[shiwanhang6868-arch]

Overview

Fix TRITON_ATTN discarding Value columns still visible to later query rows in an ordinary non-causal sliding-window tile. Preserve the union of the valid physical query rows' windows.

Claims

• Correct the non-causal V retention boundary, including partial tiles and mixed per-sequence causal flags.
• Keep scalar-causal and causal=True sequence masks unchanged; add an analytic GPU regression with zero tolerance.

Validation

RED/GREEN Test Results

Official main tested in this submission phase: 5a2633f63dd81e132f05dd14b55878fbb2169154. Copy the new test to pristine baseline production, then run in each worktree with a compatible environment and that worktree's vLLM import path:

HIP_VISIBLE_DEVICES=0 python -m pytest \
  tests/kernels/attention/test_triton_unified_attention.py \
  -k test_sliding_window_preserves_values_for_later_query_rows -q --tb=short

Baseline numerical assertions:

16 failed, 8 passed, 1917 deselected
PROCESS_EXIT=1

The fixed run included all 24 new cases plus 27 existing neighboring cases:

HIP_VISIBLE_DEVICES=2 python -m pytest \
  tests/kernels/attention/test_triton_unified_attention.py \
  -k 'sliding_window_preserves_values_for_later_query_rows or speculative_splitk or reduce_segments or clamped_mm or softcap_does_not_overflow' \
  -q --tb=short
# 51 passed, 1890 deselected; PROCESS_EXIT=0

The new test uses Q/K=0 and V=token index: every row has an analytic visible-interval mean. It covers query length 19/context 21, head 64/block 16, 2 KV heads, GQA 1/3/5/7, FP16/BF16, and causal/non-causal/mixed flags. Both sides use atol=rtol=0.

Numerical Correctness

Check	Baseline	Fixed
Current exact regression	16 failed / 8 passed	All 24 pass within 51-test selection
Current adjacent selection: split-K, empty states, graph padding, softcap, clamped multimodal masks, BF16 Q/FP8 KV	Not rerun as a baseline suite	51 passed total
Current randomized FP32-reference matrix	Not rerun as a baseline matrix	90/90, no output-guard corruption
Current public LLM.generate(), actual ordinary non-causal SW call	max abs 3.1549322605, assertion exit 1	max abs 0.0009436607, all 6 captures pass, exit 0
Earlier same-input standalone kernel replay, same main SHA	max abs 3.1549322605	max abs 0.0009436607

The 90-case matrix covers selected FP16/BF16, head 64–256, GQA 1/2/3/4/5/7/8, queries through 513, block 16/32, remapped tables, strided KV/output and actual 2D/3D launches; it is not Cartesian coverage. Nondividing-GQA random cases use short queries to avoid conflating existing ownership bugs.

Public LLM validation used real serialized weights of a locally generated 701,828-parameter DiffusionGemma, seed 20261105, token IDs [2,10,11,12], canvas 16/one denoising step, FP16/eager/TRITON_ATTN. An observer captured actual kernel inputs/outputs for FP32 comparison without replacing model computation. This checks execution and the requested ordinary window semantics, not pretrained-model accuracy or HF rectangular-mask parity. Original tensors and logs are retained locally, excluded from the commit.

Earlier validation, not rerun for submission: 497 scheduler/KV CPU tests passed at 7dca18e (relevant core files unchanged); Qwen2.5-0.5B-Instruct controls completed 48 requests across four prefix/chunk settings on earlier baseline/fixed snapshots. These are not claimed as current or matched before/after model evaluation.

Performance Regression

Current GPU0 warm-cache microbenchmark: 128 launches/graph replay, 20 warmups, 31 alternating baseline/fixed pairs per configuration in one process; no tensor-capture observer or clock changes.

Case	Base median µs	Fixed median µs	Median paired fixed/base	Pair ratio range
Causal FP16/head64/GQA1/q17	6.4850	6.4700	0.99716	0.98038–1.05471
Non-causal, same shape	6.6462	6.6650	1.00251	0.97919–1.03417
Causal 3D BF16/head128/GQA4/q1	9.8684	9.8549	1.00232	0.97307–1.03063
Non-causal 2D BF16/head128/GQA4/q65	10.9662	10.9031	0.99410	0.97711–1.01981

All batch 2/context 33. First two use block 16/W4; third block 16/W64; fourth block 32/W16. Baseline non-causal outputs are incorrect, so this compares cost rather than equivalent correct-work throughput. No material consistent regression observed in these four cases; no serving speedup or statistical-significance claim. Earlier noisy cross-process samples were preserved and are not used to claim improvement.

Hardware and Software Environment

W7900/gfx1100; Python 3.12.15, Torch 2.14.0+rocm7.2, Torch HIP 7.2.53211, host ROCm 7.2.4, Triton 3.8.0. Isolated worktrees/caches, explicit GPU bindings and timeouts. JIT source and worker extension hashes recorded. Same-Torch native extensions from 7ecc83a reused after checking relevant native cache/attention sources match. This is a locally verified development stack, not the official pinned ROCm CI stack. NVIDIA/MI300/XPU were not run locally.

Details

Root Cause

The score mask is per query row, but V sanitization is shared by the tile. Both old V window boundaries used its earliest query. Later rows could count a valid future key in softmax after its V was zeroed, corrupting results with finite, correctly addressed cache data.

Minimal Fix

For width W, the tile's union is context + qlo - W < key < context + qhi + W, where:

qhi = min(qlo + (BLOCK_M - 1) // num_queries_per_kv, query_len - 1)

Retain the left boundary from qlo and derive the non-causal right boundary from qhi. The physical-row span handles GQA not dividing BLOCK_M; the existing per-row score mask remains authoritative. No host synchronization, API, cache layout or backend-priority changes. The diff adds one test to the existing suite and a short rendered backend note.

Current dispatch gates multi-query non-causal/per-sequence cases to 2D; 3D validation covers supported single-query non-causal cases and existing causal multi-query split-K tests. It does not claim enabled non-causal multi-query 3D dispatch. Existing L4 and MI300 attention CI steps select the test and depend on the changed directory; selection is not a passing CI result.

Related Issues / PRs

Refs #62828, #62415, #65830.

• [Bug][ROCm]: ROCM_ATTN sliding-window kernels output NaN when an out-of-window KV slot holds NaN/Inf #62828/[ROCm][Bugfix] Zero out-of-window V in ROCM_ATTN's sliding-window kernels #62415 concern ROCM_ATTN reading non-finite evicted KV; closed after MRV2 block-table redirection in [Bugfix][V1] Null sliding-window block-table entries below the window #60767. This fixes valid future V being zeroed in a different kernel, still reproduced on current main.
• [Bugfix][Attention] Prevent overlapping query tile writes in Triton unified attention #65830/[XPU][Attention] Enable tensor-descriptor Q load for non-power-of-2 GQA (~1.9x Qwen2-7B) #47248 address query-tile ownership/layout. No ownership mask changes here; maintainers should review qhi alongside any future physical-row mapping change. Existing overlapping writes are outside this fix.
• [Kernel] Cover sliding window in mixed per-sequence causal attention tests #51257 adds mixed-causal tests only; no equivalent production correction.
• [Bug Fix] DiffusionGemma get different outputs compared with transformers version (mismatch sliding window attention) #49202 adds separate DiffusionGemma rectangular semantics, retaining the ordinary V-mask branch. No rectangular/HF parity claim here.
• [Kernel][Attention] Segment sliding windows in split-K; split-K for non-causal speculative rows #61760 extends window-aware split-K scheduling/dispatch, without correcting this V bound.

Open/closed searches, actual closest files patches and latest main were checked; no equivalent ordinary non-causal V-union fix found. Pre-existing multimodal/R-SWA/chunk combinations are outside scope.

AI Assistance Disclosure

OpenAI Codex assisted with investigation, implementation, test execution and review. The human submitter is responsible for reviewing and understanding every changed line and validating this contribution under vLLM's AI-assisted contribution policy. The commit includes AI attribution and the human submitter's authorized DCO signoff.

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[nikonyrh-siloai]

right_mask_v isn't defined in this branch, but it isn't used either. I'd suggest to assign (dist < SLIDING_WINDOW) & right_mask_v or dist < SLIDING_WINDOW to a variable depending on if USE_PER_SEQ_CAUSAL or not USE_CAUSAL and use that in the rest of the code. Then there is no branch where a variable remains un-declared.