fix(vllm): settle kvwarm stages through a per-step round on every attention-DP rank

[Arsene12358]

Summary

The FPM self-benchmark's KV warm-up settles each rung's chain-fleet stage through the non-blocking stage exchange added in #14029: every attention-DP rank reports its build outcome and polls the group verdict from its idle scheduler steps. On GB200 DEP16 (GLM-5.2-NVFP4, --data-parallel-size 16 --enable-expert-parallel, unpinned KV pools) every decode sweep on main aborts at the first point: rank 0's 10 s READY tolerance expires with 2–3 of 16 ranks at READY (timed out waiting for attention-DP benchmark ranks for benchmark_id=1; ready_ranks=[0, 13]; three runs, survivor sets [0, 4, 11], [0, 13], [0, 5]). The pre-merge code (all fleets settled locally in the same step) completed the same 564-point manifest 564/564.

Root cause

Under attention-DP an idle benchmark step runs vLLM's dummy forward, a group collective, so ranks step in lockstep; the benchmark overrides has_requests() so schedule() runs once per engine-loop iteration on every rank (in step and in step_with_batch_queue alike). The stage decision is a ZMQ message from rank 0 polled inside schedule(), so ranks see it in different iterations: rank 0 and any follower whose poll runs in the same iteration see it one step before the rest. Every rank idled the step it saw the verdict in and started the rung's first point in the next one. The early ranks therefore injected the point and blocked in its READY barrier (a plain ZMQ wait inside schedule()) in iteration N+1, while the late ranks returned an idle step in N+1 and entered that iteration's dummy forward, a collective that waits for the blocked ranks. A diagnostic overlay that logged every idle gate confirmed it: all 16 ranks log the verdict within 20 ms, two ranks log Benchmark decode: … batch_size=121 at once, the other 14 log nothing for the following 10 s (they are outside schedule(), in the executor), then rank 0 times out.

Fix

Settle the stage in a blocking round that every rank runs in the same scheduler step, instead of a decision that each rank reads in its own step.

On every idle decode-sweep step under attention-DP (no benchmark point active), each rank sends rank 0 its stage status: a round sequence number, the stage batch, its phase (none, building or done) and, when done, whether its fleet built. Rank 0 waits for the status of every other rank, checks that all ranks report the same sequence number and the same batch (a mismatch fails the benchmark with attention-DP warm-up ranks are not in lockstep), and answers every rank in the same round: whether all ranks are done and, if so, the group verdict ok = all ranks built. A rank whose fleet is still building keeps stepping, so a slower rank's build proceeds while the faster ranks idle through rounds; the stage settles on every rank in the round in which the last rank reports done. Because the round is a blocking exchange inside schedule() that every rank enters in the same lockstep iteration, no rank can start the rung's first point before every other rank has left the round with the same verdict, and there is no delivery window to depend on: a message that arrives late only delays the round for everyone. On a failed stage all ranks shed their fleets in that step and the rung's points take fake injection from the next one. dp_size == 1 (no synchronizer) settles locally in the step the build finishes, as before; the one visible change is that a soft timeout mid-build now records the abandoned stage as failed instead of shedding it silently.

The fallback exit still passes through the pre-existing rank-local deferred-free check one step after settlement. Under vLLM 0.28.0 a fleet shed from inside schedule() has its blocks fenced only when defer_block_free is on (a kv_consumer/kv_both connector plus max_concurrent_batches > 1), and with async scheduling that fence drains at the end of the same engine iteration, so the check is already clear on every rank by the next step; only pipeline-parallel configurations (max_concurrent_batches >= 3) could carry a deferral further. The validated setup has no KV connector, so it never defers.

An earlier revision of this PR announced a start step two iterations ahead of rank 0's decision. That works only while every rank sees the decision within two iterations, a window rather than an invariant (raised in review); the round replaces it.

Protocol change: the stage_status/stage_decision messages and the stage_report/stage_poll exchange (with _BenchmarkStageExchange) are gone; _BenchmarkSynchronizer.stage_round(seq, batch, phase, ok) returns a _BenchmarkStageRound(all_done, ok); messages of the wrong shape or an out-of-lockstep status are rejected like any other malformed message, and a coordinator failure is forwarded to every follower so no rank hangs. Cost: one message exchange per idle decode-sweep step (a few hundred microseconds against ~15–150 ms steps); measured steps are unchanged. Every settlement is logged (KVWARM: stage batch=… settled by the group (ok=…) at round …), so a run can be audited for one settlement round per stage across all ranks.

Validation

• Unit tests (need vLLM; run in the collection image on a GB200 node, vLLM 0.28.0): test_vllm_instrumented_scheduler.py + test_benchmark_points.py 263 passed (main 252 + 11 new or extended), 25 of them on the stage round. The new tests cover: a rank that keeps reporting building until every rank is done; one rank's failed build giving ok=False to all; a follower blocked until rank 0 answers; a timeout on either side without a peer; out-of-lockstep seq/batch rejected; malformed statuses and results (wrong shape, wrong rank, missing or spurious verdict) rejected with the follower notified; the status-shape check on both roles; dp_size == 1 local settlement; the scheduler's exact round sequences for build, failure, group fallback, pool shortfall and soft timeout; and a delayed-decision regression: a real inproc pair whose broadcast is delayed 0.3 s, both ranks producing the same [busy, idle, idle] step sequence with neither moving ahead (a follower that polls and treats "no reply yet" as an idle step, the pre-fix behaviour, fails this test).
• GB200 DEP16 decode sweep with this change overlaid on stock main (GLM-5.2-NVFP4, --data-parallel-size 16 --enable-expert-parallel --async-scheduling, kvwarm on, KV pool unpinned, explicit 564-point manifest over 35 batch rows 1–121): 564/564 points on all 16 ranks, 35/35 stages, 0 group fallbacks, 0 skipped points, 0 READY timeouts, 0 lockstep errors (Slurm 7201236). The same manifest on stock main aborted at point 1 in 3/3 runs.
• Lockstep audit from the new INFO line: each of the 35 stages settled on exactly one round on all 16 ranks (batch=121 … at round 194 on every rank, up to batch=1 … at round 1348). All 35 verdicts were ok=True, so the group-fallback exit was not exercised on hardware in this run; it rests on the unit tests.
• Measured values are unchanged: point by point against the pre-feat(vllm): improve FPM self-benchmark fidelity with real-KV seeding and salted real-text prompts #14029 collection of the same manifest (Slurm 7024594, same pool profile of 29,430 blocks, 559 points on real KV in both runs), the ratio has a median of 1.001 (p10–p90 0.996–1.006, min 0.94, max 1.02). One point differs by more than 5 %: batch 1 at 128 KV tokens (15.9 vs 17.0 ms, the smallest step of the grid).
• Cost: the sweep took 11.4 min from plan preparation to the results being written, against 11.3 min for the previous revision of this PR on the same manifest and hardware; the round's per-step exchange is not visible at that scale.

Related

Linear AIC-1957 (validation of #14029 on GB200 DEP16), AIC-1944 (umbrella). Follow-up on the same surface: the fake-point repeat guard still reads the rank-local pool (_bench_usable_blocks) where the stage plan reads the negotiated one (AIC-1945).

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Walkthrough

Changes

The scheduler now exchanges structured warm-up verdicts with an agreed stage-relative start_step. KV warm-up ranks track stage steps, wait for the agreed step, and reset counters between stages. Tests cover normal, delayed, fallback, and failure synchronization paths.

Attention-DP warm-up synchronization

Layer / File(s)	Summary
Stage verdict coordination components/src/dynamo/vllm/instrumented_scheduler.py	Warm-up polling now uses stage-relative steps. The coordinator broadcasts a structured verdict with a shared start_step, and followers validate it.
KV warm-up step alignment components/src/dynamo/vllm/instrumented_scheduler.py	KV warm-up tracks stage steps, records decision skew, waits until the agreed start_step, and resets counters for each stage.
Synchronization validation components/src/dynamo/vllm/tests/test_vllm_instrumented_scheduler.py	Tests cover structured verdicts, delayed startup, fallback timing, failure paths, and stage counter resets.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: 🟡 Moderate · up to 8d115

Invalid stage decisions can cause ranks to enter warm-up work out of sync and reintroduce synchronization timeouts. Validate received start steps before merge.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 54.84% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 31 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly identifies the main change: settling KV warm-up stages through a per-step round across all attention-DP ranks.
Description check	✅ Passed	The description is detailed and on-topic. It explains the failure, root cause, per-step round fix, validation, performance impact, and related work. It does not use the template headings, identify whe…

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Inline comments:
In `@components/src/dynamo/vllm/instrumented_scheduler.py`:
- Line 1136: Update the start_step validation near _kvwarm_step_busy to reject
booleans and all non-positive values, accepting only exact positive integers.
Raise an explicit exception for invalid synchronization data rather than using
assert, and add protocol tests covering True, 0, and -1.

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[Arsene12358]

Re the dynamo-review-agent note on the _BenchmarkStageVerdict docstring (#14728 (comment) — GitHub returns a server error on replies to that thread, hence this top-level comment): trimmed in 861e5bb — the docstring now states only the field's meaning and points at _coordinate_stage / _kvwarm_step_busy for the rationale.