Message-ID: <20250612181330.31236-1-bijan311@gmail.com>
Date: Thu, 12 Jun 2025 13:13:26 -0500
From: Bijan Tabatabai <bijan311@...il.com>
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Subject: [RFC PATCH 0/4] mm/damon: Add DAMOS action to interleave data across nodes

From: Bijan Tabatabai <bijantabatab@...ron.com>

A recent patch set automatically set the interleave weight for each node
according to the node's maximum bandwidth [1]. In another thread, the patch
set's author, Joshua Hahn, wondered if/how these weights should be changed
if the bandwidth utilization of the system changes [2].

This patch set adds the mechanism for dynamically changing how application
data is interleaved across nodes while leaving the policy of what the
interleave weights should be to userspace. It does this by adding a new
DAMOS action: DAMOS_INTERLEAVE. We implement DAMOS_INTERLEAVE with both
paddr and vaddr operations sets. Using the paddr version is useful for
managing page placement globally. Using the vaddr version limits tracking
to one process per kdamond instance, but the va based tracking better
captures spacial locality.

DAMOS_INTERLEAVE interleaves pages within a region across nodes using the
interleave weights at /sys/kernel/mm/mempolicy/weighted_interleave/node<N>
and the page placement algorithm in weighted_interleave_nid via
policy_nodemask. We chose to reuse the mempolicy weighted interleave
infrastructure to avoid reimplementing code. However, this has the awkward
side effect that only pages that are mapped to processes using
MPOL_WEIGHTED_INTERLEAVE will be migrated according to new interleave
weights. This might be fine because workloads that want their data to be
dynamically interleaved will want their newly allocated data to be
interleaved at the same ratio.

If exposing policy_nodemask is undesirable, we have two alternative methods
for having DAMON access the interleave weights it should use. We would
appreciate feedback on which method is preferred.
1. Use mpol_misplaced instead
  pros: mpol_misplaced is already exposed publically
  cons: Would require refactoring mpol_misplaced to take a struct vm_area
  instead of a struct vm_fault, and require refactoring mpol_misplaced and
  get_vma_policy to take in a struct task_struct rather than just using
  current. Also requires processes to use MPOL_WEIGHTED_INTERLEAVE.
2. Add a new field to struct damos, similar to target_nid for the
MIGRATE_HOT/COLD schemes.
  pros: Keeps changes contained inside DAMON. Would not require processes
  to use MPOL_WEIGHTED_INTERLEAVE.
  cons: Duplicates page placement code. Requires discussion on the sysfs
  interface to use for users to pass in the interleave weights.

This patchset was tested on an AMD machine with a NUMA node with CPUs
attached to DDR memory and a cpu-less NUMA node attached to CXL memory.
However, this patch set should generalize to other architectures and number
of NUMA nodes.

Patches Sequence
________________
The first patch exposes policy_nodemask() in include/linux/mempolicy.h to
let DAMON determine where a page should be placed for interleaving.
The second patch implements DAMOS_INTERLEAVE as a paddr action.
The third patch moves the DAMON page migration code to ops-common, allowing
vaddr actions to use it.
Finally, the fourth patch implements a vaddr version of DAMOS_INTERLEAVE.

[1] https://lore.kernel.org/linux-mm/20250520141236.2987309-1-joshua.hahnjy@gmail.com/
[2] https://lore.kernel.org/linux-mm/20250313155705.1943522-1-joshua.hahnjy@gmail.com/

Bijan Tabatabai (4):
  mm/mempolicy: Expose policy_nodemask() in include/linux/mempolicy.h
  mm/damon/paddr: Add DAMOS_INTERLEAVE action
  mm/damon: Move damon_pa_migrate_pages to ops-common
  mm/damon/vaddr: Add vaddr version of DAMOS_INTERLEAVE

 Documentation/mm/damon/design.rst |   2 +
 include/linux/damon.h             |   2 +
 include/linux/mempolicy.h         |   2 +
 mm/damon/ops-common.c             | 136 ++++++++++++++++++++
 mm/damon/ops-common.h             |   4 +
 mm/damon/paddr.c                  | 198 +++++++++++++-----------------
 mm/damon/sysfs-schemes.c          |   1 +
 mm/damon/vaddr.c                  | 124 +++++++++++++++++++
 mm/mempolicy.c                    |   4 +-
 9 files changed, 360 insertions(+), 113 deletions(-)

-- 
2.43.5

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