[Cleanup][MoE] Move FlashInfer MoE helpers under fused_moe

[TANGBUDU]

Purpose

Fixes #31414.

flashinfer_utils.py currently lives under quantization utils, but its helpers are MoE-specific and are also used by BF16 and unquantized MoE code. This PR moves it to vllm.model_executor.layers.fused_moe.flashinfer and updates in-tree callers and Buildkite test-area routing.

vllm.utils.flashinfer remains unchanged as the general compatibility/wrapper layer. The moved implementation is unchanged apart from its module docstring.

Why this approach

The alternatives were checked before submission: #45618 and #49867 keep the module under quantization utils; #42378 and #51523 put model-executor code under vllm.utils; #35440 splits the helpers across two locations. This approach keeps the change mechanical while placing the module under its owning subsystem.

#54538 subsequently proposed a rename-in-place with a compatibility shim. This PR instead makes fused_moe the canonical owner: 9 of the 11 affected production callers already live there, and fused_moe/flashinfer.py follows the existing subsystem-scoped naming convention.

Rebase and method audit

Current head: e02d8c31d, rebased onto upstream 6b5a12c0f on 2026-09-08. The 102 added/removed patch lines are identical to the previously reviewed revision; the rebase incorporates upstream changes without adding unrelated fixes.

• The helper preserves upstream's gated-padding fixes from [Model] Support Qwen3.8-Flash-Next #53896 and the TRT-LLM packing behavior from [Chore] Remove redundant _pack_topk_ids_weights_kernel in TrtLLM NvFP4 MoE #46872.
• The BF16 MoE test retains the upstream oracle abstraction instead of restoring its old direct helper import.
• Both the direct imports and the monkeypatch module alias in tests/quantization/test_fp8.py were migrated. The newer caller in the NVFP4 hidden-dimension-padding tests was also migrated.
• The B200 Kernels job retains the three-shard structure from [CI] Shard long kernel test groups #54754.
• A source audit compared all 16 changed Python files with the parent. After normalizing only the intended import targets, top-level import ordering, and helper docstring, the remaining ASTs match. This is a source-level check, not proof of runtime import-order equivalence.
• The old helper already imported fused_moe.activation, so importing the fused_moe parent package is not a newly introduced dependency. Old dotted/slash helper-path searches return zero matches.

CI routing

A follow-up audit found that moving the helper out of layers/quantization removed source-dependency matches that previously came from directory-level entries such as vllm/model_executor/layers/quantization.

The follow-up commit restores those matches with exact-file dependencies for the relocated helper. It does not change test commands or runtime code.

Validation used the dependency matcher from vllm-project/ci-infra commit 8fb314054d85497c752009deda796dc21d751c94 over the active CI configuration inputs:

• 58 active YAML files checked.
• 129 dependency lists matched the old helper path before the move.
• 12 of those matches were lost after the move.
• 0 are lost with the follow-up commit.
• Non-dependency job fields are unchanged.
• git diff --check passes.

This verifies routing equivalence, not execution of the affected GPU jobs.

Intel CI triage (2026-09-08)

The earlier Intel build 10676, on d47d5a1d9, really ran tests and failed. It is separate from the GitHub pre-run-check contributor-authorization gate.

• The two test_wna16_cuda_high_bit_skips_humming[4/8] failures also occur in the exact upstream-base build 10659, on 6fbb00b18, with the same NotImplementedError. Upstream fixed the CUDA-only test's platform guard in [CI] [Test] skip test_wna16_cuda_high_bit_skips_humming on non-CUDA platforms #55660; this rebase includes that merged fix unchanged.
• test_sampling_mask_tensors_match_finite_support[512] also fails in the independent main-branch build 10713. [CI][Intel XPU] Isolate test_outputs.py into its own pytest process in "V1 Sample + Logits" to avoid intermittent failure #55638 tracks a mitigation; its underlying race is not fully diagnosed and its changes are not copied into this PR.
• test_ipc_cache_cold_start_and_warm_restart fails in both 10676 and main-branch build 10713. The PR log reports a closed socket during weight-cache loading; the baseline's top-level exception differs. This is evidence of a pre-existing failure, not proof of an identical root cause.
• TestTritonTopkTopp::test_topk_only[32000-1024] reports a top-k support-count mismatch on XPU. Its root cause is still unconfirmed. The relevant sampler/test files are unchanged by this PR; that alone does not prove the failure unrelated.

The new head passes changed-Python compilation (16 files), git diff --check, and the official dependency-matcher audit (58 YAML files, 129 old-path matches, none lost). No new Intel runtime pass is claimed. Targeted CUDA runtime checks are recorded below. The unresolved hardware failures need supported-hardware verification; no assertions, tolerances, or CI jobs were removed to hide them.

Previously recorded GPU validation

The results in this section were recorded before the 2026-09-07 server rebase. They must not be read as fresh GPU results for e02d8c31d.

Environment: Linux, Python 3.12.14, PyTorch 2.13.0+cu132, FlashInfer 0.6.18, RTX 5070 Ti Laptop GPU (SM120).

Targeted module-move smoke:

.venv/bin/python -m pytest \
  tests/quantization/test_fp8.py \
  tests/quantization/test_trtllm_nvfp4_hidden_dim_padding.py \
  tests/kernels/moe/test_flashinfer.py \
  tests/kernels/moe/test_routed_experts_capture_monolithic.py \
  tests/kernels/moe/test_trtllm_bf16_moe.py \
  -q \
  -k "test_prepare_gated_trtllm_fp8_moe_weights_pads_each_projection or test_shared_nvfp4_input_scales_have_writable_storage or test_align_trtllm_fp4_moe or test_convert_moe_weights_to_flashinfer_trtllm_block_layout"

Result: 7 passed, 2 skipped, 43 deselected.

.venv/bin/python -m pytest tests/quantization/test_trtllm_nvfp4_hidden_dim_padding.py -q

Result: 4 passed.

An earlier full five-file run on f747ddd collected 82 tests: 41 failed, 38 passed, 3 skipped. A runtime-only pytest helper set the default GPU memory utilization to 0.88 for the 12 GiB device; repository test sources and skip conditions were unchanged. All 41 failures encountered unavailable FlashInfer TRT-LLM cubins on SM120. Six representative failures across different kernel families reproduced on a clean upstream worktree. The supported selection completed with 38 passed, 3 skipped, 41 deselected. These results do not validate the unsupported kernel families.

Before the latest server rebase, changed-file compilation, pinned Ruff, applicable pre-commit hooks including Python 3.10 mypy, manual Python 3.12 mypy, and isolated imports of twelve production modules passed. The earlier no-overlap rebase from a41cfd44 to c7b85be83 had an identical stable patch ID.

Latest-head runtime validation (2026-09-08)

Re-ran the existing module-move smoke on e02d8c31dfd7e00756e5131954946bde8e3ccac8:

PYTHONPATH="$PWD" .venv/bin/python -m pytest \
  tests/quantization/test_fp8.py \
  tests/quantization/test_trtllm_nvfp4_hidden_dim_padding.py \
  tests/kernels/moe/test_flashinfer.py \
  tests/kernels/moe/test_routed_experts_capture_monolithic.py \
  tests/kernels/moe/test_trtllm_bf16_moe.py -q \
  -k "test_prepare_gated_trtllm_fp8_moe_weights_pads_each_projection or test_shared_nvfp4_input_scales_have_writable_storage or test_align_trtllm_fp4_moe or test_convert_moe_weights_to_flashinfer_trtllm_block_layout"

• Result: 7 passed, 2 skipped, 48 deselected.
• The complete test_trtllm_nvfp4_hidden_dim_padding.py file: 4 passed.
• All 12 changed production modules imported successfully in separate fresh Python processes.

Environment: WSL Ubuntu 22.04, Python 3.12.14, PyTorch 2.13.0+cu132, FlashInfer 0.6.18, RTX 5070 Ti Laptop GPU (SM120, 12 GiB). Python sources came from the exact PR head. Native extensions and the generated FlashMLA interface were reused from the existing local installation; native vLLM extensions were not rebuilt for this source revision. Initial collection errors from missing build artifacts were resolved before the successful runs.

These results cover the module move's targeted runtime checks. They do not establish a full supported-hardware GPU suite pass or an Intel CI pass. The GitHub pre-run-check failure is the contributor-authorization gate; the Intel job is separate.

AI Assistance

AI assistance was used for rebase/conflict analysis, patch preparation, source auditing, CI routing analysis, and validation scripting. Historical runtime results and latest-head source checks are separated above.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Team

Run ID: b9d377be-f739-47fe-a312-9274c97b5c81

📥 Commits

Reviewing files that changed from the base of the PR and between 0ac7d8c and d47d5a1.

📒 Files selected for processing (7)

• .buildkite/intel_jobs/lm_eval_intel.yaml
• .buildkite/intel_jobs/quantization.yaml
• .buildkite/test_areas/compile.yaml
• .buildkite/test_areas/kernels.yaml
• .buildkite/test_areas/lm_eval.yaml
• .buildkite/test_areas/plugins.yaml
• .buildkite/test_areas/quantization.yaml

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Walkthrough

Changes

The PR moves FlashInfer fused MoE helper imports to fused_moe.flashinfer, updates affected tests, and tracks the module in Buildkite source dependencies. Runtime logic and test behavior remain unchanged.

FlashInfer Import Consolidation

Layer / File(s)	Summary
FlashInfer helper import migration vllm/model_executor/layers/fused_moe/..., vllm/model_executor/layers/quantization/utils/...	Production modules import FlashInfer helpers from vllm.model_executor.layers.fused_moe.flashinfer.
Test import updates tests/kernels/moe/*, tests/quantization/*	Tests use the new helper import paths without changing test logic.
Buildkite dependency updates .buildkite/intel_jobs/*, .buildkite/test_areas/*	Kernel, evaluation, plugin, compile, and quantization jobs track fused_moe/flashinfer.py as a source dependency.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to d47d5

MoE-specific FlashInfer helper imports and CI routing now use the fused MoE module, with no remaining concrete merge-readiness risk identified.

Suggested reviewers: zxd1997066

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 16 files. (7 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The description references issue #31414 and explains how the module relocation addresses the issue's ownership and naming concerns.
Out of Scope Changes check	✅ Passed	The changes match the stated objective. They update imports, add the relocated module docstring, and restore CI dependency coverage without unrelated runtime changes.
Title check	✅ Passed	The title clearly and concisely describes the main change: moving MoE-specific FlashInfer helpers under the fused_moe module.
Description check	✅ Passed	The description directly explains the module move, updated callers, CI routing, rationale, and validation. It is clearly related to the changeset.

Full details: Docstring Coverage

Explanation

Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 16 files. (7 skipped: 7 unsupported.)

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

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