files:
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sysfs files for memory hotplug
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==============================
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All memory blocks have their device information in sysfs. Each memory block
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is described under /sys/devices/system/memory as:
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/sys/devices/system/memory/memoryXXX
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(XXX is the memory block id.)
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For the memory block covered by the sysfs directory. It is expected that all
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memory sections in this range are present and no memory holes exist in the
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range. Currently there is no way to determine if there is a memory hole, but
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the existence of one should not affect the hotplug capabilities of the memory
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block.
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For example, assume 1GiB memory block size. A device for a memory starting at
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0x100000000 is /sys/device/system/memory/memory4::
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(0x100000000 / 1Gib = 4)
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This device covers address range [0x100000000 ... 0x140000000)
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Under each memory block, you can see 5 files:
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- /sys/devices/system/memory/memoryXXX/phys_index
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- /sys/devices/system/memory/memoryXXX/phys_device
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- /sys/devices/system/memory/memoryXXX/state
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- /sys/devices/system/memory/memoryXXX/removable
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- /sys/devices/system/memory/memoryXXX/valid_zones
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=================== ============================================================
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``phys_index`` read-only and contains memory block id, same as XXX.
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``state`` read-write
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- at read: contains online/offline state of memory.
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- at write: user can specify "online_kernel",
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"online_movable", "online", "offline" command
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which will be performed on all sections in the block.
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``phys_device`` read-only: designed to show the name of physical memory
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device. This is not well implemented now.
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``removable`` read-only: contains an integer value indicating
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whether the memory block is removable or not
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removable. A value of 1 indicates that the memory
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block is removable and a value of 0 indicates that
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it is not removable. A memory block is removable only if
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every section in the block is removable.
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``valid_zones`` read-only: designed to show which zones this memory block
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can be onlined to.
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The first column shows it`s default zone.
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"memory6/valid_zones: Normal Movable" shows this memoryblock
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can be onlined to ZONE_NORMAL by default and to ZONE_MOVABLE
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by online_movable.
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"memory7/valid_zones: Movable Normal" shows this memoryblock
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can be onlined to ZONE_MOVABLE by default and to ZONE_NORMAL
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by online_kernel.
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=================== ============================================================
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.. note::
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These directories/files appear after physical memory hotplug phase.
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If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed
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via symbolic links located in the /sys/devices/system/node/node* directories.
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For example:
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/sys/devices/system/node/node0/memory9 -> ../../memory/memory9
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A backlink will also be created:
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/sys/devices/system/memory/memory9/node0 -> ../../node/node0
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.. _memory_hotplug_physical_mem:
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Physical memory hot-add phase
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=============================
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Hardware(Firmware) Support
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--------------------------
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On x86_64/ia64 platform, memory hotplug by ACPI is supported.
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In general, the firmware (ACPI) which supports memory hotplug defines
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memory class object of _HID "PNP0C80". When a notify is asserted to PNP0C80,
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Linux's ACPI handler does hot-add memory to the system and calls a hotplug udev
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script. This will be done automatically.
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But scripts for memory hotplug are not contained in generic udev package(now).
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You may have to write it by yourself or online/offline memory by hand.
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Please see :ref:`memory_hotplug_how_to_online_memory` and
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:ref:`memory_hotplug_how_to_offline_memory`.
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If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004",
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"PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handler
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calls hotplug code for all of objects which are defined in it.
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If memory device is found, memory hotplug code will be called.
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Notify memory hot-add event by hand
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-----------------------------------
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On some architectures, the firmware may not notify the kernel of a memory
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hotplug event. Therefore, the memory "probe" interface is supported to
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explicitly notify the kernel. This interface depends on
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CONFIG_ARCH_MEMORY_PROBE and can be configured on powerpc, sh, and x86
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if hotplug is supported, although for x86 this should be handled by ACPI
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notification.
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Probe interface is located at
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/sys/devices/system/memory/probe
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You can tell the physical address of new memory to the kernel by::
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% echo start_address_of_new_memory > /sys/devices/system/memory/probe
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Then, [start_address_of_new_memory, start_address_of_new_memory +
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memory_block_size] memory range is hot-added. In this case, hotplug script is
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not called (in current implementation). You'll have to online memory by
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yourself. Please see :ref:`memory_hotplug_how_to_online_memory`.
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Logical Memory hot-add phase
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============================
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State of memory
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---------------
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To see (online/offline) state of a memory block, read 'state' file::
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% cat /sys/device/system/memory/memoryXXX/state
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- If the memory block is online, you'll read "online".
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- If the memory block is offline, you'll read "offline".
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.. _memory_hotplug_how_to_online_memory:
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How to online memory
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--------------------
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When the memory is hot-added, the kernel decides whether or not to "online"
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it according to the policy which can be read from "auto_online_blocks" file::
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% cat /sys/devices/system/memory/auto_online_blocks
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The default depends on the CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE kernel config
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option. If it is disabled the default is "offline" which means the newly added
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memory is not in a ready-to-use state and you have to "online" the newly added
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memory blocks manually. Automatic onlining can be requested by writing "online"
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to "auto_online_blocks" file::
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% echo online > /sys/devices/system/memory/auto_online_blocks
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This sets a global policy and impacts all memory blocks that will subsequently
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be hotplugged. Currently offline blocks keep their state. It is possible, under
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certain circumstances, that some memory blocks will be added but will fail to
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online. User space tools can check their "state" files
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(/sys/devices/system/memory/memoryXXX/state) and try to online them manually.
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If the automatic onlining wasn't requested, failed, or some memory block was
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offlined it is possible to change the individual block's state by writing to the
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"state" file::
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% echo online > /sys/devices/system/memory/memoryXXX/state
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This onlining will not change the ZONE type of the target memory block,
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If the memory block doesn't belong to any zone an appropriate kernel zone
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(usually ZONE_NORMAL) will be used unless movable_node kernel command line
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option is specified when ZONE_MOVABLE will be used.
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You can explicitly request to associate it with ZONE_MOVABLE by::
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% echo online_movable > /sys/devices/system/memory/memoryXXX/state
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.. note:: current limit: this memory block must be adjacent to ZONE_MOVABLE
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Or you can explicitly request a kernel zone (usually ZONE_NORMAL) by::
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% echo online_kernel > /sys/devices/system/memory/memoryXXX/state
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.. note:: current limit: this memory block must be adjacent to ZONE_NORMAL
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An explicit zone onlining can fail (e.g. when the range is already within
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and existing and incompatible zone already).
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After this, memory block XXX's state will be 'online' and the amount of
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available memory will be increased.
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This may be changed in future.
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Logical memory remove
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=====================
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Memory offline and ZONE_MOVABLE
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-------------------------------
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Memory offlining is more complicated than memory online. Because memory offline
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has to make the whole memory block be unused, memory offline can fail if
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the memory block includes memory which cannot be freed.
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In general, memory offline can use 2 techniques.
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(1) reclaim and free all memory in the memory block.
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(2) migrate all pages in the memory block.
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In the current implementation, Linux's memory offline uses method (2), freeing
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all pages in the memory block by page migration. But not all pages are
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migratable. Under current Linux, migratable pages are anonymous pages and
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page caches. For offlining a memory block by migration, the kernel has to
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guarantee that the memory block contains only migratable pages.
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Now, a boot option for making a memory block which consists of migratable pages
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is supported. By specifying "kernelcore=" or "movablecore=" boot option, you can
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create ZONE_MOVABLE...a zone which is just used for movable pages.
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(See also Documentation/admin-guide/kernel-parameters.rst)
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Assume the system has "TOTAL" amount of memory at boot time, this boot option
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creates ZONE_MOVABLE as following.
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1) When kernelcore=YYYY boot option is used,
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Size of memory not for movable pages (not for offline) is YYYY.
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Size of memory for movable pages (for offline) is TOTAL-YYYY.
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2) When movablecore=ZZZZ boot option is used,
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Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ.
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Size of memory for movable pages (for offline) is ZZZZ.
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.. note::
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Unfortunately, there is no information to show which memory block belongs
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to ZONE_MOVABLE. This is TBD.
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.. _memory_hotplug_how_to_offline_memory:
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How to offline memory
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---------------------
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You can offline a memory block by using the same sysfs interface that was used
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in memory onlining::
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% echo offline > /sys/devices/system/memory/memoryXXX/state
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If offline succeeds, the state of the memory block is changed to be "offline".
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If it fails, some error core (like -EBUSY) will be returned by the kernel.
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Even if a memory block does not belong to ZONE_MOVABLE, you can try to offline
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it. If it doesn't contain 'unmovable' memory, you'll get success.
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A memory block under ZONE_MOVABLE is considered to be able to be offlined
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easily. But under some busy state, it may return -EBUSY. Even if a memory
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block cannot be offlined due to -EBUSY, you can retry offlining it and may be
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able to offline it (or not). (For example, a page is referred to by some kernel
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internal call and released soon.)
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Consideration:
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Memory hotplug's design direction is to make the possibility of memory
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offlining higher and to guarantee unplugging memory under any situation. But
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it needs more work. Returning -EBUSY under some situation may be good because
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the user can decide to retry more or not by himself. Currently, memory
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offlining code does some amount of retry with 120 seconds timeout.
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Physical memory remove
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======================
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Need more implementation yet....
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- Notification completion of remove works by OS to firmware.
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- Guard from remove if not yet.
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Memory hotplug event notifier
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=============================
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Hotplugging events are sent to a notification queue.
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There are six types of notification defined in include/linux/memory.h:
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MEM_GOING_ONLINE
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Generated before new memory becomes available in order to be able to
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prepare subsystems to handle memory. The page allocator is still unable
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to allocate from the new memory.
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MEM_CANCEL_ONLINE
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Generated if MEMORY_GOING_ONLINE fails.
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MEM_ONLINE
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Generated when memory has successfully brought online. The callback may
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allocate pages from the new memory.
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MEM_GOING_OFFLINE
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Generated to begin the process of offlining memory. Allocations are no
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longer possible from the memory but some of the memory to be offlined
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is still in use. The callback can be used to free memory known to a
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subsystem from the indicated memory block.
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MEM_CANCEL_OFFLINE
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Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from
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the memory block that we attempted to offline.
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MEM_OFFLINE
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Generated after offlining memory is complete.
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A callback routine can be registered by calling::
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hotplug_memory_notifier(callback_func, priority)
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Callback functions with higher values of priority are called before callback
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functions with lower values.
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A callback function must have the following prototype::
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int callback_func(
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struct notifier_block *self, unsigned long action, void *arg);
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The first argument of the callback function (self) is a pointer to the block
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of the notifier chain that points to the callback function itself.
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The second argument (action) is one of the event types described above.
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The third argument (arg) passes a pointer of struct memory_notify::
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struct memory_notify {
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unsigned long start_pfn;
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unsigned long nr_pages;
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int status_change_nid_normal;
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int status_change_nid_high;
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int status_change_nid;
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}
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- start_pfn is start_pfn of online/offline memory.
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- nr_pages is # of pages of online/offline memory.
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- status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask
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is (will be) set/clear, if this is -1, then nodemask status is not changed.
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- status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask
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is (will be) set/clear, if this is -1, then nodemask status is not changed.
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- status_change_nid is set node id when N_MEMORY of nodemask is (will be)
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set/clear. It means a new(memoryless) node gets new memory by online and a
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node loses all memory. If this is -1, then nodemask status is not changed.
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If status_changed_nid* >= 0, callback should create/discard structures for the
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node if necessary.
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The callback routine shall return one of the values
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NOTIFY_DONE, NOTIFY_OK, NOTIFY_BAD, NOTIFY_STOP
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defined in include/linux/notifier.h
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NOTIFY_DONE and NOTIFY_OK have no effect on the further processing.
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NOTIFY_BAD is used as response to the MEM_GOING_ONLINE, MEM_GOING_OFFLINE,
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MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops
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further processing of the notification queue.
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NOTIFY_STOP stops further processing of the notification queue.
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Future Work
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===========
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- allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like
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sysctl or new control file.
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- showing memory block and physical device relationship.
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- test and make it better memory offlining.
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- support HugeTLB page migration and offlining.
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- memmap removing at memory offline.
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- physical remove memory.
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