[Bugfix][GDN] Reclassify a partial final speculative group as prefill at the max-model-len boundary - #55516
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WalkthroughThe GDN metadata builder converts a truncated final speculative group into a stateful non-spec prefill. It clears speculative metadata, sizes non-spec tensors to the reclassified rows, aligns initial-state flags, and adds targeted tests. ChangesGDN partial speculative group handling
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to Truncated final speculative groups at the model-length boundary are now handled as stateful non-speculative prefills, including padded batches, with no current merge-blocking risk identified. Sequence Diagram(s)sequenceDiagram
participant GDNMetadataBuilder
participant SpecMetadata
participant NonSpecMetadata
GDNMetadataBuilder->>GDNMetadataBuilder: Detect actual spec tokens below expected group size
GDNMetadataBuilder->>SpecMetadata: Clear speculative metadata
GDNMetadataBuilder->>NonSpecMetadata: Set prefill counts and row-sized tensors
GDNMetadataBuilder->>NonSpecMetadata: Slice has_initial_state to reclassified rows
Suggested reviewers: 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches 💡 1🛠️ Fix failing CI checks 💡
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… at the max-model-len boundary When the scheduler hands the final speculative step fewer than num_speculative_tokens + 1 query tokens per sequence at the max-model-len boundary, the shared GDN metadata builder reclassified the batch as pure spec decode while the trailing partial group left the request unable to complete without padding. The builder now reclassifies the partial final spec group as a stateful non-spec prefill and slices the non-spec metadata (state indices, query-start-loc CPU views, initial-state masks) to the reclassified rows only. Regression tests pin the reclassified metadata shapes and content on CPU tensors, including zero-length padded batches. Strip hardware-name marker from reclassification comment (no behavior change; AST-identical). Signed-off-by: J. Gavin Ray <git@jgavinray.com>
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End-to-end verification on real hardware (Intel Arc Pro B70, 32 GB) — same request, both sides, single A/B variable: Setup. Pinned image BEFORE (image as-is, unpatched builder): at the boundary step the engine dies with the kernel's own invariant: Completion never delivered — same signature as the field failure (a 131,072-token MTP4 generation that stalled at 124/128 outputs). AFTER (single-file overlay of this PR's Full-length completion, engine healthy ( Kernel-level, direct Fast path unaffected: ordinary non-boundary generation on the patched tree: 70.12 tok/s on the reference complex prompt — inside the pre-fix band for this exact config (62.3–62.7 baseline with the same async adapter + GPU-prep + draft-INT4 layers, identical md5s on both sides; the 82.1 figure is a different, fp16-draft config and explicitly not the comparison basis). Zero reclassification log lines during the timing run — the branch is dead code off the boundary. Caveats, stated honestly: wheel |
1 participant
Problem
A completion that ends at exactly
max-model-lenyields one incomplete speculative token group (fewer thannum_speculative_tokens + 1tokens).GDNAttentionMetadataBuilder.build()still classifies the batch as pure spec decode and hands the fused GDN kernels a batch that violates their complete-group invariant (spec_token == num_spec_decodes * (num_speculative_tokens + 1)), so the final tokens of an exact full-context generation can never complete.Repro: MTP4 on a GDN-hybrid Qwen checkpoint — a 131,072-token completion stalled at 124/128 outputs because the last step produced a 4-token group instead of 5.
Fix
In the pure-spec branch, detect the truncated final group and reclassify it as a stateful non-spec prefill (the prefill path already handles initial state). Complete groups are untouched. The classification lives in the shared builder, so every backend benefits; the kernel-side complete-group TORCH_CHECK stays intact as a live guard.
Sizing invariants of the reclassified batch,
N= reclassified spec rows — sized byNalone, never the padded batch size (which may carry trailing zero-length sequences):num_spec_decodes=0, spec fieldsNone,num_prefills=N,non_spec_query_start_loc(_cpu) = query_start_loc(_cpu)[:N + 1],non_spec_state_indices = block_table[:N, 0],has_initial_state = (computed_tokens > 0)[:N],num_accepted_tokens=None.Tests (3 new)
test_gdn_build_classification[partial_final_spec_group_at_max_len_uses_full_group]— truncated step classifies asnum_prefills=2,num_spec_decodes=0.test_partial_final_spec_group_reclassified_as_prefill— no spec metadata leaks;non_spec_query_start_loc.tolist() == [0, 3, 5].test_partial_final_spec_group_padded_batch_shapes— with a trailing zero-length padded sequence, asserts the fused-op sizing above (values[0, 3, 5],[True, True]) — the padded row is sliced out, not leaked.Verification
CPU-only (no GPU) in
vllm/vllm-openai-xpu@sha256:f01e24f6c7ff01f1e0662234255a1372297d1dbd89d003cf13c8fad3eab1ba4f(vllm 0.27.2rc1.dev77+gac7509e2b, torch2.13.0+xpu):pytest -p no:cacheprovider -v tests/v1/attention/test_gdn_metadata_builder.py.Regression-first — padded test against a build without the
[:N]slices (padded row leaks[0, 3, 5, 5]):With the slices:
12 passed, 22 warnings in 43.27s, exit 0.Kernel-contract checks (
TORCH_CHECK(spec_token == num_spec_decodes * (num_speculative_tokens + 1))etc.) verified againstvllm-xpu-kernelsgdn_attn_interface.cpp; on-device evidence follows in a comment.Not #391 (vllm-xpu-kernels)
#391 (open, 2026-06-03, @ehartford/QuixiAI, fixes #389) relaxes metadata shape checks for graph-padded buffers whose groups are still complete, leaving the uniform-stride invariant intact. This PR handles a genuinely ragged final group by reclassifying it out of the spec path upstream of the kernel.
Related, none claiming this fix (checked 2026-09-05)
#50623 (cudagraph padding branch, different condition) · #50021 (accepted-token bounds, kernel-side) · #37052 (
block_tableOOB, different bug) · #55404 (merged; rewrotebuild_for_cudagraph_captureabove this region — diff regenerated after it, applies at exact hunk positions).