[TRTLLM-8376][feat] top-p optimization (removes redundant softmax)

[ixlmar]

Description

Avoid recomputing softmax after top-p prob masking.

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[ixlmar]

/bot run

[tensorrt-cicd]

PR_Github #25575 [ run ] triggered by Bot. Commit: 716e9f7

[tensorrt-cicd]

PR_Github #25575 [ run ] completed with state FAILURE. Commit: 716e9f7 /LLM/main/L0_MergeRequest_PR pipeline #19372 completed with status: 'FAILURE'

[ixlmar]

/bot run

[tensorrt-cicd]

PR_Github #25579 [ run ] triggered by Bot. Commit: 716e9f7

[tensorrt-cicd]

PR_Github #25579 [ run ] completed with state FAILURE. Commit: 716e9f7 /LLM/main/L0_MergeRequest_PR pipeline #19376 completed with status: 'FAILURE'

[ixlmar]

/bot run --disable-fail-fast

[tensorrt-cicd]

PR_Github #25587 [ run ] triggered by Bot. Commit: 716e9f7

[tensorrt-cicd]

PR_Github #25587 [ run ] completed with state SUCCESS. Commit: 716e9f7 /LLM/main/L0_MergeRequest_PR pipeline #19382 completed with status: 'SUCCESS' Pipeline passed with automatic retried tests. Check the rerun report for details.

[ixlmar]

According to benchmarks, this provides a small speed-up (observed 1.7 %).

[coderabbitai[bot]]

📝 Walkthrough

Walkthrough

This PR refactors the top-p masking logic in top_k_top_p_sampling_batch to compute masks in cumulative-probability space and reconstruct probability distributions aligned to original token order. The function now returns reconstructed probabilities instead of softmax distributions. An early-exit condition is added to assert_no_cuda_sync when CUDA_LAUNCH_BLOCKING is set.

Changes

Cohort / File(s)	Summary
Sampling probability distribution refactoring tensorrt_llm/_torch/pyexecutor/sampling_utils.py	Reworked top-p masking to use mask_to_remove and cumulative probabilities instead of boolean scatter approach. Probability handling now masks in sorted-probability space and reconstructs distribution aligned to original token order. Function return value changed from (next_tokens, softmax) to (next_tokens, probs). Wrapper functions implicitly propagate new probability distribution.
Testing utilities enhancement tests/unittest/utils/util.py	Added early-exit path in assert_no_cuda_sync when CUDA_LAUNCH_BLOCKING environment variable is set, skipping synchronization-check logic and logging control.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

• tensorrt_llm/_torch/pyexecutor/sampling_utils.py: Verify correctness of new cumulative probability masking logic, probability reconstruction and renormalization, and that wrapper functions properly propagate the new probs return value
• tests/unittest/utils/util.py: Confirm early-exit logic with CUDA_LAUNCH_BLOCKING is correct and test coverage exists for both code paths

Pre-merge checks and finishing touches

❌ 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%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main optimization: removing redundant softmax computation after top-p masking.
Description check	✅ Passed	The PR description provides the key motivation (avoiding redundant softmax recomputation), and the checklist is completed with the box checked. However, the description lacks specific details about test coverage and other template sections.

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[ixlmar]

/bot run --skip-test

[tensorrt-cicd]

PR_Github #25727 [ run ] triggered by Bot. Commit: 0a63721

[tensorrt-cicd]

PR_Github #25727 [ run ] completed with state SUCCESS. Commit: 0a63721 /LLM/main/L0_MergeRequest_PR pipeline #19509 (Partly Tested) completed with status: 'SUCCESS' Pipeline passed with automatic retried tests. Check the rerun report for details.

[ixlmar]

/bot skip --comment "No functional changes"

[tensorrt-cicd]

PR_Github #25741 [ skip ] triggered by Bot. Commit: 0a63721

[tensorrt-cicd]

PR_Github #25741 [ skip ] completed with state SUCCESS. Commit: 0a63721 Skipping testing for commit 0a63721