[https://nvbugs/5624818][fix] Fix GPT-OSS with non-paged_context_fmha

[pengbowang-nv]

Summary by CodeRabbit

• Bug Fixes
  • Added runtime validation to ensure attention sinks are only compatible with flash attention.
  • Strengthened constraints in non-flash-attention code paths to prevent invalid configuration combinations.

Description

Fixed a GPT-OSS output error with enable_kvcache_reuse=False. It's due to selecting a kernel that does not support attention sink when seqlen is small.

After fix:

> python ./examples/llm-api/quickstart_advanced.py --model_dir /home/scratch.trt_llm_data/llm-models/gpt_oss/gpt-oss-20b --tp_size 1 --moe_ep_size 1 --moe_tp_size 1 --max_seq_len 512  --disable_kv_cache_reuse

[0] Prompt: 'Hello, my name is', Generated text: ' John Doe. I am a software engineer with a passion for developing innovative solutions. I have experience in Java, Python, and JavaScript.'

> python ./examples/llm-api/quickstart_advanced.py --model_dir /home/scratch.trt_llm_data/llm-models/gpt_oss/gpt-oss-20b --tp_size 1 --moe_ep_size 1 --moe_tp_size 1 --max_seq_len 512

[0] Prompt: 'Hello, my name is', Generated text: ' John Doe. I am a software engineer with a passion for developing innovative solutions. I have experience in Java, Python, and JavaScript.'

Test Coverage

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[pengbowang-nv]

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[coderabbitai]

📝 Walkthrough

Walkthrough

The changes enforce compatibility constraints between attention sinks and flash attention across two kernel files. One file adds a runtime assertion validating that attention sinks are only used when flash attention is enabled, while the other restricts the non-flash-attention fallback path to exclude cases where attention sinks are present.

Changes

Cohort / File(s)	Summary
Attention Sinks Validation cpp/kernels/fmha_v2/src/fused_multihead_attention.cpp, cpp/tensorrt_llm/kernels/contextFusedMultiHeadAttention/fmhaRunner.cpp	Added runtime assertion and conditional constraints to ensure attention sinks are only used with flash attention enabled. The assertion checks compatibility, and the fallback path now explicitly excludes attention sink scenarios.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 inconclusive)

Check name	Status	Explanation	Resolution
Description check	❓ Inconclusive	PR description addresses the issue and solution but lacks required sections (title format, test coverage details) and checklist completion clarity.	Add properly formatted PR title following [type] convention, provide specific test case names that validate the fix, and clarify which checklist items apply to this change.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main fix: preventing selection of a kernel that doesn't support attention sinks in non-paged context FMHA scenarios.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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[tensorrt-cicd]

PR_Github #34956 [ run ] triggered by Bot. Commit: e06f802

[tensorrt-cicd]

PR_Github #34956 [ run ] completed with state SUCCESS. Commit: e06f802
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