[None][fix] Size the cross-attention KV cache from the encoder's decl…

[ravo42-maker]

…ared length, not max_input_len

For encoder-decoder models the cross KV pool was sized from llm_args.max_input_len (default 1024), which overwrote the engine length unconditionally; the config-derived encoder limit was then applied with min() and could only shrink it further. Whisper's encoder emits max_source_positions=1500 frames, so every request failed in prepare_cross_metadata with "max KV cache length of input sequences (1500) exceeds the KV cache manager's maximum supported length (1024)".

Treat the encoder's declared position limit (now including max_source_positions) as authoritative for the cross pool and fall back to max_input_len only when the model declares no encoder length.

Fixes #19514.

Verified on RTX 5070 Ti (sm_120) with openai/whisper-large-v3 (release 1.3.0rc25 + #19450): the default-argument reproduction from #18609 transcribes.

Dev Engineer Review

_get_cross_kv_cache_layout now recognizes max_source_positions and gives declared encoder limits precedence over max_input_len. A positive max_input_len remains the fallback when no encoder limit exists. No public API changes are reported.

QA Engineer Review

Modified tests/unittest/_torch/executor/test_cross_kv_cache_layout.py with CPU-only regression coverage for Whisper sizing, encoder-limit precedence, supported encoder-limit attributes, and fallback behavior. Coverage is sufficient for the reported defect. The test directory is skipped by the broad tests/integration/test_lists/waives.txt waiver for NVBugs 6800103. No CI or manual-QA test-list entry was found.

Per-File QA Perspective

• tensorrt_llm/_torch/pyexecutor/_util.py: Verify that declared encoder limits, including max_source_positions, size the cross-attention KV cache and are not reduced by max_input_len. Verify fallback behavior when no encoder limit exists.
• tests/unittest/_torch/executor/test_cross_kv_cache_layout.py: Covers the Whisper regression, precedence rules, encoder-limit lookup names, and fallback paths. The test directory is covered by the unittest/_torch/executor waiver in tests/integration/test_lists/waives.txt.

Description

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Fixes #19514. Follow-up to #18609 / #19450.

Problem

For encoder-decoder models, _get_cross_kv_cache_layout() (tensorrt_llm/_torch/pyexecutor/_util.py) sizes the cross-attention KV pool from llm_args.max_input_len (default 1024), which overwrites the engine sequence length unconditionally; the config-derived encoder limit is then applied with min() and can only shrink it further. Whisper declares its encoder length as max_source_positions (1500), which was not in the lookup list, so every openai/whisper-large-v3 request with default arguments failed in prepare_cross_metadata:

The max KV cache length of input sequences (1500) exceeds the KV cache manager's maximum supported length (1024).

Change

• Add max_source_positions to the encoder-length lookup.
• When the model declares an encoder length, use it as the cross pool length (the pool holds the encoder output; that limit is authoritative).
• Fall back to max_input_len only when the model declares no encoder length, so models without any of those config attributes keep today's behaviour.

One function, +14 −6.

Verification

RTX 5070 Ti (sm_120), nvcr.io/nvidia/tensorrt-llm/release:1.3.0rc25 with the #19450 guard, openai/whisper-large-v3, LLM API directly:

config	before	after
defaults (max_input_len 1024, block reuse on)	1500 exceeds 1024	transcribes, 30 s clip in 3.5 s
max_input_len=1500 (workaround)	transcribes	transcribes

Verified against the 1.3.0rc25 release rather than a main build; the function is byte-identical between the two. Run matrix and logs are in my comments on #19450 and in #19514.

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Walkthrough

The cross-attention KV cache layout now prioritizes encoder position limits, including max_source_positions. It uses a positive decoder-side max_input_len only when no encoder limit exists. CPU-only tests cover precedence and fallback behavior.

Changes

Cross-KV cache sizing

Layer / File(s)	Summary
Encoder limit precedence tensorrt_llm/_torch/pyexecutor/_util.py	The layout uses encoder-specific limits directly, including max_source_positions and encoder_max_position_embeddings. It falls back to a positive max_input_len only when no encoder limit exists.
Cross-KV layout validation tests/unittest/_torch/executor/test_cross_kv_cache_layout.py	CPU-only tests verify Whisper encoder limits, supported configuration attributes, and fallback selection when encoder and decoder limits are unavailable or unusable.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~15 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: bowenfu

Merge Risk: 🔵 Low · up to a87de

This change makes cross-attention KV-cache sizing honor encoder lengths such as Whisper’s 1500 positions instead of the decoder’s 1024 default. The reported regression cases are not independently established and the test directory remains waived, leaving low residual merge risk until that coverage runs.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the fix and the primary change: sizing the cross-attention KV cache from the encoder's declared length. It follows the required [None][fix] format.
Description check	✅ Passed	The description explains the problem, root cause, implementation, regression coverage, and verification results. The required sections and checklist template are included, although the checklist remai…
Linked Issues check	✅ Passed	The PR satisfies issue #19514. _get_cross_kv_cache_layout recognizes max_source_positions and uses a declared encoder limit as the authoritative cross-attention cache length. A declared encoder li…
Out of Scope Changes check	✅ Passed	The changes are limited to cross-attention KV-cache sizing and focused regression tests. These changes directly support issue #19514. No unrelated implementation or test changes are identified.
Docstring Coverage	✅ Passed	Docstring coverage is 85.71% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 2 files.

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Actionable comments posted: 1

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Inline comments:
In `@tensorrt_llm/_torch/pyexecutor/_util.py`:
- Around line 2217-2232: The change lacks regression coverage for
KvCacheCreator._get_cross_kv_cache_layout. Add focused tests under tests/**
asserting max_source_positions=1500 takes precedence over max_input_len=1024,
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[ravo42-maker]

Added the regression coverage CodeRabbit asked for: tests/unittest/_torch/executor/test_cross_kv_cache_layout.py (cpu_only, no engine needed — it builds a KvCacheCreator with only the state _get_cross_kv_cache_layout reads).

It asserts:

• Whisper-large-v3's config (max_source_positions=1500) sizes the cross pool to 1500 with max_input_len=1024, and the geometry is (32 layers, 20 KV heads, head_dim 64).
• The encoder limit is authoritative: neither max_seq_len (512/1024/4096), max_input_len (None/0/512/1024/2048) nor an explicit fallback_max_seq_len can move it.
• Every name in the encoder-limit lookup is recognized, including the new max_source_positions.
• Without an encoder limit a positive max_input_len is the fallback, and without that the engine's max_seq_len / explicit fallback is used.

Verified against the release:1.3.0rc25 container: 23 of 28 cases fail on the unpatched code (they hit the min(max_input_len, ...) clamp and the missing max_source_positions lookup) and all 28 pass with this PR applied.

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Inline comments:
In `@tests/unittest/_torch/executor/test_cross_kv_cache_layout.py`:
- Line 103: Extend the cross-KV cache layout tests around
_get_cross_kv_cache_layout to cover precedence when no encoder limit exists:
with max_input_len=1024 and fallback_max_seq_len=2048, assert the layout value
is 1024. Keep the existing fallback cases for unusable max_input_len values.
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[ravo42-maker]

Thanks, both points are fair.

Fallback precedence — added in the latest commit. test_max_input_len_is_the_fallback_without_an_encoder_limit is now parametrized over max_input_len 512/1024/2048 and asserts the result against an explicit fallback_max_seq_len of 2048 and 256 as well as the bare call, so a fallback-first ordering can no longer pass. The file is 30 cases now; on release:1.3.0rc25 23 of 30 fail unpatched and all 30 pass with this PR's diff applied.

Waiver — confirmed: waives.txt line 257 waives unittest/_torch/executor as a whole against https://nvbugs/6800103, and apply_waives expands a directory entry to every node under it, so this file (and every other executor test, on every stage) is skipped while that line stands. That waiver landed in #19040 on 2026-09-21, the commit this branch is based on, against an internal bug I can't see, so I have deliberately not narrowed or removed it in this PR. The test needs nothing from the executor package beyond KvCacheCreator, so if the maintainers would rather it run now than when 6800103 is closed, I'm happy to either move it out from under the waived directory (e.g. tests/unittest/_torch/test_cross_kv_cache_layout.py with an l0_cpu.yml entry) or add a narrower waiver in place of the directory one — whichever you prefer.