[Bugfix][MoE] Respect padded row strides in top-k routing by Id545 · Pull Request #57823 · vllm-project/vllm

Id545 · 2026-09-20T19:03:57Z

Purpose

Inductor can produce router logits with padded physical rows, for example shape
[M, 60] and stride (64, 1). The native MoE top-k path used
blockIdx.x * num_experts to locate each input row, so it read the wrong logits
whenever the physical row stride differed from num_experts. This silently
changed expert routing under compilation.

This change:

propagates gating_output.stride(0) to the native top-k launcher;
uses the existing unfused softmax/sigmoid plus top-k path for padded rows;
allocates the required workspace for padded power-of-two expert counts;
keeps packed rows on the existing specialized path; and
adds regression coverage for softmax and sigmoid with 60/64 and 64/72
logical/physical row widths.

The input-row offset is int64_t because PyTorch tensor strides are 64-bit;
this also avoids 32-bit wraparound in the input address calculation. Open PR
#53131 independently widens
packed-row pointer arithmetic more broadly, but it does not propagate
stride(0) or handle padded rows. The two changes overlap only on the
input-offset width; this PR addresses the separate functional stride bug.

No open PR referencing #56900 or implementing padded-row support for this
kernel was found before submission.

Test Plan

uvx pre-commit run --files \
  csrc/libtorch_stable/moe/topk_softmax_kernels.cu \
  tests/kernels/moe/test_fused_topk.py

.venv/bin/python -m pytest -q \
  tests/kernels/moe/test_fused_topk.py \
  -k padded_row_stride

EXPECTED_VLLM="$(.venv/bin/python -c 'import vllm; print(vllm.__version__)')"
.venv/bin/python \
  /workspace/fp-atomics-census/probes/diagnostics/moe_compile/moe_compile_reproducer.py \
  --output-dir /workspace/results-56900/patched-e2e \
  --runners v2 \
  --compiles on off \
  --graphs off \
  --expected-vllm "$EXPECTED_VLLM"

The same focused cases were also run with a standalone kernel reproducer before
and after rebuilding vLLM from source. The end-to-end command uses the
issue reproducer.
Each end-to-end arm ran in a fresh process with separate vLLM, Inductor, and
Triton cache roots.

Test Result

Check	Baseline	Patched
Kernel reproducer	failed as expected; minimum 3/33 matching rows	passed; 33/33 in all four cases
Focused padded-stride pytest	not applicable	4 passed
Compiled duplicate-prompt pairs	0/8	8/8
Compiled short-cycle outputs	9	0
Eager duplicate-prompt pairs	8/8	8/8
Eager short-cycle outputs	0	0

The end-to-end control used
Qwen/Qwen1.5-MoE-A2.7B-Chat at revision
ec052fda178e241c7c443468d2fa1db6618996be with the 16-prompt duplicate
batch from #56900. The compiled and eager arms held the model runner and
selected backends fixed; only compilation mode changed.

Validation environment:

NVIDIA H100 80 GB HBM3, compute capability 9.0
CUDA 13.0 (nvcc 13.0.88), driver 580.126.09
PyTorch 2.13.0+cu130
TORCH_CUDA_ARCH_LIST=9.0
vLLM source commit 0748d3bd57cb5c4303e58c8465e21f069629480d

Pre-commit passed on both changed files. The validated source files are
unchanged between the tested commit and the current PR base.

AI assistance

AI assistance was used for root-cause analysis, implementation drafting, and
validation planning. I reviewed every changed line and ran the tests and model
evaluation reported above.

Propagate the router-logit row stride through the native top-k kernels and use the stride-aware unfused path for padded rows. Add regression coverage for softmax and sigmoid routing with padded power-of-two and non-power-of-two expert counts. Assisted-by: OpenAI Signed-off-by: Id545 <idder.ghanbaja@gmail.com>

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github-actions · 2026-09-20T19:04:06Z

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mergify · 2026-10-13T20:35:20Z

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @Id545.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

…ow-stride Signed-off-by: Id545 <idder.ghanbaja@gmail.com>

mergify · 2026-10-20T00:00:42Z

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @Id545.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

…moe-topk-padded-row-stride Signed-off-by: Id545 <idder.ghanbaja@gmail.com> # Conflicts: # csrc/moe/topk_softmax_kernels.cu

bnellnm · 2026-10-26T20:52:32Z

 {
+    STD_TORCH_CHECK(gating_output.dim() == 2,
+                    "gating_output must be a 2D tensor");
+    STD_TORCH_CHECK(gating_output.stride(1) == 1,


This assert might reject edge case tensors, e.g. (M, 1). It could be updated to num_experts == 1 || gating_output.stride(1) == 1.

Thanks, good catch. Both topk_softmax and topk_sigmoid now check num_experts == 1 || gating_output.stride(1) == 1 (a1b3997).

bnellnm · 2026-10-26T20:52:46Z

 {
+    STD_TORCH_CHECK(gating_output.dim() == 2,
+                    "gating_output must be a 2D tensor");
+    STD_TORCH_CHECK(gating_output.stride(1) == 1,


Done in a1b3997

bnellnm

LGTM except for a minor suggestion about the assertions.

A (M, 1) gating output has a contiguous last dimension whatever its stride(1) reports, so the check now only requires stride(1) == 1 when there is more than one expert, as suggested in review. Signed-off-by: Id545 <idder.ghanbaja@gmail.com>

github-actions · 2026-11-05T23:37:52Z

✅ @Id545, CI is now available for this PR.

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Id545 · 2026-11-06T04:39:01Z

/ci run

github-actions · 2026-11-06T04:39:35Z

✅ Triggered Buildkite CI #100455 for commit 359e42a7149f.

ℹ️ This PR is 1343 commits behind upstream main. Merged upstream main into this branch as 359e42a7149f before starting CI. Pull the branch before pushing more changes.

Id545 · 2026-11-06T16:03:25Z

/ci retry

github-actions · 2026-11-06T16:03:46Z

✅ Queued 1 failed job(s) for retry in Buildkite CI #100455.

Id545 requested review from AndreasKaratzas, WoosukKwon, mgoin, tlrmchlsmth, yewentao256 and zyongye as code owners September 20, 2026 19:03

claude Bot reviewed Sep 20, 2026

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mergify Bot added the bug Something isn't working label Sep 20, 2026

jlqibm mentioned this pull request Oct 1, 2026

[Bug]: CUTLASS fp8 MoE corrupts valid tokens when CUDA-graph padding routes are -1 (wrong expert_first_token_offset from moe_permute) #59686

Closed

2 tasks done

mergify Bot added the needs-rebase label Oct 13, 2026

Merge remote-tracking branch 'origin/main' into fix/moe-topk-padded-r…

95670fb

…ow-stride Signed-off-by: Id545 <idder.ghanbaja@gmail.com>

mergify Bot removed the needs-rebase label Oct 14, 2026

mergify Bot added the needs-rebase label Oct 20, 2026

Merge branch 'main' of https://github.com/vllm-project/vllm into fix/…

8c73608

…moe-topk-padded-row-stride Signed-off-by: Id545 <idder.ghanbaja@gmail.com> # Conflicts: # csrc/moe/topk_softmax_kernels.cu

Id545 requested a review from bnellnm as a code owner October 20, 2026 06:00

mergify Bot removed the needs-rebase label Oct 20, 2026

kimseunghyun-kr mentioned this pull request Oct 21, 2026

[Perf][MoE] Select full-warp top-k rows with redux.sync and keep selections in registers (SM80+) #62912

Open

6 tasks done

bnellnm reviewed Oct 26, 2026

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bnellnm requested changes Oct 26, 2026

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Id545 requested a review from bnellnm October 27, 2026 05:48

bnellnm approved these changes Nov 5, 2026

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bnellnm added the ready ONLY add when PR is ready to merge/full CI is needed label Nov 5, 2026

Merge branch 'main' into fix/moe-topk-padded-row-stride

359e42a

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Id545 commented Sep 20, 2026

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