Fix strided reduction block count

[achirkin]

Bound strided reduction kernel block count to fix cudaErrorInvalidValue.

[coderabbitai]

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• cpp/tests/linalg/strided_reduction.cu

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes

  • Improved reliability and stability for large-scale strided reductions.
  • Safely handles very large input dimensions during reduction processing.
  • Supports accurate results for both compensated summation and wider-precision generic reductions.

• Tests

  • Added coverage for reductions spanning millions of rows.
  • Validated multiple reduction modes with large datasets and resource-managed execution.

Walkthrough

The change bounds CUDA reduction grids on the y-dimension and adds coverage for reductions over 8.4 million rows using float and double outputs.

Changes

Strided reduction grid bounds

Layer / File(s)	Summary
Apply bounded reduction launch grids cpp/include/raft/linalg/detail/strided_reduction.cuh	Adds the shared kMaxBlocksDimY limit and applies safe rounded division and y-dimension capping to summation and generic reduction launches.
Validate large reduced dimensions cpp/tests/linalg/strided_reduction.cu	Updates the test helper to use a caller-provided resources handle and dry-run checks. Adds coverage for 8.4 million rows with float and double outputs.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: 🟡 Moderate · up to 45c04

The change bounds strided-reduction block counts, but empty-dimension inputs can still lead to invalid CUDA launches or a zero-divisor calculation. This bounded correctness risk should be fixed or explicitly accepted before merging.

Suggested reviewers: cjnolet

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: bounding the strided reduction block count.
Description check	✅ Passed	The description directly states that the change bounds the kernel block count to prevent cudaErrorInvalidValue.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@cpp/include/raft/linalg/detail/strided_reduction.cuh`:
- Around line 164-166: Validate that reduction dimensions D and N are positive
before initializing or computing launch grids in the affected reduction path,
using RAFT_EXPECTS (or the existing supported empty-result behavior) to prevent
zero grid dimensions and zero-divisor calculations. Anchor the change near the
grid construction and elemsPerThread logic, and add coverage for D == 0 and N ==
0.

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ℹ️ Review info

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Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 8012bfe3-9143-4748-9c72-48c17d449d4b

📥 Commits

Reviewing files that changed from the base of the PR and between 06cb693 and 9a4f96a.

📒 Files selected for processing (2)

• cpp/include/raft/linalg/detail/strided_reduction.cuh
• cpp/tests/linalg/strided_reduction.cu

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[achirkin]

/merge