[Core] Add unit tests for BlockTable slot mapping

[CiaranCw]

Purpose

BlockTable.compute_slot_mapping maps logical token positions to physical KV cache slots. An off-by-one in this path silently corrupts KV cache contents instead of raising, which is hard to trace back from garbled output. It also carries several easy-to-break branches: hybrid KV/kernel block sizes, decode context parallel (DCP) interleaving, CUDA graph padding, and the SlotMappingMode.NONE short circuit used by Mamba-style state caches.

On the base revision before this change, the tests directory had no direct coverage of BlockTable.compute_slot_mapping:

$ git grep -l "compute_slot_mapping" 24c939c47df59d0d3e9af2fe63046e21e65b5924 -- tests/
$ # (no matches)

tests/v1/worker/test_gpu_block_table.py covers the MRV2 BlockTables staged-write and move_row behaviour, and tests/v1/worker/test_gpu_model_runner.py::test_hybrid_block_table_initialization covers hybrid block table initialization (asserting the expanded row against map_to_kernel_blocks itself). Neither computes a slot mapping.

This PR adds tests/v1/worker/test_block_table_slot_mapping.py, exercising BlockTable.compute_slot_mapping. It is test-only: no production code is modified.

Test plan

.venv/bin/python -m pytest tests/v1/worker/test_block_table_slot_mapping.py -v
.venv/bin/pre-commit run --files tests/v1/worker/test_block_table_slot_mapping.py

Test result

18 passed, 14 warnings in 5.34s

.venv/bin/pre-commit run --files tests/v1/worker/test_block_table_slot_mapping.py: all applicable hooks pass (ruff check, ruff format, typos, mypy, SPDX headers, and the rest).

Environment: RTX 5080 (sm_120), driver 610.62, Python 3.12.13, torch 2.13.0+cu130, triton 3.7.1, against 24c939c47df59d0d3e9af2fe63046e21e65b5924.

What is covered

Area	Cases
Hand-computed golden slots, anchoring both the kernel output and the reference	plain mapping (2: single request block boundaries, multiple request rows)
Block boundaries, ragged batches, prefix offsets	single token, first/last position in a block, spanning many blocks, non-contiguous block ids, ragged multi-request, absolute non-zero positions after a cached prefix (6)
CUDA graph padding across a Triton tile	1030 tokens crossing the kernel's 1024-token tile with a hand-computed anchor on positions 1023-1029, padded tail crossing a second tile
Decode context parallel partitioning	world size {2, 4} × interleave {1, 4}, every rank, plus "each token is owned by exactly one rank"
Hand-computed DCP golden crossing a virtual block	world size 2, rank 1, positions up to 33 so the second KV block is used
Hybrid blocks	KV/kernel sizes (32, 16) and (64, 16), end to end through compute_slot_mapping
Hand-computed hybrid golden crossing the second kernel block	32/16 blocks, positions up to 48 so both kernel sub-blocks of a KV block are used
SlotMappingMode.NONE	buffer is left untouched (checked with a sentinel distinct from PAD_SLOT_ID)

The reference implementation independently expands KV manager block IDs into kernel block IDs and rebuilds the expected mapping from the inputs passed to BlockTable. It does not call BlockTable.map_to_kernel_blocks, does not read the block table that add_row produced, and does not reuse the kernel's indexing implementation. This matters in practice: with a deliberate off-by-one injected into map_to_kernel_blocks, an earlier draft whose reference reused that helper still passed the hybrid cases, while the current version fails all of them. The plain hand-computed cases assert the expected slots against both the kernel output and the reference, so neither can drift unnoticed.

Why this is not a duplicate

• No open or merged PR adds direct tests for BlockTable.compute_slot_mapping (searched compute_slot_mapping, BlockTable slot mapping, test_block_table, DCP slot mapping, hybrid block table).
• The open slot-mapping PRs are unrelated in scope: [Attention] Remove unused slot mapping from standard metadata #41270 / [Attention] Remove unused slot mapping from FlashAttention metadata #41272 remove slot mapping from attention metadata; [Bugfix][V1] Warm up slot-mapping kernel through BlockTable #46446 / [Bugfix] Pre-compile _zero_kv_blocks_kernel and _compute_slot_mapping_kernel during warmup #45245 / [Bugfix] precompile Triton KV-zero and slot-mapping kernels (#43009) #43879 pre-compile the kernel during warmup; [RFC+prototype] Compile the decode step e2e incl. KV-cache management (B.1: slot-mapping in the decode cudagraph) #47924 is an RFC/prototype.
• Open PR [Bugfix] Zero block_table row tail to fix concurrent variable-length prefill non-determinism (#39589) #39591 adds a tests/v1/worker/test_block_table.py covering a different behaviour: zeroing the block table row tail to fix concurrent variable-length prefill non-determinism ([Bug]: KV Cache Read/Write Index Corruption Under Concurrent Prefill of Variable-Length Sequences (vLLM V1, FlashInfer) #39589). Its tests exercise append_row / add_row / move_row / clear_row and never call compute_slot_mapping. To keep the two independent, this PR uses the separate file name test_block_table_slot_mapping.py; happy to merge the files if maintainers prefer a single module.
• The closed [UT][PCP&DCP] UT for block_table.py #30683 by @pisceskkk previously explored direct tests for this area, and credit for identifying the testing direction goes there. It was not merged and targets an older API: it used compute_slot_mapping(req_indices, positions) and assumed slot mapping was sharded by pcp_world_size * dcp_world_size. On current main the signature is (num_reqs, query_start_loc, positions) and the kernel is sharded by DCP only, so these tests were written against the current API and additionally cover hybrid blocks, SlotMappingMode, and tile-crossing padding.

AI assistance disclosure

AI assistance (Claude Code) was used to research the current implementation, draft and refine the tests, and prepare validation materials. I reviewed every changed line, verified the hand-computed golden cases by hand, and reproduced the test results locally (18 passed; changed-file pre-commit clean). I take responsibility for this change.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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