memory-hotplug.rst: document the "auto-movable" online policy
Commit e83a437faa ("mm/memory_hotplug: introduce "auto-movable" online
policy") introduced a new memory online policy to automatically select a
zone for memory blocks to be onlined. It added a way to set the active
online policy and tunables for the auto-movable online policy.
Follow-up commits tweaked the "auto-movable" policy to also consider
memory device details when selecting zones for memory blocks to be
onlined.
Let's document the new toggles and how the two online policies we have
work.
[david@redhat.com: updates]
Link: https://lkml.kernel.org/r/20211011082058.6076-4-david@redhat.com
Link: https://lkml.kernel.org/r/20210930144117.23641-4-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
David Hildenbrand
Linus Torvalds
parent
a8db400f99
commit
9e122cc1bd
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@ -165,9 +165,8 @@ Or alternatively::
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% echo 1 > /sys/devices/system/memory/memoryXXX/online
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The kernel will select the target zone automatically, usually defaulting to
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``ZONE_NORMAL`` unless ``movable_node`` has been specified on the kernel
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command line or if the memory block would intersect the ZONE_MOVABLE already.
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The kernel will select the target zone automatically, depending on the
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configured ``online_policy``.
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One can explicitly request to associate an offline memory block with
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ZONE_MOVABLE by::
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@ -198,6 +197,9 @@ Auto-onlining can be enabled by writing ``online``, ``online_kernel`` or
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% echo online > /sys/devices/system/memory/auto_online_blocks
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Similarly to manual onlining, with ``online`` the kernel will select the
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target zone automatically, depending on the configured ``online_policy``.
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Modifying the auto-online behavior will only affect all subsequently added
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memory blocks only.
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@ -393,11 +395,16 @@ command line parameters are relevant:
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======================== =======================================================
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``memhp_default_state`` configure auto-onlining by essentially setting
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``/sys/devices/system/memory/auto_online_blocks``.
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``movable_node`` configure automatic zone selection in the kernel. When
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set, the kernel will default to ZONE_MOVABLE, unless
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other zones can be kept contiguous.
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``movable_node`` configure automatic zone selection in the kernel when
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using the ``contig-zones`` online policy. When
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set, the kernel will default to ZONE_MOVABLE when
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onlining a memory block, unless other zones can be kept
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contiguous.
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======================== =======================================================
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See Documentation/admin-guide/kernel-parameters.txt for a more generic
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description of these command line parameters.
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Module Parameters
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------------------
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@ -414,20 +421,114 @@ and they can be observed (and some even modified at runtime) via::
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The following module parameters are currently defined:
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======================== =======================================================
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``memmap_on_memory`` read-write: Allocate memory for the memmap from the
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added memory block itself. Even if enabled, actual
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support depends on various other system properties and
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should only be regarded as a hint whether the behavior
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would be desired.
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================================ ===============================================
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``memmap_on_memory`` read-write: Allocate memory for the memmap from
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the added memory block itself. Even if enabled,
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actual support depends on various other system
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properties and should only be regarded as a
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hint whether the behavior would be desired.
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While allocating the memmap from the memory block
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itself makes memory hotplug less likely to fail and
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keeps the memmap on the same NUMA node in any case, it
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can fragment physical memory in a way that huge pages
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in bigger granularity cannot be formed on hotplugged
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memory.
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======================== =======================================================
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While allocating the memmap from the memory
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block itself makes memory hotplug less likely
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to fail and keeps the memmap on the same NUMA
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node in any case, it can fragment physical
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memory in a way that huge pages in bigger
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granularity cannot be formed on hotplugged
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memory.
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``online_policy`` read-write: Set the basic policy used for
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automatic zone selection when onlining memory
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blocks without specifying a target zone.
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``contig-zones`` has been the kernel default
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before this parameter was added. After an
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online policy was configured and memory was
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online, the policy should not be changed
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anymore.
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When set to ``contig-zones``, the kernel will
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try keeping zones contiguous. If a memory block
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intersects multiple zones or no zone, the
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behavior depends on the ``movable_node`` kernel
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command line parameter: default to ZONE_MOVABLE
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if set, default to the applicable kernel zone
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(usually ZONE_NORMAL) if not set.
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When set to ``auto-movable``, the kernel will
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try onlining memory blocks to ZONE_MOVABLE if
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possible according to the configuration and
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memory device details. With this policy, one
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can avoid zone imbalances when eventually
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hotplugging a lot of memory later and still
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wanting to be able to hotunplug as much as
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possible reliably, very desirable in
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virtualized environments. This policy ignores
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the ``movable_node`` kernel command line
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parameter and isn't really applicable in
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environments that require it (e.g., bare metal
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with hotunpluggable nodes) where hotplugged
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memory might be exposed via the
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firmware-provided memory map early during boot
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to the system instead of getting detected,
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added and onlined later during boot (such as
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done by virtio-mem or by some hypervisors
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implementing emulated DIMMs). As one example, a
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hotplugged DIMM will be onlined either
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completely to ZONE_MOVABLE or completely to
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ZONE_NORMAL, not a mixture.
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As another example, as many memory blocks
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belonging to a virtio-mem device will be
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onlined to ZONE_MOVABLE as possible,
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special-casing units of memory blocks that can
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only get hotunplugged together. *This policy
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does not protect from setups that are
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problematic with ZONE_MOVABLE and does not
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change the zone of memory blocks dynamically
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after they were onlined.*
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``auto_movable_ratio`` read-write: Set the maximum MOVABLE:KERNEL
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memory ratio in % for the ``auto-movable``
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online policy. Whether the ratio applies only
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for the system across all NUMA nodes or also
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per NUMA nodes depends on the
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``auto_movable_numa_aware`` configuration.
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All accounting is based on present memory pages
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in the zones combined with accounting per
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memory device. Memory dedicated to the CMA
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allocator is accounted as MOVABLE, although
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residing on one of the kernel zones. The
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possible ratio depends on the actual workload.
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The kernel default is "301" %, for example,
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allowing for hotplugging 24 GiB to a 8 GiB VM
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and automatically onlining all hotplugged
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memory to ZONE_MOVABLE in many setups. The
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additional 1% deals with some pages being not
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present, for example, because of some firmware
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allocations.
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Note that ZONE_NORMAL memory provided by one
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memory device does not allow for more
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ZONE_MOVABLE memory for a different memory
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device. As one example, onlining memory of a
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hotplugged DIMM to ZONE_NORMAL will not allow
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for another hotplugged DIMM to get onlined to
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ZONE_MOVABLE automatically. In contrast, memory
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hotplugged by a virtio-mem device that got
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onlined to ZONE_NORMAL will allow for more
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ZONE_MOVABLE memory within *the same*
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virtio-mem device.
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``auto_movable_numa_aware`` read-write: Configure whether the
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``auto_movable_ratio`` in the ``auto-movable``
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online policy also applies per NUMA
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node in addition to the whole system across all
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NUMA nodes. The kernel default is "Y".
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Disabling NUMA awareness can be helpful when
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dealing with NUMA nodes that should be
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completely hotunpluggable, onlining the memory
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completely to ZONE_MOVABLE automatically if
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possible.
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Parameter availability depends on CONFIG_NUMA.
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================================ ===============================================
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ZONE_MOVABLE
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============
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