Patch series "mm/demotion: Memory tiers and demotion", v15.

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 highest 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 implementation 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-backed
  memory-only node on a virtual machine) and that 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 nodes with shortest distance
  on the next lower tier as defined by the demotion path, not any other
  node from any lower tier.  This strict, 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.

This patch series make the creation of memory tiers explicit under the
control of device driver.

Memory Tier Initialization
==========================

Linux kernel presents memory devices as NUMA nodes and each memory device
is of a specific type.  The memory type of a device is represented by its
abstract distance.  A memory tier corresponds to a range of abstract
distance.  This allows for classifying memory devices with a specific
performance range into a memory tier.

By default, all memory nodes are assigned to the default tier with
abstract distance 512.

A device driver can move its memory nodes from the default tier.  For
example, PMEM can move its memory nodes below the default tier, whereas
GPU can move 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.


This patch (of 10):

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 highest tier,
and builds the tier hierarchy by establishing the per-node demotion
targets based on the distances between nodes.

This current memory tier kernel implementation 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-backed memory-only node on a
virtual machine) that 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 or GPU devices, the memory-only NUMA nodes mapping
these devices should be in the top tier, and DRAM nodes with CPUs are
better to be placed into the next lower tier.

With current kernel higher tier node can only be demoted to nodes with
shortest distance on the next lower tier as defined by the demotion path,
not any other node from any lower tier.  This strict, 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), 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.

This patch series address the above by defining memory tiers explicitly.

Linux kernel presents memory devices as NUMA nodes and each memory device
is of a specific type.  The memory type of a device is represented by its
abstract distance.  A memory tier corresponds to a range of abstract
distance.  This allows for classifying memory devices with a specific
performance range into a memory tier.

This patch configures the range/chunk size to be 128.  The default DRAM
abstract distance is 512.  We can have 4 memory tiers below the default
DRAM with abstract distance range 0 - 127, 127 - 255, 256- 383, 384 - 511.
Faster memory devices can be placed in these faster(higher) memory tiers.
Slower memory devices like persistent memory will have abstract distance
higher than the default DRAM level.

[akpm@linux-foundation.org: fix comment, per Aneesh]
Link: https://lkml.kernel.org/r/20220818131042.113280-1-aneesh.kumar@linux.ibm.com
Link: https://lkml.kernel.org/r/20220818131042.113280-2-aneesh.kumar@linux.ibm.comSigned-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Acked-by: Wei Xu <weixugc@google.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Bharata B Rao <bharata@amd.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Hesham Almatary <hesham.almatary@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Tim Chen <tim.c.chen@intel.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Jagdish Gediya <jvgediya.oss@gmail.com>
Cc: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>