Re: [PATCH v2 3/3] memory-hotplug.rst: document the "auto-movable" online policy

Mike Rapoport
2021-10-11T09:31:31Z

On Mon, Oct 11, 2021 at 10:20:58AM +0200, David Hildenbrand wrote:
> 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.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Acked-by: Mike Rapoport <rppt@linux.ibm.com>

> ---
> .../admin-guide/mm/memory-hotplug.rst | 141 +++++++++++++++---
> 1 file changed, 121 insertions(+), 20 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
> index ee00b70dedde..0f56ecd8ac05 100644
> --- a/Documentation/admin-guide/mm/memory-hotplug.rst
> +++ b/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 ``online``, ``online_kernel`` or
>
> % 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 modified at runtime) via::
>
> 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
> ============
> --
> 2.31.1
>

--
Sincerely yours,
Mike.