[Bugfix][Core] Stop zero-progress preemption cascades for deferred KV frees

[LearningMachine621]

Fixes #49674

Scope

This PR fixes one known zero-progress transition in the running-request
allocation retry. It does not change the deferred-free safety fence, model
output semantics, or FCFS/PRIORITY victim-selection policy.

Change

The scheduler now uses the existing deferred-free conditions through one
predicate. Before mutating the running queue, it applies that predicate to the
victim already selected by FCFS/PRIORITY. If the victim's free is fenced, the
current allocation retry stops. After output processing advances the fence,
the normal preemption/allocation path remains available.

def _should_defer_request_block_free(self, request: Request) -> bool:
    return (
        self.defer_block_free
        and request.last_sched_seq > self.processed_step_seq
    )

The predicate names the known deferred path. It does not claim that every
immediate free must increase the pool; shared references and other pins remain
outside this patch.

Correctness boundary

• The existing safety invariant is unchanged: no KV block returns for reuse
  before its in-flight write is processed.
• A guarded retry performs no victim removal, budget rollback, request-status
  transition, or preemption accounting.
• Processing constructed model-runner outputs through the real scheduler update
  path advances the fence and restores the existing
  preemption/allocation path.
• The fix: resolve silent request skipping in PRIORITY scheduling #49206 victim_index/req_index correction is preserved.
• The guard is disabled when deferred freeing is disabled.

The implementation deliberately stops on the current policy-selected victim;
it does not scan for another candidate.

A-priori design trade-off

Stopping can leave the remainder of one scheduling pass unused even when a
different request might be immediately reclaimable. The possible costs are
head-of-line delay and transient under-utilization. This is the explicit,
pre-measurement trade-off for preserving current victim order in a narrow
correctness fix.

Alternative-victim scanning, capacity-aware admission, and generic checks for
other zero-delta causes are future policy questions:
separate future-policy RFC.

Validation

• Base: f25c2692fe3081bdf89b58208d6bb995b43e92f0
• Implementation: f4341c95dfd97f34b782d900337dc37357527d42
• GPU-tested head: 8299effb6a555cbb9ae2d786a5b3d1dfd94ad316
• Refined tests/helper tree: 8523fb4a831d08093552f034575bd1cf6e88aeb3
• Publication head: 71521360db9f11494773f262248df0bbc668a59b

At the 2026-08-26 16:36 CST refresh, upstream main was 2267d3b112. The
relevant scheduler/test/helper paths were unchanged from the pinned base. The
publication head produced a clean merge tree (68dd1aacad81) with that moving
main; the production scheduler code in it is unchanged from the GPU-tested
head.

Deterministic red/green

The new case extends the existing deferred-free suite and injects exactly one
allocation failure. Before the fence advances, it asserts no preemption and
that the trigger is not scheduled. It then validates that ordinary preemption,
the real allocator, and request scheduling all resume after constructed
model-runner outputs pass through the scheduler's real output-update path.
The PRIORITY parameter forces victim_index < req_index to cover #49206.

CUDA_VISIBLE_DEVICES="" PYTHONNOUSERSITE=1 PYTHONOPTIMIZE=0 \
VLLM_NO_USAGE_STATS=1 VLLM_DO_NOT_TRACK=1 \
PYTHONPATH="${VLLM_CHECKOUT}" \
"${PYTHON}" -m pytest -q \
  tests/v1/core/test_deferred_block_free.py \
  tests/v1/core/test_priority_preemption_bug.py

Here ${VLLM_CHECKOUT} is the clean checkout at the refined publication tree
and ${PYTHON} is the isolated interpreter recorded in the artifact; the exact
resolved paths are preserved there.

• Refined fix tree: 15 passed, 15 warnings in 17.94s.
• Pinned base plus refined test-only tree
  c50dfb5019b5a359f4b74cb393d13d990e4a4446:
  2 failed, 13 passed, 15 warnings in 17.65s as expected.
• FCFS made 3 allocator calls after one injected failure; the guard allows 1.
• PRIORITY made 4 allocator calls when the single failure was injected on
  allocator call 2; the guard allows 2.

The CPU tests use upstream's existing public facebook/opt-* IDs; network was
limited to Hugging Face metadata/config resolution, with no model weights.

Repository checks

pre-commit run --files \
  vllm/v1/core/sched/scheduler.py \
  tests/v1/core/test_deferred_block_free.py \
  tests/v1/core/utils.py

All applicable hooks passed, including ruff, mypy, SPDX, forbidden-import, and
repository validation hooks.

Frozen pinned-base GPU experiment

The experiment uses upstream-main snapshot f25c2692, fetched on 2026-08-26,
and the official cu130 native wheel for that exact SHA. The PR changes Python
scheduler code and tests only. The non-confirmatory base preflight passed its
80/80 exact-OSL gate with 1051 preemptions; it only validates that the frozen
pressure point triggers and is excluded from the confirmatory table.

Artifact: release bundle, raw runs, environment, and checksums

Run validity

Role	Affected runs	Required result
Pinned base	5	80/80 in every run; zero failures; exact OSL 1792
Fix head	5	80/80 in every run; zero failures; exact OSL 1792

The required result is 80/80 requests, zero failures, empty errors, and exact
output length 1792 in every run.

Mechanism

Endpoint	Pinned base mean ± sample SD (n=5)	Fix mean ± sample SD (n=5)	Fix/base
Preemptions	1099.800 ± 48.721	32.000 ± 0.000	0.0291 (34.4× lower)

The deterministic scheduler test validates the progress invariant;
preemption deltas measure the mechanism.

Workload-specific performance trade-off

Endpoint	Pinned base mean ± sample SD (n=5)	Fix mean ± sample SD (n=5)	Fix/base or Δ
Output throughput (tok/s)	2310.671 ± 6.516	2354.637 ± 4.197	1.0190 (+1.90%)
P50 TTFT (ms)	995.702 ± 122.633	1016.034 ± 68.829	+20.332 ms
P99 TTFT (ms)	1547.926 ± 79.245	1554.195 ± 95.045	1.0041 (+0.41%)
P99 TPOT (ms)	33.662 ± 0.075	33.016 ± 0.030	-0.646 ms
P99 E2EL (ms)	61835.696 ± 171.789	60686.367 ± 116.669	0.9814 (-1.86%)

All three pre-registered point-estimate bounds passed. On this frozen
workload, stop-and-wait did not lower the output-throughput point estimate;
P99 TTFT moved by +0.41% and P99 E2EL by -1.86%. Individual paired P99 TTFT
effects ranged from -11.11% to +9.21%, so these measurements characterize this
workload rather than establish a universal latency result.

The pre-specified point-estimate guardrails are fix/base ratios of at least
0.95 for output throughput and at most 1.10 for P99 TTFT and P99 E2EL. These
are descriptive workload-specific thresholds, not confidence intervals,
hypothesis tests, or a universal no-regression claim. Per-pair effects and all
raw values are included.

The three A/B scope sentinels (async off, connector off, output length 1280)
are reported separately and are not pooled with the five affected pairs:

Sentinel	Base preemptions / output tok/s	Fix preemptions / output tok/s
Async off	45 / 1963.015	32 / 2074.462
Connector off	33 / 2338.411	45 / 2133.500
Output length 1280	666 / 2793.017	15 / 2839.666

Each sentinel is one pair, not an equivalence test. Their raw variation is why
performance interpretation is restricted to the five interleaved affected
pairs. The internal condition ID below_onset is not an outcome claim; the
newly reconstructed workload still triggered 666 base preemptions at OSL 1280.

Artifact contents

The artifact records the exact model revision, newly reconstructed frozen
workload and hashes, connector JSON, run order, each raw benchmark result and
Prometheus snapshot, server logs, environment/package checks, official
wheel/native comparison, aggregation code, and SHA-256 checksums. Historical
v0.24/v0.26/former-base measurements are context only and are not pooled with
this pinned-base experiment.

Non-goals and future policy

This PR does not choose a new victim when the current victim is fenced and does
not address shared-reference or connector-pin zero deltas. Those choices may
change fairness, starvation, latency, and throughput and require a separate
policy review in the
future-policy RFC.

Related work

• [Bug]: performance regression caused by frequently preempting and resuming a request #25538 reports related preemption churn without this consumer-fence
  mechanism.
• [Bugfix][MoRIIO] Prevent remote KV reload after decoder preemption #53250 concerns a different MoRIIO remote-KV reload path after preemption.
• A downstream commit,
  nvyutwu/vllm@19a2c1a,
  independently carries an equivalent guard but explicitly reports no hardware
  validation; it is not validation evidence here.

The 2026-08-26 16:36 CST publication search found no replacement for this
deferred-free retry guard. Exact defer_block_free, deferred-free, and
zero-progress preemption searches returned #49674/#49675; the other results
concern encoder-cache retention, admission/QoS, output corruption, routed
experts, or the separate default-off #53723 LCF victim-policy proposal.

AI assistance

Claude assisted with the original implementation and tests. GPT assisted with
the current-main review, test reduction and refinement, experiment design,
harness hardening, analysis, and documentation.

On 2026-08-26 the human submitter confirmed review of every changed line and
reported result, accepted responsibility for the contribution, and approved
this disclosure. The commits retain AI Co-authored-by attribution where
applicable. Their Signed-off-by trailers separately record the human DCO
sign-off; they are not AI attribution.

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[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[LearningMachine621]

Final validation update:

• Rebased onto current main at 5c4fe4b.
• Current-main Scheduler tests: 17 passed, including explicit
  fence-advancement coverage.
• Current-main E2E A/B ×3: 1083.7 ± 280.8 → 23.7 ± 0.6
  preemptions (46× reduction), with 80/80 successful requests in all runs.
• Local code/style checks passed. The all-files pip-compile hook could not
  complete because the local environment could not reach PyPI.

The PR is ready for maintainer review. Could a maintainer apply the
ready or verified label to enable CI when convenient?

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @LearningMachine621.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[LearningMachine621]

Maintainer handoff after the 2026-08-26 refresh:

• The branch is now 71521360db9f11494773f262248df0bbc668a59b. GitHub reports it as mergeable. At the 16:36 CST refresh, main was 2267d3b112; the relevant scheduler/test/helper paths were unchanged from the pinned base, and the publication head produced a clean merge tree (68dd1aacad81).
• Targeted deterministic validation is 15 passed on the fix. The pinned base plus the refined test-only tree is 2 failed / 13 passed as expected. All applicable local pre-commit hooks passed.
• The public reproducibility release contains 16/16 gated raw runs, environment/native provenance, logs, aggregation/validation code, and checksums. In the five affected pairs, preemptions changed from 1099.800 ± 48.721 to 32.000 ± 0.000 (34.4x lower). Output throughput changed by +1.90%, P99 TTFT by +0.41%, and P99 E2EL by -1.86%; these are workload-specific descriptive results, not a universal no-regression claim.
• The PR body now separates the narrow correctness/progress guard, its explicit stop-and-wait performance trade-off, and the future-policy RFC.

The latest pre-run-check failed only because this account has 0 merged PRs and the PR lacks verified, ready, or ready-run-all-tests; the actual upstream pre-commit job was skipped. If the evidence is sufficient, could a maintainer reassess the stale needs-rebase label and add the appropriate authorization label so upstream CI can run?

[njhill]

@LearningMachine621 thank you for this fix and sorry for the delay in reviewing!

I made a small modification to change the method name for clarity.

[github-actions]

✅ @LearningMachine621, CI is now available for this PR.

• /ci run starts upstream CI; /amd-ci run starts AMD CI only.
• /ci retry retries failed jobs in the CI build for the current PR head. If the current head has no CI build, it starts a new CI build for the current head containing only jobs that failed in the latest earlier CI build for this PR.
• /amd-ci retry retries failed jobs in AMD CI for the current PR head. Use /amd-ci run when the current head has no AMD CI build.
• /ci cancel cancels scheduled or running CI builds for this PR branch; /amd-ci cancel does the same for AMD CI only.

[njhill]

/ci run

[github-actions]

✅ Triggered Buildkite CI #88187 for commit 16970895e901.

[njhill]

/ci run

[github-actions]

✅ Triggered Buildkite CI #88198 for commit 75aa7fadf001.

[njhill]

/ci run

[github-actions]

✅ Triggered Buildkite CI #88214 for commit 36e0b41dc849.