[Bugfix][KV Offload] Preserve reachable tails for hybrid SWA groups

[coltonottley]

Purpose

Fix CPU KV offload store pruning for hybrid sliding-window attention when:

1. the group is EAGLE/MTP and lookup requires one extra chunk before dropping the volatile draft tail; or
2. the final alignment segment is shorter than the configured full-attention alignment.

The current store predicate retains only sliding_window_size_in_chunks at the end of every configured alignment segment:

pos_in_segment = abs_chunk_idx % alignment_chunk_count
if pos_in_segment < alignment_chunk_count - tail:
    continue

That disagrees with lookup in two ways:

• EAGLE/MTP lookup requires tail + 1 contiguous chunks before popping the volatile trailing chunk.
• The final segment may be partial, so its reachable tail is relative to the actual segment end, not the hypothetical full alignment end.

On DeepSeek V4 Flash with MTP2, this made native CPU offload write-only after a real GPU-cache miss: GPU→CPU stores completed, but external-prefix lookup converged to zero because required EAGLE/SWA keys had never been stored.

The fix computes each chunk's actual segment length from the current storable chunk count and retains tail + int(is_eagle_group) chunks at that actual segment end.

This interacts directly with the DeepSeek V4 + MTP path carried by @jasl in #41834. The mismatch is between the EAGLE extra-window lookup semantics from #44784 and the store-pruning predicate currently present after #48150.

Test Plan

• Add direct parameterized scheduler regressions for:
  • full ordinary SWA segment;
  • full EAGLE/MTP SWA segment (tail + 1);
  • partial 48-of-64 ordinary SWA tail;
  • partial 48-of-64 EAGLE/MTP tail;
  • a later partial segment;
  • no-alignment and tail-saturation behavior.
• Run the focused OffloadingConnector scheduler tests.
• Run changed-file formatting/lint and git diff --check.

Test Result

pytest -q tests/v1/kv_connector/unit/offloading_connector/test_scheduler.py \
  -k is_store_reachable_swa_chunk

Result: 12 passed, 87 deselected.

Changed-file Ruff format/check and git diff --check pass. A full scheduler-file run in the older production image is not comparable to current main: 87 existing tests fail before scheduler construction because that image's installed VllmConfig rejects the current-main fixture's explicit hybrid-KV setting. The new pure-helper selection imports current source and passes independently.

Hardware validation on 2× RTX PRO 6000 Blackwell (TP=2), DeepSeek V4 Flash, MTP2, FP8 KV, 1M context, native OffloadingConnector + 64 GiB CPU tier:

• pre-fix: GPU miss + 0 external tokens + 0 CPU→GPU bytes; all 18,877 prompt tokens recomputed;
• post-fix: 0 local GPU-hit tokens, 18,688 external tokens, 189 recomputed tokens, 197,539,200 CPU→GPU bytes, exact output preserved;
• 100,006-token replay: 99,840 external tokens, 166 recomputed, 856,696,320 CPU→GPU bytes; TTFT 23.563s cold → 0.611s first CPU hit → 0.404s second hit;
• canonical decode remained 190.40 tok/s versus ~192 tok/s baseline;
• six fictional prefix-dependent semantic probes passed strict cold-control + CPU-hop validation (PASS_WITH_STRICT_SUPPLEMENT; one probe required a narrow supplemental rerun after a null cold final, and no wrong/unstable cold result was accepted).

AI Assistance

AI assistants were used for trace analysis, implementation review, regression-test drafting, and benchmark analysis. The final source change and test evidence were independently reviewed against captured per-group OffloadKey traces and live hardware results.

---

Essential Elements of an Effective PR Description Checklist

• Purpose and affected path explained.
• Test plan provided.
• Unit and live hardware results provided.
• No documentation update is required for this bugfix.

[claude]

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[coltonottley]

Follow-up from live TP=2 validation on two independent RTX PRO 6000 Blackwell hosts using the fixed source lineage:

A narrower short-prefix geometry still produces a zero external-hit result after a successful cold store and selective GPU-only reset:

Geometry	Host	Result
17,975 prompt tokens, exact SHA-256 ff2657f7971e2c88cd3ed0892fa9187f2d322fd1e00937e558ff1f5b8646a0cd	Queen	MISS: 17,975 local compute, 0 external tokens/load bytes after 3,509,959,680 cold-store bytes
Same exact bytes and sequence	Giga Worker	Same MISS
18,877-token qualification target	Giga Worker	PASS: 18,688 external tokens
21,579-token target	Queen	PASS: 21,504 external tokens, 220,412,160 load bytes
Two exact copies of the failing text, 35,947 tokens	Giga Worker	PASS: 35,840 external tokens, 336,856,320 load bytes, 0.292s

Each selective-reset sequence was: reset GPU+external, cold prime, 30s no-inference drain, reset_external=false, exact replay. The same 17,975-token bytes fail on both hosts, while doubling them passes, so this is not host hardware, build provenance, prompt content, or general reset failure.

I attempted to reduce this to the current RequestRunner. A simple partial-segment store-count test is not a valid reproduction: it passes with this PR and still produces nonzero mocked lookup hits. The remaining zero-hit behavior appears to require incremental-prefill / request-state convergence that the single-step mock does not model. I am deliberately not adding a vacuous green test or speculative second fix to this PR.

Operational impact is narrow (~2.2s recompute for this short geometry); longer production agent prefixes restore correctly. I’ll follow up with a direct failing regression once the first terminal group/key mismatch is captured.

[coltonottley]

Thanks @jasl — the clean cherry-pick and independent focused results are excellent confirmation.

The SpeculativeConfig.use_eagle() point is important and I agree it should be explicit here: method="mtp" takes this path, so the affected geometry is the default DeepSeek-V4 MTP + hybrid-SWA configuration, not an EAGLE-only edge case.

The third store/protect-vs-lookup +1 skew is also useful corroboration that this invariant is currently distributed across call sites rather than enforced canonically. I’ll keep #48911 scoped to the two proven offloading reachability defects, but that repeated pattern is a strong case for a follow-up invariant/helper cleanup after this lands.

Thank you for testing the exact commit and for stating the unit-vs-live evidence boundary clearly. Your 12/12 + 141 + 223 focused results complement our TP=2 live replay evidence well.

[orozery]

LGTM! Thanks @coltonottley !