Re: [PATCH v8 00/12] mm/demotion: Memory tiers and demotion

Aneesh Kumar K.V
2022-07-11T15:30:37Z

"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> writes:

> The current kernel has the basic memory tiering support: Inactive
> pages on a higher tier NUMA node can be migrated (demoted) to a lower
> tier NUMA node to make room for new allocations on the higher tier
> NUMA node. Frequently accessed pages on a lower tier NUMA node can be
> migrated (promoted) to a higher tier NUMA node to improve the
> performance.
>
> In the current kernel, memory tiers are defined implicitly via a
> demotion path relationship between NUMA nodes, which is created during
> the kernel initialization and updated when a NUMA node is hot-added or
> hot-removed. The current implementation puts all nodes with CPU into
> the top tier, and builds the tier hierarchy tier-by-tier by establishing
> the per-node demotion targets based on the distances between nodes.
>
> This current memory tier kernel interface needs to be improved for
> several important use cases:
>
> * The current tier initialization code always initializes
> each memory-only NUMA node into a lower tier. But a memory-only
> NUMA node may have a high performance memory device (e.g. a DRAM
> device attached via CXL.mem or a DRAM-backed memory-only node on
> a virtual machine) and should be put into a higher tier.
>
> * The current tier hierarchy always puts CPU nodes into the top
> tier. But on a system with HBM (e.g. GPU memory) devices, these
> memory-only HBM NUMA nodes should be in the top tier, and DRAM nodes
> with CPUs are better to be placed into the next lower tier.
>
> * Also because the current tier hierarchy always puts CPU nodes
> into the top tier, when a CPU is hot-added (or hot-removed) and
> triggers a memory node from CPU-less into a CPU node (or vice
> versa), the memory tier hierarchy gets changed, even though no
> memory node is added or removed. This can make the tier
> hierarchy unstable and make it difficult to support tier-based
> memory accounting.
>
> * A higher tier node can only be demoted to selected nodes on the
> next lower tier as defined by the demotion path, not any other
> node from any lower tier. This strict, hard-coded demotion order
> does not work in all use cases (e.g. some use cases may want to
> allow cross-socket demotion to another node in the same demotion
> tier as a fallback when the preferred demotion node is out of
> space), and has resulted in the feature request for an interface to
> override the system-wide, per-node demotion order from the
> userspace. This demotion order is also inconsistent with the page
> allocation fallback order when all the nodes in a higher tier are
> out of space: The page allocation can fall back to any node from
> any lower tier, whereas the demotion order doesn't allow that.
>
> * There are no interfaces for the userspace to learn about the memory
> tier hierarchy in order to optimize its memory allocations.
>
> This patch series make the creation of memory tiers explicit under
> the control of userspace or device driver.
>
> Memory Tier Initialization
> ==========================
>
> By default, all memory nodes are assigned to the default tier with
> tier ID value 200.
>
> A device driver can move up or down its memory nodes from the default
> tier. For example, PMEM can move down its memory nodes below the
> default tier, whereas GPU can move up its memory nodes above the
> default tier.
>
> The kernel initialization code makes the decision on which exact tier
> a memory node should be assigned to based on the requests from the
> device drivers as well as the memory device hardware information
> provided by the firmware.
>
> Hot-adding/removing CPUs doesn't affect memory tier hierarchy.
>
> Memory Allocation for Demotion
> ==============================
> This patch series keep the demotion target page allocation logic same.
> The demotion page allocation pick the closest NUMA node in the
> next lower tier to the current NUMA node allocating pages from.
>
> This will be later improved to use the same page allocation strategy
> using fallback list.
>
> Sysfs Interface:
> -------------
> Listing current list of memory tiers details:
>
> :/sys/devices/system/memtier$ ls
> default_tier max_tier memtier1 power uevent
> :/sys/devices/system/memtier$ cat default_tier
> memtier200
> :/sys/devices/system/memtier$ cat max_tier
> 400
> :/sys/devices/system/memtier$
>
> Per node memory tier details:
>
> For a cpu only NUMA node:
>
> :/sys/devices/system/node# cat node0/memtier
> :/sys/devices/system/node# echo 1 > node0/memtier
> :/sys/devices/system/node# cat node0/memtier
> :/sys/devices/system/node#
>
> For a NUMA node with memory:
> :/sys/devices/system/node# cat node1/memtier
> 1
> :/sys/devices/system/node# ls ../memtier/
> default_tier max_tier memtier1 power uevent
> :/sys/devices/system/node# echo 2 > node1/memtier
> :/sys/devices/system/node#
> :/sys/devices/system/node# ls ../memtier/
> default_tier max_tier memtier1 memtier2 power uevent
> :/sys/devices/system/node# cat node1/memtier
> 2
> :/sys/devices/system/node#
>
> Removing a memory tier
> :/sys/devices/system/node# cat node1/memtier
> 2
> :/sys/devices/system/node# echo 1 > node1/memtier
> :/sys/devices/system/node#
> :/sys/devices/system/node# cat node1/memtier
> 1
> :/sys/devices/system/node#
> :/sys/devices/system/node# ls ../memtier/
> default_tier max_tier memtier1 power uevent
> :/sys/devices/system/node#
>
> The above resulted in removal of memtier2 which was created in the earlier step.
>
> Changes from v7:
> * Fix kernel crash with demotion.
> * Improve documentation.
>
> Changes from v6:
> * Drop the usage of rank.
> * Address other review feedback.
>
> Changes from v5:
> * Remove patch supporting N_MEMORY node removal from memory tiers. memory tiers
> are going to be used for features other than demotion. Hence keep all N_MEMORY
> nodes in memory tiers irrespective of whether they want to participate in promotion or demotion.
> * Add NODE_DATA->memtier
> * Rearrage patches to add sysfs files later.
> * Add support to create memory tiers from userspace.
> * Address other review feedback.
>
>
> Changes from v4:
> * Address review feedback.
> * Reverse the meaning of "rank": higher rank value means higher tier.
> * Add "/sys/devices/system/memtier/default_tier".
> * Add node_is_toptier
>
> v4:
> Add support for explicit memory tiers and ranks.
>
> v3:
> - Modify patch 1 subject to make it more specific
> - Remove /sys/kernel/mm/numa/demotion_targets interface, use
> /sys/devices/system/node/demotion_targets instead and make
> it writable to override node_states[N_DEMOTION_TARGETS].
> - Add support to view per node demotion targets via sysfs
>
> v2:
> In v1, only 1st patch of this patch series was sent, which was
> implemented to avoid some of the limitations on the demotion
> target sharing, however for certain numa topology, the demotion
> targets found by that patch was not most optimal, so 1st patch
> in this series is modified according to suggestions from Huang
> and Baolin. Different examples of demotion list comparasion
> between existing implementation and changed implementation can
> be found in the commit message of 1st patch.
>
>
> Aneesh Kumar K.V (10):
> mm/demotion: Add support for explicit memory tiers
> mm/demotion: Move memory demotion related code
> mm/demotion/dax/kmem: Set node's memory tier to MEMORY_TIER_PMEM
> mm/demotion: Add hotplug callbacks to handle new numa node onlined
> mm/demotion: Build demotion targets based on explicit memory tiers
> mm/demotion: Expose memory tier details via sysfs
> mm/demotion: Add per node memory tier attribute to sysfs
> mm/demotion: Add pg_data_t member to track node memory tier details
> mm/demotion: Update node_is_toptier to work with memory tiers
> mm/demotion: Add sysfs ABI documentation
>
> Jagdish Gediya (2):
> mm/demotion: Demote pages according to allocation fallback order
> mm/demotion: Add documentation for memory tiering
>
> .../ABI/testing/sysfs-kernel-mm-memory-tiers | 61 ++
> Documentation/admin-guide/mm/index.rst | 1 +
> .../admin-guide/mm/memory-tiering.rst | 192 +++++
> drivers/base/node.c | 42 +
> drivers/dax/kmem.c | 6 +-
> include/linux/memory-tiers.h | 72 ++
> include/linux/migrate.h | 15 -
> include/linux/mmzone.h | 3 +
> include/linux/node.h | 5 -
> mm/Makefile | 1 +
> mm/huge_memory.c | 1 +
> mm/memory-tiers.c | 791 ++++++++++++++++++
> mm/migrate.c | 453 +---------
> mm/mprotect.c | 1 +
> mm/vmscan.c | 59 +-
> mm/vmstat.c | 4 -
> 16 files changed, 1215 insertions(+), 492 deletions(-)
> create mode 100644 Documentation/ABI/testing/sysfs-kernel-mm-memory-tiers
> create mode 100644 Documentation/admin-guide/mm/memory-tiering.rst
> create mode 100644 include/linux/memory-tiers.h
> create mode 100644 mm/memory-tiers.c
>

Gentle ping. Any objections for this series?

-aneesh