[SM120][Bugfix] Fix packed KV block zeroing stride

[wangxian001]

Purpose

Apply the packed KV cache zeroing fix to ds4-sm120-preview-dev. The upstream counterpart is vllm-project#50276.

The zeroing path currently uses a segment's page size for both its logical block stride and its zero span. Packed DeepSeek KV views can have a larger physical stride than an individual segment's page, so clearing a high block ID may touch adjacent packed data or cross the end of the allocation.

This change:

• tracks logical block stride independently from page size;
• represents virtual kernel blocks as independent zeroing segments;
• derives the zero span from the tensor shape and strides; and
• adds a final-block regression test for a packed backing allocation.

The SM120 prefill/decode paths, DeepSeek-V4 kernels, and DSpark speculative decoding behavior are unchanged.

Duplicate-work check

The matching upstream fix is vllm-project#50276. Searches of open JASL issues and PRs for KVBlockZeroer and packed KV zeroing found no other implementation on this branch.

Test Plan

• .venv/Scripts/python.exe -m py_compile vllm/v1/worker/utils.py tests/v1/worker/test_kv_block_zeroer.py
• ruff 0.14.0 check vllm/v1/worker/utils.py tests/v1/worker/test_kv_block_zeroer.py
• ruff 0.14.0 format --check vllm/v1/worker/utils.py tests/v1/worker/test_kv_block_zeroer.py
• pytest -q tests/v1/worker/test_kv_block_zeroer.py on CUDA
• End-to-end DeepSeek-V4 serving with FP8 KV cache, block size 256, prefix caching, DSpark speculative decoding, and mixed long-context refill and short structured requests

Test Result

• Python 3.12 syntax checks passed.
• Ruff lint and format checks passed.
• Targeted CUDA regression suite: 3 passed.
• End-to-end validation passed on two RTX PRO 6000 Blackwell GPUs under repeated cache pressure; the API remained healthy and no CUDA Xid was observed after the fix.

AI assistance disclosure: OpenAI Codex helped prepare the patch and PR text. The submitter participated in the diagnosis, reviewed the change, and validated it end-to-end.

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[jasl]

Sorry, I missed the PR, will handle it now

[jasl]

Merged as 3dd81226c7; ds4-sm120-preview-dev and codex/ds4-sm120-min-enable are both at 67f0057da4, tagged sm120-pr-41834-stable-preview-20260802d. Your commit 409defc6fd is in with authorship intact. Thank you — the diagnosis is right and the fix is the right shape.

I resolved the conflict rather than asking you to rebase, because the branch moved a long way while this was open (a 41-commit upstream merge carrying DSv4 sequence parallelism and the qnorm_rope_kv_insert op split, then FlashInfer 0.6.16). Two notes on what that involved.

The textual conflict was the harmless part. It was only in tests/v1/worker/test_kv_block_zeroer.py, and it was the ordinary kind — both sides appended a test at the same spot. Kept both.

The part that did not show up as a conflict is the one that mattered. This PR changes KVBlockZeroer._meta from a 5-tuple to a 6-tuple, inserting seg_block_strides second. You correctly updated the two tests that existed on your base (d64074e6f0). Since then our tree picked up upstream vllm-project#49903, which added two more hand-built _meta fixtures (test_warmup_compiles_every_n_blocks_specialization, test_warmup_respects_available_block_count). Those still built 5-tuples and would have died at the single unpack site with ValueError: not enough values to unpack. Nothing you could have known about; both are now 6-tuples, with a comment noting their fixtures are contiguous so block stride equals page size there, and pointing at your new packed test for the interesting case.

vllm/v1/worker/utils.py auto-merged cleanly and I checked it is semantically sound rather than just textually clean: upstream vllm-project#49903's warmup() delegates to zero_block_ids() instead of unpacking _meta itself, so there is exactly one unpack site and your patch updated it.

Verified on GB10 (SM121): tests/v1/worker/test_kv_block_zeroer.py 5 passed, and the full DSv4 unit suite at the merge head is green (229 / 6 / 57 / 715 / 232+1 / 2 / 44 / 16 — the one failure needs two GPUs in a single node, which GB10 does not have).

One scoping note for the record: vllm-project#50276 is still open, so this is a local activation of an unmerged upstream fix rather than a duplicate. We will drop it in favour of upstream's version if and when that lands, the same way we retired our local vllm-project#48304 / vllm-project#48911 / vllm-project#48959 deltas.