[https://nvbugs/5787453][fix] Better align MLA chunking with indexer chunking when chunked prefill enabled for DSV32

[chang-l]

Summary by CodeRabbit

• Bug Fixes
  • Improved multi-head attention chunked prefill activation for more consistent context processing.
  • Optimized prefill chunking strategy to better leverage cached context information and reduce computational overhead.

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Description

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[coderabbitai]

📝 Walkthrough

Walkthrough

Modified the MLA chunked prefill activation logic by removing a cached token presence check, allowing it to activate based solely on enable_context_mla_with_cached_kv and chunked_prefill flags. Added clarifying comments distinguishing between single-chunk and multi-chunk prefill paths.

Changes

Cohort / File(s)	Summary
MLA Chunked Prefill Logic tensorrt_llm/_torch/attention_backend/sparse/dsa.py	Removed host_cached_tokens.sum().item() > 0 condition from has_mla_chunked_prefill calculation; simplified activation to depend solely on feature flags. Updated single-chunk path to use prepare_one_prefill_chunk. Added explanatory comments in fallback branch describing indexer's chunking behavior for long sequences.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The PR description is largely empty, containing only the template structure without substantive content in required sections like Description and Test Coverage.	Fill in the Description section explaining what the issue is and the solution. Add Test Coverage section listing relevant tests that safeguard these changes to dsa.py.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Title check	✅ Passed	The title clearly and specifically describes the main change: improving alignment between MLA chunking and indexer chunking for chunked prefill in DSV32, with a valid NVBugs ticket reference.

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✨ Finishing touches

• 📝 Generate docstrings

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[chang-l]

/bot run

[tensorrt-cicd]

PR_Github #31115 [ run ] triggered by Bot. Commit: 2c4492f

[tensorrt-cicd]

PR_Github #31115 [ run ] completed with state SUCCESS. Commit: 2c4492f
/LLM/main/L0_MergeRequest_PR pipeline #24032 completed with status: 'FAILURE'

⚠️ Action Required:

• Please check the failed tests and fix your PR
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[chang-l]

/bot run --disable-fail-fast

[tensorrt-cicd]

PR_Github #31126 [ run ] triggered by Bot. Commit: 2c4492f

[tensorrt-cicd]

PR_Github #31126 [ run ] completed with state SUCCESS. Commit: 2c4492f
/LLM/main/L0_MergeRequest_PR pipeline #24043 completed with status: 'SUCCESS'