[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

Test Coverage

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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 uses an available encoder position limit as its sequence length. If no encoder limit exists, it uses a positive max_input_len or falls back to the engine or explicit fallback length. CPU-only tests cover these cases.

Changes

Cross-KV cache sizing

Layer / File(s)	Summary
Encoder limit precedence tensorrt_llm/_torch/pyexecutor/_util.py	The layout uses the first recognized encoder limit directly. A positive max_input_len applies only when no encoder limit exists.
Cross-KV layout validation tests/unittest/_torch/executor/test_cross_kv_cache_layout.py	CPU-only tests cover Whisper encoder limits, supported encoder-limit attributes, and fallback selection for positive or unusable max_input_len values.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: brnguyen2

Merge Risk: ⚪ Minimal · up to 77612

The default Whisper path can use all 1500 encoder positions, and regression coverage is eligible in CI. No actionable merge risk remains.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title follows the required [None][fix] format and clearly identifies the primary change: sizing the cross-attention KV cache from the encoder-declared length.
Description check	✅ Passed	The description clearly explains the problem, root cause, implementation change, issue reference, and verification results. It includes relevant regression coverage, although the repository template r…
Linked Issues check	✅ Passed	The PR meets the coding requirements in issue #19514. _get_cross_kv_cache_layout recognizes max_source_positions, uses a declared encoder limit as the cross-pool length, and does not let `max_inpu…
Out of Scope Changes check	✅ Passed	The changes stay within issue #19514. The implementation changes cross-attention KV-cache sizing, and the new tests directly verify the requested precedence and fallback rules. No unrelated implementa…
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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Actionable comments posted: 2

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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.

[ravo42-maker]

Rebased onto current main (147f70f9, 2026-10-08) — no conflicts, the diff is byte-identical to the previously reviewed version (a87debf5): 3 commits, _util.py +14/-6 and the new test_cross_kv_cache_layout.py. All commits keep their Signed-off-by for DCO.

Re-ran the regression test with the rebased hunk applied to nvcr.io/nvidia/tensorrt-llm/release:1.3.0rc25:

• stock rc25: 23 failed / 7 passed (the expected pre-fix state)
• with this patch: 30 passed

Two notes for whoever picks this up:

• [https://nvbugs/6713231][fix] Guard Whisper cross-KV reuse without encoder tokens #19302 (the maintainer fix for [Bug]: Whisper (PyTorch backend) fails with RuntimeError: bad optional access in the capacity scheduler — TensorRT-LLM 1.3.0rc25, RTX 5070 Ti (sm_120) #18609 / [Bug]: Whisper (PyTorch backend) fails on any repeated request when KV block reuse is on: "Request requires multimodal_embed_mask_cumsum for chunked prefill or KV-cache reuse" #19515, now with three approvals) no longer touches _util.py, and its description keeps the encoder-length sizing out of scope — this PR remains the fix for [Bug]: cross Kv sizing with whisper #19514 and the two are complementary.
• The blanket unittest/_torch/executor waiver is no longer in waives.txt, so the new test will actually run in CI once a maintainer triggers the bot.

[ravo42-maker]

@pranav-nvidia — congratulations on landing #19302, and thanks for fixing #18609 and the reuse path.

This PR was closed at the same moment #19302 merged, without a note, so I want to check whether that was intentional. #19302's description says the encoder-length sizing issue (#19514, this PR) "remains outside this PR", and the merged diff does not touch _util.py: on current main, _get_cross_kv_cache_layout() still sizes the cross-KV pool from llm_args.max_input_len (default 1024), and the encoder-limit fallback list still does not include Whisper's max_source_positions. The new docs text ("Set max_input_len to at least 1500 … the cross-KV pool is sized from max_input_len") documents the workaround, so #19514 is still open on main as far as I can tell.

Two ways forward, and I am fine with either:

1. The explicit max_input_len=1500 requirement is the intended design. Then I will close [Bug]: cross Kv sizing with whisper #19514 as won't-fix and, if useful, turn item 5 of [Feature]: Whisper serving parity in trtllm-serve — /v1/audio/transcriptions, long-form audio, verbose_json segments, decoder biasing #19994 into a small PR that validates the setting and fails fast with a clear message instead of the current silent phrase loss at max_batch_size=8.
2. A fix is still welcome. This branch (ravo42-maker:fix/cross-kv-encoder-length, 3 signed-off commits, _util.py +14/-6 plus a cpu-only unit test, 30 cases) is unchanged since the CodeRabbit-clean review, merges onto today's main (321a38cd) with no conflicts, and the blanket unittest/_torch/executor waiver that would have skipped its test is gone from waives.txt, so the regression test now runs in CI. Say the word and I will reopen and rebase.

Either way, a one-line answer here lets me tidy up #19514 so it does not sit open indefinitely.