[Bugfix][Core] Prevent token lengths exceeding max_model_len in V0

[22quinn]

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Purpose

Fix #19337

Test Plan

pytest tests/entrypoints/llm/test_generate.py -k 'test_max_model_len'

Test Result

Unit test passed

(Optional) Documentation Update

[gemini-code-assist]

Hello @22quinn, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

Summary of Changes

Hello! Gemini or gemini-code-assist here, providing a summary of this pull request. This PR addresses a bug where sequences could potentially exceed the configured max_model_len by one token. The core change involves adjusting the length check in the stop condition logic to ensure sequences are capped precisely at max_model_len. A new test case has been added to specifically verify this behavior, ensuring that the total number of tokens (prompt + generated) does not exceed the maximum allowed length.

Highlights

• Bug Fix: Fixes issue #19337 by preventing sequences from exceeding max_model_len.
• Length Capping Logic: The condition for capping sequences based on length has been updated from strictly greater than (>) to greater than or equal to (>=) max_model_len.
• New Test Case: A dedicated test (test_max_model_len) is introduced to verify that the total sequence length (prompt + output) correctly adheres to the specified max_model_len.

Changelog

• tests/entrypoints/llm/test_generate.py
  • Added a new pytest fixture v1 to ensure the test runs with both engine versions.
  • Introduced test_max_model_len to specifically test the max_model_len capping behavior, asserting that the final sequence length equals max_model_len when max_tokens is set higher.
• vllm/engine/output_processor/stop_checker.py
  • Modified the length check condition in maybe_stop_sequence from seq.get_len() > self._get_max_model_len(lora_req) to seq.get_len() >= self._get_model_len(lora_req) to correctly cap sequences at max_model_len.

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[gemini-code-assist]

Code Review

This pull request effectively addresses the bug where token lengths could exceed max_model_len by one token in V0. The fix in vllm/engine/output_processor/stop_checker.py by changing the comparison operator from > to >= is correct and directly resolves the issue.

The accompanying test, test_max_model_len in tests/entrypoints/llm/test_generate.py, is well-constructed. It clearly sets up a scenario to trigger the max_model_len capping and correctly asserts that the total token length (prompt + output) adheres to this limit. The use of a small max_model_len and max_tokens greater than what max_model_len would allow for output tokens ensures the capping mechanism is properly tested.

The addition of the v1 autouse fixture to run tests with both engines is also a good practice for ensuring broader compatibility and correctness.

Overall, the changes are clear, concise, and the PR includes appropriate testing. Great job!

Summary of Findings

• Correction of max_model_len Enforcement: The PR successfully corrects an off-by-one error in vllm/engine/output_processor/stop_checker.py that allowed sequences to exceed max_model_len by one token. The change from seq.get_len() > self._get_max_model_len(lora_req) to seq.get_len() >= self._get_max_model_len(lora_req) ensures strict adherence to the specified maximum model length.
• Effective Unit Testing: A new unit test, test_max_model_len, has been added. This test effectively verifies the fix by ensuring that the total length of prompt and generated tokens does not exceed the configured max_model_len, even when SamplingParams.max_tokens would otherwise allow for a longer sequence.

Merge Readiness

The pull request appears to be in good shape and addresses the identified bug effectively with appropriate tests. I am unable to approve pull requests, but based on this review, the changes are sound and ready for further review and merging by authorized maintainers.

[github-actions]

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[houseroad]

Looks good. Thanks for the fix with test.

[runzhen]

Hi @22quinn , the test function test_max_model_len you added in this PR cause a CI failure, could you please take a look at the error log ?

[22quinn]

@runzhen Will be fixed in #19451