[bugfix] ascend schedule encountered an incorrect req block length in…

[liziyu179]

… the check_watermark_for_prefill function

What this PR does / why we need it?

Does this PR introduce any user-facing change?

How was this patch tested?

• vLLM version: v0.10.0
• vLLM main: vllm-project/vllm@6603288

[gemini-code-assist]

Code Review

This PR fixes a bug in the watermark check by using the correct block length. The fix is correct in principle, but the implementation is not robust and can lead to crashes for new requests. I've added a critical comment with a suggested improvement to prevent potential KeyError and IndexError exceptions.

[gemini-code-assist]

The change from len(req_blocks) to len(req_blocks[0]) is conceptually correct for getting the number of allocated blocks. However, the current implementation is not robust and can lead to runtime errors:

1. self.kv_cache_manager.coordinator.get_blocks(request.request_id) will raise a KeyError if a request has no blocks allocated yet, which is the case for new requests from the waiting queue.
2. If get_blocks were to return an empty list for a new request, req_blocks[0] would then raise an IndexError.

This can cause the scheduler to crash. The implementation should be updated to handle these cases gracefully.

        req_blocks = self.kv_cache_manager.coordinator.req_to_blocks.get(
            request.request_id, [])
        num_allocated_blocks = len(req_blocks[0]) if req_blocks else 0
        num_new_blocks = (num_required_blocks - num_allocated_blocks -
                          len(computed_blocks))

[github-actions]

👋 Hi! Thank you for contributing to the vLLM Ascend project. The following points will speed up your PR merge:‌‌

• A PR should do only one thing, smaller PRs enable faster reviews.
• Every PR should include unit tests and end-to-end tests ‌to ensure it works and is not broken by other future PRs.
• Write the commit message by fulfilling the PR description to help reviewer and future developers understand.

If CI fails, you can run linting and testing checks locally according Contributing and Testing.