[patch 185/262] memory-hotplug.rst: document the "auto-movable" online policy

Andrew Morton
2021-11-05T20:44:20Z

From: David Hildenbrand <david@redhat.com>
Subject: memory-hotplug.rst: document the "auto-movable" online policy

In commit e83a437faa62 ("mm/memory_hotplug: introduce "auto-movable"
online policy") we introduced a new memory online policy to automatically
select a zone for memory blocks to be onlined. We added a way to set the
active online policy and tunables for the auto-movable online policy. In
follow-up commits we 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>
---

Documentation/admin-guide/mm/memory-hotplug.rst | 141 ++++++++++++--
1 file changed, 121 insertions(+), 20 deletions(-)

--- a/Documentation/admin-guide/mm/memory-hotplug.rst~memory-hotplugrst-document-the-auto-movable-online-policy
+++ a/Documentation/admin-guide/mm/memory-hotplug.rst
@@ -165,9 +165,8 @@ Or alternatively::

% echo 1 > /sys/devices/system/memory/memoryXXX/online

-The kernel will select the target zone automatically, usually defaulting to
-``ZONE_NORMAL`` unless ``movable_node`` has been specified on the kernel
-command line or if the memory block would intersect the ZONE_MOVABLE already.
+The kernel will select the target zone automatically, depending on the
+configured ``online_policy``.

One can explicitly request to associate an offline memory block with
ZONE_MOVABLE by::
@@ -198,6 +197,9 @@ Auto-onlining can be enabled by writing

% echo online > /sys/devices/system/memory/auto_online_blocks

+Similarly to manual onlining, with ``online`` the kernel will select the
+target zone automatically, depending on the configured ``online_policy``.
+
Modifying the auto-online behavior will only affect all subsequently added
memory blocks only.

@@ -393,11 +395,16 @@ command line parameters are relevant:
======================== =======================================================
``memhp_default_state`` configure auto-onlining by essentially setting
``/sys/devices/system/memory/auto_online_blocks``.
-``movable_node`` configure automatic zone selection in the kernel. When
- set, the kernel will default to ZONE_MOVABLE, unless
- other zones can be kept contiguous.
+``movable_node`` configure automatic zone selection in the kernel when
+ using the ``contig-zones`` online policy. When
+ set, the kernel will default to ZONE_MOVABLE when
+ onlining a memory block, unless other zones can be kept
+ contiguous.
======================== =======================================================

+See Documentation/admin-guide/kernel-parameters.txt for a more generic
+description of these command line parameters.
+
Module Parameters
------------------

@@ -414,20 +421,114 @@ and they can be observed (and some even

The following module parameters are currently defined:

-======================== =======================================================
-``memmap_on_memory`` read-write: Allocate memory for the memmap from the
- added memory block itself. Even if enabled, actual
- support depends on various other system properties and
- should only be regarded as a hint whether the behavior
- would be desired.
-
- While allocating the memmap from the memory block
- itself makes memory hotplug less likely to fail and
- keeps the memmap on the same NUMA node in any case, it
- can fragment physical memory in a way that huge pages
- in bigger granularity cannot be formed on hotplugged
- memory.
-======================== =======================================================
+================================ ===============================================
+``memmap_on_memory`` read-write: Allocate memory for the memmap from
+ the added memory block itself. Even if enabled,
+ actual support depends on various other system
+ properties and should only be regarded as a
+ hint whether the behavior would be desired.
+
+ While allocating the memmap from the memory
+ block itself makes memory hotplug less likely
+ to fail and keeps the memmap on the same NUMA
+ node in any case, it can fragment physical
+ memory in a way that huge pages in bigger
+ granularity cannot be formed on hotplugged
+ memory.
+``online_policy`` read-write: Set the basic policy used for
+ automatic zone selection when onlining memory
+ blocks without specifying a target zone.
+ ``contig-zones`` has been the kernel default
+ before this parameter was added. After an
+ online policy was configured and memory was
+ online, the policy should not be changed
+ anymore.
+
+ When set to ``contig-zones``, the kernel will
+ try keeping zones contiguous. If a memory block
+ intersects multiple zones or no zone, the
+ behavior depends on the ``movable_node`` kernel
+ command line parameter: default to ZONE_MOVABLE
+ if set, default to the applicable kernel zone
+ (usually ZONE_NORMAL) if not set.
+
+ When set to ``auto-movable``, the kernel will
+ try onlining memory blocks to ZONE_MOVABLE if
+ possible according to the configuration and
+ memory device details. With this policy, one
+ can avoid zone imbalances when eventually
+ hotplugging a lot of memory later and still
+ wanting to be able to hotunplug as much as
+ possible reliably, very desirable in
+ virtualized environments. This policy ignores
+ the ``movable_node`` kernel command line
+ parameter and isn't really applicable in
+ environments that require it (e.g., bare metal
+ with hotunpluggable nodes) where hotplugged
+ memory might be exposed via the
+ firmware-provided memory map early during boot
+ to the system instead of getting detected,
+ added and onlined later during boot (such as
+ done by virtio-mem or by some hypervisors
+ implementing emulated DIMMs). As one example, a
+ hotplugged DIMM will be onlined either
+ completely to ZONE_MOVABLE or completely to
+ ZONE_NORMAL, not a mixture.
+ As another example, as many memory blocks
+ belonging to a virtio-mem device will be
+ onlined to ZONE_MOVABLE as possible,
+ special-casing units of memory blocks that can
+ only get hotunplugged together. *This policy
+ does not protect from setups that are
+ problematic with ZONE_MOVABLE and does not
+ change the zone of memory blocks dynamically
+ after they were onlined.*
+``auto_movable_ratio`` read-write: Set the maximum MOVABLE:KERNEL
+ memory ratio in % for the ``auto-movable``
+ online policy. Whether the ratio applies only
+ for the system across all NUMA nodes or also
+ per NUMA nodes depends on the
+ ``auto_movable_numa_aware`` configuration.
+
+ All accounting is based on present memory pages
+ in the zones combined with accounting per
+ memory device. Memory dedicated to the CMA
+ allocator is accounted as MOVABLE, although
+ residing on one of the kernel zones. The
+ possible ratio depends on the actual workload.
+ The kernel default is "301" %, for example,
+ allowing for hotplugging 24 GiB to a 8 GiB VM
+ and automatically onlining all hotplugged
+ memory to ZONE_MOVABLE in many setups. The
+ additional 1% deals with some pages being not
+ present, for example, because of some firmware
+ allocations.
+
+ Note that ZONE_NORMAL memory provided by one
+ memory device does not allow for more
+ ZONE_MOVABLE memory for a different memory
+ device. As one example, onlining memory of a
+ hotplugged DIMM to ZONE_NORMAL will not allow
+ for another hotplugged DIMM to get onlined to
+ ZONE_MOVABLE automatically. In contrast, memory
+ hotplugged by a virtio-mem device that got
+ onlined to ZONE_NORMAL will allow for more
+ ZONE_MOVABLE memory within *the same*
+ virtio-mem device.
+``auto_movable_numa_aware`` read-write: Configure whether the
+ ``auto_movable_ratio`` in the ``auto-movable``
+ online policy also applies per NUMA
+ node in addition to the whole system across all
+ NUMA nodes. The kernel default is "Y".
+
+ Disabling NUMA awareness can be helpful when
+ dealing with NUMA nodes that should be
+ completely hotunpluggable, onlining the memory
+ completely to ZONE_MOVABLE automatically if
+ possible.
+
+ Parameter availability depends on CONFIG_NUMA.
+================================ ===============================================

ZONE_MOVABLE
============
_