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@Linda-Stadter Linda-Stadter commented Sep 1, 2025

Description

This change fixes a bug in the trt backend workflow that caused non-deterministic behavior for an FP8 engine that has biases enabled in the MLP layer. See bug description

Fix: Adding logic to handle the bias term when fusing gate and fc layers in combination with FP8 quantization. Without this step, the bias term will contain random values.

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Summary by CodeRabbit

  • Bug Fixes
    • Corrects bias handling in the FP8 fused gate-MLP path, aligning it with non-FP8 behavior. This ensures biases are applied when present, improving accuracy and consistency for FP8-quantized models during inference and reducing unexpected output discrepancies across precisions.
  • Reliability
    • Enhances numerical parity and stability for models using fused FP8 layers, leading to more predictable results without changing APIs or non-FP8 behavior.

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PR_Github #17244 [ run ] triggered by Bot

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PR_Github #17244 [ run ] completed with state FAILURE
/LLM/main/L0_MergeRequest_PR pipeline #12965 completed with status: 'FAILURE'

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PR_Github #17335 [ run ] triggered by Bot

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PR_Github #17335 [ run ] completed with state FAILURE
/LLM/main/L0_MergeRequest_PR pipeline #13028 completed with status: 'FAILURE'

@Linda-Stadter Linda-Stadter marked this pull request as ready for review September 5, 2025 12:49
@Linda-Stadter Linda-Stadter requested a review from QiJune September 5, 2025 12:52
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coderabbitai bot commented Sep 5, 2025

📝 Walkthrough

Walkthrough

Updated FP8 quantization path in fuse_gate_mlp to set fused gate-mlp bias when present by concatenating mlp.gate.bias and mlp.fc.bias along axis 0, aligning with existing non-FP8 behavior. No signature changes; only tensorrt_llm/models/modeling_utils.py modified.

Changes

Cohort / File(s) Summary
FP8 fused MLP bias handling
tensorrt_llm/models/modeling_utils.py
In FP8 branch of fuse_gate_mlp, when mlp.bias exists, set fused_layer.fused_fc.bias.value to concat(mlp.gate.bias.raw_value, mlp.fc.bias.raw_value, axis=0), mirroring non-FP8 path. No other logic or API changes.

Sequence Diagram(s)

sequenceDiagram
    participant C as Caller
    participant MU as modeling_utils.fuse_gate_mlp
    participant FL as fused_layer
    Note over C,MU: Fuse gate+MLP
    C->>MU: fuse_gate_mlp(mlp, quant_cfg=FP8, ...)
    MU->>MU: Create fused_layer
    MU->>FL: Assign fused weights (FP8)
    alt FP8 path with bias present
        MU->>MU: concat(gate.bias, fc.bias) along axis 0
        MU->>FL: Set fused_fc.bias.value
    else No bias
        MU->>FL: Leave bias unset
    end
    MU-->>C: return fused_layer
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Actionable comments posted: 1

🧹 Nitpick comments (1)
tensorrt_llm/models/modeling_utils.py (1)

1237-1237: Be explicit when checking for bias presence.

If mlp.bias can be a Parameter, prefer is not None to avoid truthiness quirks: if mlp.bias is not None:

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File: tests/unittest/_torch/auto_deploy/_utils_test/_model_test_utils.py:269-275
Timestamp: 2025-08-27T16:59:12.325Z
Learning: In FP8 quantized linear layers, bias should be kept in high precision (typically float32) rather than being quantized to FP8 or cast to half precision, as bias is added after the matrix multiplication and high precision bias helps maintain numerical accuracy.
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tensorrt_llm/models/modeling_utils.py (1)

1237-1240: Fix restores fused bias in FP8 path — good catch.

Concatenating gate/fc biases for the FP8-fused layer mirrors the non-FP8 path and should eliminate the previous non-determinism from uninitialized bias.

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LGTM

@MartinMarciniszyn MartinMarciniszyn merged commit 9cb5410 into NVIDIA:main Sep 9, 2025
5 checks passed
gergely-magyar pushed a commit to gergely-magyar/TensorRT-LLM that referenced this pull request Sep 9, 2025
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4 participants