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[None][feat] Add SiTU support to Rubin FC12 MoE backend - #19468

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@xguannv xguannv commented Sep 21, 2026 •

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Dev Engineer Review

  • Adds Rubin FC12 support for ActivationType.SiTu while preserving Swiglu and clamp behavior.
  • Propagates activation and SiTU beta values through the operator, runner, generated kernel, and autotuner identities.
  • Validates supported activations, SiTU parameters, softcaps, and incompatible clamp settings.
  • Adds CUTEDSL_FC12 as a strict Kimi K3 SiTU backend.
  • Review finding counts are unavailable.
  • CI follow-up remains required for the Rubin capability skip condition, the merge from main, and focused Rubin test reruns.

QA Engineer Review

  • Updates FC12 backend, Kimi K3 SiTU, and activation-resolution tests.
  • Covers SiTU softcaps, nonuniform-softcap rejection, SwiGLU clamp behavior, cache-key separation, FC12 preparation, unsupported activations, and CUDA Graph replay.
  • Renames the CI test-list entry to test_codegen_baked_situ_softcaps_are_uniform_scalars.
  • Coverage verdict: needs follow-up because CI reported an unstable L0 result and pending Rubin test updates.

Per-File QA Perspective

  • tensorrt_llm/_torch/custom_ops/cute_dsl_custom_ops.py: Verify activation and SiTU beta defaults, schema changes, and forwarding.
  • tensorrt_llm/_torch/cute_dsl_kernels/rubin/moe/rubin_contiguous_grouped_blockscaled_gemm_fused_fc12.py: Verify SiTU epilogues, validation, clamp rejection, and SwiGLU behavior.
  • tensorrt_llm/_torch/moe/fused_moe/fused_moe_cute_dsl_fc12.py: Verify capability reporting, autotuner-key separation, and FC12 preparation.
  • tensorrt_llm/_torch/models/modeling_kimi_linear.py: Verify strict Kimi K3 routing to CUTEDSL_FC12.
  • tests/integration/test_lists/test-db/l0_b200.yml: Renames the SiTU softcap test entry in the CI test database.
  • tests/unittest/_torch/moe/test_kimi_k3_situ_moe.py: Covers Rubin FC12 execution, capability gating, CUDA Graph replay, and backend preservation. CI-list coverage is not established.
  • tests/unittest/_torch/moe/test_moe_backend.py: Covers softcaps, cache identities, clamp behavior, and FC12 preparation. CI-list coverage is not established.
  • tests/unittest/_torch/moe/test_moe_impl.py: Covers SiTU acceptance and Relu2 rejection. CI-list coverage is not established.

Description

PR #18357 added the Rubin FC12 MoE backend, but its fused epilogue supports only SwiGLU. Kimi K3 uses SiTU, so the resolver cannot select FC12 for those MoE layers.

This change:

  • specializes the FC12 epilogue for SwiGLU or SiTU;
  • passes the SiTU softcaps through the backend and custom op; and
  • includes the activation and softcaps in both compiled-kernel and outer-autotuner identities.

Existing SwiGLU and clamp behavior remains supported.

Test Coverage

Added tests cover:

  • FC12 activation selection and rejection of unsupported activations;
  • validation that SiTU softcaps are uniform scalars;
  • compiled-kernel and autotuner cache-key separation by activation and softcaps;
  • FC12 preparation behavior; and
  • a Rubin-only real FC12 SiTU comparison with CUDA Graph replay and a plain-SwiGLU negative reference.

Validation:

  • All pre-commit hooks applicable to the seven modified files passed, including both repository-wide test-list checks.
  • python3 -m py_compile passed for all six modified Python files.
  • Hecate SM107, implementation commit 51ec5e46b792ef64fd9e5db96dc49edb09bc7ba8, Slurm job 621253: the TensorRT-LLM build with CMAKE_CUDA_ARCHITECTURES=107-real passed. The three implementation-file blobs are unchanged in the current rebased commit 666fc3a35459369f9bf23463d99a3f4fa4ecd4d6.
  • FC12 registry and activation tests: 4 passed.
  • FC12 backend and NVFP4 shape tests: 16 passed; 10 locality-domain variants skipped by their existing condition.
  • Real CUTEDSL_FC12 SiTU kernel and CUDA Graph test: passed. SiTU reference: cosine 0.972989, relative L2 0.232667. Incorrect SwiGLU reference: cosine 0.065067, relative L2 0.998031.

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  • CODEOWNERS updated if ownership changes

  • Documentation updated as needed

  • Update tava architecture diagram if there is a significant design change in PR.

  • The reviewers assigned automatically/manually are appropriate for the PR.

  • Please check this after reviewing the above items as appropriate for this PR.

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@xguannv
xguannv force-pushed the user/xguan/fc12-situ-main branch 2 times, most recently from 51ec5e4 to 10313e1 Compare September 21, 2026 05:05
Signed-off-by: Xin Guan <294044352+xguannv@users.noreply.github.com>
@xguannv
xguannv force-pushed the user/xguan/fc12-situ-main branch from 10313e1 to 666fc3a Compare September 21, 2026 05:53
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xguannv marked this pull request as ready for review September 21, 2026 07:54
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Walkthrough

Rubin NVFP4 fused FC12 now supports SwiGLU and SiTU activation modes. Activation parameters flow through the public operator, runner, tactic keys, autotuner, and generated kernel. Backend selection and tests cover FC12 SiTU execution and degradation rules.

Changes

Rubin FC12 activation support

Layer / File(s) Summary
Operator parameter wiring
tensorrt_llm/_torch/custom_ops/cute_dsl_custom_ops.py
The fused operator and runner expose activation type and optional SiTU beta values. The values are stored, cached, and passed to the generated kernel.
Kernel activation validation and execution
tensorrt_llm/_torch/cute_dsl_kernels/rubin/moe/rubin_contiguous_grouped_blockscaled_gemm_fused_fc12.py
The kernel validates SwiGLU and SiTU configurations. It implements both epilogues and selects the configured path at trace time.
Backend preparation and autotuning
tensorrt_llm/_torch/moe/fused_moe/fused_moe_cute_dsl_fc12.py
The FC12 backend accepts both activation types, includes activation parameters in autotuning identity, prepares supported tiles, and forwards the configuration.
Backend selection and validation
tensorrt_llm/_torch/models/modeling_kimi_linear.py
The Kimi runtime recognizes CUTEDSL_FC12 as a SiTU backend and rejects degradation when the backend is unavailable.
Activation coverage and regression tests
tests/unittest/_torch/moe/*, tests/integration/test_lists/test-db/l0_b200.yml
Tests cover SiTU execution, activation resolution, invalid configurations, tuning-key separation, tile preparation, backend availability, CUDA Graph replay, and renamed integration coverage.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant TorchOperator
  participant FC12Runner
  participant Autotuner
  participant RubinKernel
  participant ActivationEpilogue
  TorchOperator->>FC12Runner: pass activation type and beta values
  FC12Runner->>Autotuner: identify configured workload
  FC12Runner->>RubinKernel: execute fused FC12
  RubinKernel->>ActivationEpilogue: dispatch SiTU or SwiGLU
  ActivationEpilogue-->>TorchOperator: return fused output
Loading

Merge Risk: 🔵 Low · up to 0f893

The FC12 SiTU validation tests do not cover nonuniform linear softcaps, so a regression could go undetected. This is a bounded follow-up risk.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name Status Explanation Resolution
Docstring Coverage ⚠️ Warning Docstring coverage is 54.55% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 33 functions across 6 files. Write docstrings for the functions missing them to satisfy the coverage threshold.
✅ Passed checks (4 passed)
Check name Status Explanation
Title check ✅ Passed The title follows the required format and clearly identifies the main change: adding SiTU support to the Rubin FC12 MoE backend.
Description check ✅ Passed The description explains the problem and solution, lists relevant test coverage, reports validation results, and includes the required checklist sections. It is sufficiently complete for review.
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.
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Actionable comments posted: 1


  • 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
In `@tests/unittest/_torch/moe/test_kimi_k3_situ_moe.py`:
- Around line 1723-1726: Update the FC12 test’s skip condition to also require
the explicit MoED.CUTEDSL_RUBIN capability, while preserving the existing CUDA
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PR_Github #74768 [ run ] completed with state SUCCESS. Commit: 666fc3a
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Blocked for Rubin 1.3 on two mechanical items.

Please update the FC12 SiTU test skip condition to require explicit MoED.CUTEDSL_RUBIN capability while retaining the CUDA/SM107 checks, then merge current main to resolve the conflict and rerun the focused Rubin backend/shape and real-kernel tests. The hardware results in the description are useful; once these two items are on the live head, this can return to review.

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One correctness guard must be fixed before re-review.

The FC12 SiTU test currently gates on CUDA and SM107 but not on the explicit MoED.CUTEDSL_RUBIN capability it exercises. Please add that capability requirement while preserving the existing architecture checks, then resolve the main conflict and rerun the focused Rubin backend/shape and real-kernel tests. This is required for this PR; otherwise unsupported configurations can enter a capability-specific test path.

@svc-trtllm-gh-bot svc-trtllm-gh-bot added the Community want to contribute PRs initiated from Community label Sep 21, 2026

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Approved on functionality; the bot’s remaining test-guard item is non-blocking.

The Rubin FC12 SiTU implementation has focused backend/shape coverage and reported real-kernel plus CUDA-graph validation. Adding MoED.CUTEDSL_RUBIN to the test skip condition is worthwhile test hygiene, but it does not invalidate the supported SM107 path. The branch still must resolve its main conflict before merge.

…u-main

Signed-off-by: Xin Guan <294044352+xguannv@users.noreply.github.com>

# Conflicts:
#	tests/unittest/_torch/moe/test_kimi_k3_situ_moe.py

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Actionable comments posted: 1


  • 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
In `@tests/unittest/_torch/moe/test_moe_backend.py`:
- Line 1307: Update the SiTuActivation validation test to parameterize separate
cases where gate_softcap and linear_softcap are each nonuniform, ensuring both
nonuniform inputs are rejected. Preserve the existing expected error-path
assertion and use scalar values for the other softcap in each case.

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PR_Github #74923 [ run ] triggered by Bot. Commit: 9a7cda2 Link to invocation

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xguannv requested a review from a team as a code owner September 22, 2026 05:47
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PR_Github #74923 [ run ] completed with state SUCCESS. Commit: 9a7cda2
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Actionable comments posted: 1


  • 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
In `@tensorrt_llm/_torch/models/modeling_kimi_linear.py`:
- Line 1406: Add "CUTEDSL_FC12" to the non-degradable set used by create_moe so
unavailable FC12 raises instead of selecting the Cutlass fallback. Preserve the
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No attention change, approve to unblock it

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The reported Rubin numerics contradict the test at head, and CI cannot catch it. The description says the real CUTEDSL_FC12 SiTU test passed with relative L2 0.232667 against the SiTU reference, but _SITU_NVFP4_REL_L2_MAX in test_kimi_k3_situ_moe.py is 0.20 (from #19003, which landed in this branch via the merge from main, after the Slurm run on 51ec5e46). At the current head that number fails the assertion. Since the test skips on every SM except 107, the manual Rubin run is the only merge gate for this path, and it is stale. Please rerun test_nvfp4_kernel_actually_applies_situ[CUTEDSL_FC12] at head and post cosine, rel_l2 and |k|/|ref|.

Beyond pass/fail, the number itself is suspicious. The bound's comment derives it as sqrt(stages)×0.095; the other three NVFP4 backends measure 0.161–0.166 (sqrt(3)×eps). 0.2327/0.095 ≈ 2.45 ≈ sqrt(6), i.e. roughly three extra quantization-stage-equivalents, and it also exceeds the measured plain-SwiGLU distance (≥0.205). The SiTU epilogue math is copied verbatim from the Blackwell act-fusion kernel that scores 0.16, so the excess is more likely in the FC12-specific path (in-kernel requant/SFC generation, alpha application) than in the activation formula. Comparing FC12 SwiGLU vs two-op CUTEDSL SwiGLU on the same shape would tell whether this is SiTU-specific or an FC12-wide precision characteristic. Please do not loosen the bound without that analysis; the comment above it says why.

Smaller items:

  • The description is truncated mid-sentence ("Incorrect SwiGLU ref") — that number is exactly what is needed for the analysis above.
  • [None] on a feature PR that adds a new kernel epilogue and backend capability: please attach a JIRA.
  • The CuteDslFc12FusedMoENvfp4Runner.forward / do_preparation override is a behavior change (FC12 preparation went from a no-op to priming both tiles) that the description does not mention.
  • CUTEDSL_FC12 is not mentioned anywhere under docs/source (pre-existing since #18357). A line in the MoE backend docs listing it and its SwiGLU/SiTU support would help.

"ActivationType.SiTu requires both situ_beta and "
f"situ_linear_beta, got {situ_beta} and {situ_linear_beta}."
)
if situ_beta <= 0 or situ_linear_beta <= 0:

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<= 0 admits inf and nan (every comparison with nan is False). Both are then folded into the epilogue constants: with beta=inf, 2.0/beta, 2.0*beta, -beta become 0, inf, -inf and every output element is inf*sigmoid(0) - inf = NaN. The Blackwell act-fusion kernel checks 0 < situ_beta < _INF and 0 < situ_linear_beta < _INF for exactly this reason; please use the same bounded check here (or math.isfinite) so a bad softcap raises at construction instead of producing NaNs at runtime.

)

@cute.jit
def _apply_situ_epilogue(

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This method and the nested _sigmoid are byte-for-byte copies of _apply_situ_epilogue in blackwell/blockscaled_contiguous_gather_grouped_gemm_act_fusion.py:2836-2924. Copying is defensible if the Rubin kernel must stay self-contained, but a numerics fix in one will not reach the other. Consider lifting the packed-f32x2 sigmoid/tanh helpers into cute_dsl_kernels/utils (next to silu_f32/sigmoid_f32) and calling them from both kernels, or at minimum add a cross-reference comment in both places so the drift risk is visible.


router_logits = gate.compute_logits(x)
actual = moe.forward(x, router_logits, all_rank_num_tokens=None).float()
assert torch.isfinite(actual).all()

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This test asserts situ_l2 < _SITU_NVFP4_REL_L2_MAX (0.20, line 1836) further down, but the PR description reports rel_l2 0.232667 for CUTEDSL_FC12. That run predates the merge of #19003, which introduced the calibrated bound. Because the FC12 parametrization skips on every SM except 107, no CI lane will exercise this; please rerun at head on Rubin and report cosine, rel_l2 and |k|/|ref|. If it fails, follow the comment above the bound: 0.233 ≈ sqrt(6)*0.095 vs the other backends' sqrt(3)*0.095, which points at extra quantization stages in the FC12 path rather than at the bound.

- unittest/_torch/moe/test_moe_backend.py::test_trtllm_gen_nvfp4_situ_selects_padded_quant_method
- unittest/_torch/moe/test_moe_backend.py::test_trtllm_gen_nvfp4_situ_fc31_scale_c_drops_dequant_scale
- unittest/_torch/moe/test_moe_backend.py::test_megamoe_bakes_situ_softcaps_as_uniform_scalars
- unittest/_torch/moe/test_moe_backend.py::test_codegen_baked_situ_softcaps_are_uniform_scalars

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The rename is complete (only this list had the old name), but the four new GPU-free tests in test_moe_backend.py (test_fc12_rejects_nonuniform_situ_softcaps, test_fc12_swiglu_keeps_clamp_without_situ_constants, test_fc12_outer_tuning_separates_activation_and_softcaps, test_fc12_preparation_primes_fused_tiles_without_two_op_memset_knob) and test_moe_impl.py::test_fc12_pinned_activation_support are not registered here or in any other l0 list, so CI never runs them. This file lists test_moe_backend.py per-function, and test_moe_impl.py is not in any list at all. Please add the new functions here alongside this entry.

tactic: Optional[int],
do_preparation: bool = False,
) -> torch.Tensor:
if do_preparation:

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Worth stating in the docstring that this override exists because setting workload_identity (new in this PR) turns on the parent's priming loop, which passes overlap_moe_output_memset=False, a kwarg run_moe_nvfp4_impl does not accept. Before this PR FC12 preparation was effectively a no-op (identity was None), so this is a behavior change for the SwiGLU path too, not just SiTU; the PR description should mention it.

Signed-off-by: Xin Guan <294044352+xguannv@users.noreply.github.com>
@xguannv
xguannv requested a review from a team as a code owner September 27, 2026 14:42
@xguannv
xguannv requested a review from chang-l September 27, 2026 14:42

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8 participants