fix(sched): tolerate late or duplicate KV-xfer completions - #192
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_update_from_kv_xfer_finished asserted that every finished_recving or finished_sending request id is still tracked and in an expected status. A worker-side connector can violate both: an abort can race an async KV load so the completion arrives after the request was freed, and a connector can report the same request twice (seen with the LMCache MP connector under kv_both plus async scheduling; upstream reports in LMCache/LMCache#2356, partial fix for one producer in LMCache/LMCache#4136). Each occurrence killed EngineCore with an AssertionError. Warn and skip unknown request ids, and skip the block free for a live request that is neither WAITING_FOR_REMOTE_KVS nor finished, since freeing the blocks of a live request would corrupt scheduler state.
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📝 WalkthroughWalkthroughThe scheduler now tolerates late and duplicate KV-transfer completion signals, ignoring unknown requests and protecting live request state. New tests cover unknown completions, active requests receiving receive completions, and duplicate send completions after block cleanup. ChangesLate KV completion handling
Estimated code review effort: 3 (Moderate) | ~20 minutes Suggested reviewers: 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
✨ Finishing Touches🧪 Generate unit tests (beta)
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Actionable comments posted: 1
🧹 Nitpick comments (1)
tests/v1/kv_connector/unit/test_late_kv_xfer_completions.py (1)
31-36: 📐 Maintainability & Code Quality | 🔵 Trivial | 💤 Low valueDocument the test helper with a Google-style docstring.
Add
Args:andReturns:sections describing the forwardedKVConnectorOutputfields. As per coding guidelines,**/*.pyrequires Google-style docstrings withArgs:/Returns:/Raises:sections.🤖 Prompt for AI Agents
Verify each finding against current code. Fix only still-valid issues, skip the rest with a brief reason, keep changes minimal, and validate. In `@tests/v1/kv_connector/unit/test_late_kv_xfer_completions.py` around lines 31 - 36, Add a Google-style docstring to _deliver_kv_connector_output describing the forwarded KVConnectorOutput keyword fields in an Args: section and documenting its None return value in a Returns: section; leave the helper behavior unchanged.Source: Coding guidelines
🤖 Prompt for all review comments with AI agents
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.
Inline comments:
In `@vllm/v1/core/sched/scheduler.py`:
- Around line 2718-2729: Update the finished_sending handling in the scheduler
loop to call _free_blocks only for tracked requests in a finished state; warn
and ignore requests with any other status, including RUNNING, while preserving
the missing-request handling. Add a regression test matching the receive-side
protection that verifies a running request’s KV completion is ignored without
aborting and does not free its blocks.
---
Nitpick comments:
In `@tests/v1/kv_connector/unit/test_late_kv_xfer_completions.py`:
- Around line 31-36: Add a Google-style docstring to
_deliver_kv_connector_output describing the forwarded KVConnectorOutput keyword
fields in an Args: section and documenting its None return value in a Returns:
section; leave the helper behavior unchanged.
🪄 Autofix (Beta)
Fix all unresolved CodeRabbit comments on this PR:
- Push a commit to this branch (recommended)
- Create a new PR with the fixes
ℹ️ Review info
⚙️ Run configuration
Configuration used: Organization UI
Review profile: CHILL
Plan: Pro Plus
Run ID: e22bcc2e-af01-44c6-b582-17af859422b6
📒 Files selected for processing (2)
tests/v1/kv_connector/unit/test_late_kv_xfer_completions.pyvllm/v1/core/sched/scheduler.py
| for req_id in kv_connector_output.finished_sending or (): | ||
| logger.debug("Finished sending KV transfer for request %s", req_id) | ||
| assert req_id in self.requests | ||
| self._free_blocks(self.requests[req_id]) | ||
| req = self.requests.get(req_id) | ||
| if req is None: | ||
| logger.warning( | ||
| "Finished sending KV transfer for request %s, but the " | ||
| "request is no longer tracked; ignoring late/duplicate " | ||
| "completion.", | ||
| req_id, | ||
| ) | ||
| continue | ||
| self._free_blocks(req) |
There was a problem hiding this comment.
🩺 Stability & Availability | 🔴 Critical | ⚡ Quick win
Guard finished_sending against live requests.
A tracked RUNNING request reaches _free_blocks() at Line 2729, whose finished-state assertion aborts the scheduler. Only free finished requests; warn and ignore every other status, matching the receive-side protection. Add the equivalent running-request regression test.
Proposed fix
if req is None:
logger.warning(
"Finished sending KV transfer for request %s, but the "
"request is no longer tracked; ignoring late/duplicate "
"completion.",
req_id,
)
continue
- self._free_blocks(req)
+ if RequestStatus.is_finished(req.status):
+ self._free_blocks(req)
+ else:
+ logger.warning(
+ "Finished sending KV transfer for request %s in "
+ "unexpected status %s; ignoring late/duplicate "
+ "completion.",
+ req_id,
+ req.status,
+ )📝 Committable suggestion
‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.
| for req_id in kv_connector_output.finished_sending or (): | |
| logger.debug("Finished sending KV transfer for request %s", req_id) | |
| assert req_id in self.requests | |
| self._free_blocks(self.requests[req_id]) | |
| req = self.requests.get(req_id) | |
| if req is None: | |
| logger.warning( | |
| "Finished sending KV transfer for request %s, but the " | |
| "request is no longer tracked; ignoring late/duplicate " | |
| "completion.", | |
| req_id, | |
| ) | |
| continue | |
| self._free_blocks(req) | |
| for req_id in kv_connector_output.finished_sending or (): | |
| logger.debug("Finished sending KV transfer for request %s", req_id) | |
| req = self.requests.get(req_id) | |
| if req is None: | |
| logger.warning( | |
| "Finished sending KV transfer for request %s, but the " | |
| "request is no longer tracked; ignoring late/duplicate " | |
| "completion.", | |
| req_id, | |
| ) | |
| continue | |
| if RequestStatus.is_finished(req.status): | |
| self._free_blocks(req) | |
| else: | |
| logger.warning( | |
| "Finished sending KV transfer for request %s in " | |
| "unexpected status %s; ignoring late/duplicate " | |
| "completion.", | |
| req_id, | |
| req.status, | |
| ) |
🤖 Prompt for AI Agents
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.
In `@vllm/v1/core/sched/scheduler.py` around lines 2718 - 2729, Update the
finished_sending handling in the scheduler loop to call _free_blocks only for
tracked requests in a finished state; warn and ignore requests with any other
status, including RUNNING, while preserving the missing-request handling. Add a
regression test matching the receive-side protection that verifies a running
request’s KV completion is ignored without aborting and does not free its
blocks.
|
The source branch was deleted, so this PR was auto-closed. It is superseded by #196, which preserves the original commit and adds the send-side live-request guard plus the matching regression test requested by CodeRabbit. |
2 participants
Summary
Scheduler._update_from_kv_xfer_finishedasserted that everyfinished_recving/finished_sendingrequest id isstill tracked and in an expected status. Late or duplicate KV-transfer completions violate both assumptions and each
occurrence killed EngineCore with an
AssertionError. The asserts become warn-and-skip guards:finished_recvingorfinished_sending: log a warning and skipWAITING_FOR_REMOTE_KVSnor finished: log a warning and skip the block free, sincefreeing the blocks of a live request would corrupt scheduler state
Intention
A worker-side connector can legitimately produce these completions: a client abort can race an async KV load so the
completion arrives after the request was freed, and a connector can report the same request id twice. Both were
observed with the LMCache MP connector under
kv_role=kv_bothwith async scheduling (upstream reports inLMCache/LMCache#2356; LMCache/LMCache#4136 fixes one producer of the duplicates on the LMCache side, this guard stays
as defense in depth for any other producer). Upstream vLLM main still has the bare asserts, so this is a fork-side
hardening, not a backport.
Validation
Run in the r8 image against this branch:
tests/v1/kv_connector/unit/test_late_kv_xfer_completions.py: 4 passed; the same 4 tests fail withAssertionErrorwhen the guard commit is revertedtests/v1/kv_connector/unit/test_remote_decode_lifecycle.py: 4 passedThe guarded code has additionally been running in production as a startup source patch on the GLM-5.2 TP8 + DCP4
deployment since 2026-07-25 (v10/v11 of the runtime patch) with no assertion deaths since.
Related
Summary by CodeRabbit