[Bugfix][KV Offload] Fix SWA store reachability during chunked prefill - #54362
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Assisted-by: GPT-5.6-sol Signed-off-by: Will Hampson <whamp@ggl.slmail.me>
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Thanks @orozery. #51886 has landed, so I rebased this PR onto current main and adapted the fix to the new reachable_block_mask path. The updated patch projects the final prompt horizon during chunked prefill, revisits newly reachable SWA tail chunks when a request finishes or aborts, and avoids treating an active decode frontier as final. EAGLE groups retain the behavior from #51886, and DCP uses the already-resolved group block size. Local validation:
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Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review. 📝 SummarySummary by CodeRabbit
WalkthroughAdds final-segment support to KV cache reachability masks and updates SWA offload scheduling for partial segments. Tests cover completed and aborted requests, active decode, EAGLE behavior, and DCP block indexing. ChangesSWA offload horizon handling
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: ⚪ Minimal · up to This change makes sliding-window KV offloading retain newly reachable final prompt-tail blocks during chunked prefill while preserving EAGLE behavior. The covered completion, abort, decode, and DCP cases leave no actionable merge-blocking risk. Sequence Diagram(s)sequenceDiagram
participant OffloadingConnectorScheduler
participant SlidingWindowManager
participant StoreJobBuilder
OffloadingConnectorScheduler->>OffloadingConnectorScheduler: Compute prompt and final-segment horizons
OffloadingConnectorScheduler->>SlidingWindowManager: Request reachable_block_mask with final segment end
SlidingWindowManager-->>OffloadingConnectorScheduler: Return reachable block mask
OffloadingConnectorScheduler->>StoreJobBuilder: Build store ranges and store jobs
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
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✅ @Whamp, CI is now available for this PR.
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/ci run |
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✅ Triggered Buildkite CI #87434 for commit |
vllm-project#54362) Signed-off-by: Will Hampson <whamp@ggl.slmail.me> Co-authored-by: Will Hampson <whamp@ggl.slmail.me> Co-authored-by: Or Ozeri <oro@il.ibm.com> Signed-off-by: Jyotirmoy Roy <jyotirmoyroy649@gmail.com>
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Purpose
During chunked prefill, the offloading scheduler used the current step's SWA chunk count as the store-reachability horizon. This treated each intermediate frontier as the prompt's final partial segment and submitted chunks that the load path could never request.
Use the prompt's final offloadable length while prefill remains active. If the request aborts before completing its prompt, reconsider the computed frontier as the final segment and submit any newly reachable SWA tail chunks exactly once.
The unpatched scheduler submitted 152 SWA chunks where the completed-prompt regression expected 150. The first mismatch was chunk 248. The abort regression also caught two missing tail chunks in the earlier PR revision and catches duplicate resubmission of chunks already stored before abort.
Test plan
Exercise the same mixed full-attention/SWA request through three lifecycles:
Each case asserts the complete ordered set of SWA chunks submitted for storage.
Test results
TestEagleselection: 34 passed. The CPU-only environment required disabling the two accelerator cache-cleanup calls during teardown; test behavior was unchanged.git diff --checkpassed.Duplicate work
I searched open vLLM PRs for chunked-prefill SWA store reachability, sliding-window store reachability, and offload horizons. I found no matching PR.
Model evaluation
No live GPU serving evaluation was run. This change only changes which SWA chunks are written to the offload tier. It does not change load decisions, model execution, or model output. The scheduler regression asserts the complete store decision across all three request lifecycles.
AI assistance
GPT-5.6-sol assisted with the implementation, tests, and review. I reviewed every changed line and personally ran the three-case regression; all three cases passed.