[Bugfix][Metrics] Fix RayPrometheusMetric.labels() returning shared labeled child#40369
[Bugfix][Metrics] Fix RayPrometheusMetric.labels() returning shared labeled child#40369marwan116 wants to merge 1 commit intovllm-project:mainfrom
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Code Review
This pull request refactors the Ray metrics wrappers to ensure that calling .labels() returns independent labeled children rather than mutating the base metric's default tags. This change aligns the behavior with the prometheus_client contract, preventing label clobbering where increments were incorrectly attributed to the last used label set. The implementation introduces _LabeledRayMetric subclasses for counters, gauges, and histograms to handle per-call tag forwarding. Comprehensive regression tests have been added to verify independent tag sets, tag forwarding, and label arity validation. I have no feedback to provide as the review comments were evaluative or confirmatory in nature.
…hild RayPrometheusMetric.labels() mutated the wrapped Ray metric's default tags in place and returned self, so every .labels(...) call on a given wrapper instance returned the same object. The initialization loop in PrometheusStatLogger iterates over FinishReason and uses counter.labels(model, idx, str(reason)) to create a "child" per reason; under the Ray wrapper, all five children pointed at the same underlying Ray counter whose default tags were set by the last iteration. Every .inc() call landed on finished_reason="repetition", regardless of the request's actual finish reason. The same flaw affected every other .labels(...)-partitioned counter, gauge, and histogram in the Ray metrics path (per-engine splits via create_metric_per_engine, spec decoding / KV connector / perf / NIXL metrics, etc.), silently falsifying any multi-bucket dashboard or alert derived from vLLM metrics on Ray. Fix: .labels() now returns an independent _LabeledRayMetric that carries its own tag dict and forwards .inc()/.set()/.observe() to the underlying Ray metric with tags=self._tags on every call. Per Ray's metric API, per-call tags take precedence over any default tags, so concurrent labeled children no longer clobber each other. This matches the prometheus_client.Metric.labels() contract callsites rely on. Adds regression tests covering Counter, Gauge, and Histogram wrappers: labels() returns distinct children, per-child tags forward to the underlying metric, non-string label values are coerced, and arity validation still fires. Co-authored-by: Claude <noreply@anthropic.com> Signed-off-by: Marwan Sarieddine <sarieddine.marwan@gmail.com>
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Thanks @marwan116! LGTM.
| def inc(self, value: int | float = 1.0): | ||
| if value == 0: | ||
| return | ||
| return self._wrapper.metric.inc(value, tags=self._tags) |
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This duplicates the inc() in RayCounterWrapper (same for the other classes too)
Can we not have labels() return a new instance of the RayPrometheusMetric subclass rather than adding a new class hierarchy?
| # Ray metric's default tags in place and returned self, so every | ||
| # labeled "child" shared the last-set label values -- e.g. every | ||
| # vllm:request_success increment was attributed to the last | ||
| # FinishReason iterated (REPETITION). |
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I'm not a fan of "earlier versions [did this other thing]" that coding models tend to spit out - I think it just adds clutter
| """Regression test: RayCounterWrapper.labels() must return distinct | ||
| labeled children that each carry their own tag set. | ||
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| Prior to the fix, labels() mutated the wrapped Ray counter's default |
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Ditto on this "prior to the fix" comment
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Discussed with @marwan116 offline, I will pick this up in #40840. We can close this PR. |
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Closing in favor of #40840 |
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Purpose
When vLLM runs with the Ray Prometheus path (Ray Serve,
ray.data.llm, etc.),vllm:request_success{finished_reason=...}only ever increments therepetitionbucket regardless of the request's actual finish reason;stop,length,abort, anderrorstay at zero.Root cause.
RayPrometheusMetric.labels()mutated the wrapped Ray metric's default tags in place (viaset_default_tags) and returnedself, so every.labels(...)call on a given wrapper returned the same object. InPrometheusStatLogger, the initialization loop partitionscounter_request_successoverFinishReason; all five entries end up pointing at the same wrapper, whose default tags get frozen at the last-iterated member (REPETITION). Subsequent.inc()calls record under that tag, no matter the request's actualfinish_reason.The same flaw affects every other
.labels(...)-partitioned counter, gauge, and histogram on the Ray path — per-engine splits viacreate_metric_per_engine, plus spec-decoding, KV-connector, perf, and NIXL metrics. Any alerting, SLO, or capacity-planning built on multi-bucket vLLM-on-Ray metrics is silently wrong.Fix.
labels()now returns an independent_LabeledRayMetricthat carries its own tag dict and forwards.inc()/.set()/.observe()to the underlying Ray metric withtags=self._tagson every call. Per Ray's metric API, per-call tags take precedence over default tags, so concurrent labeled children cannot clobber each other. This matches theprometheus_client.Metric.labels()contract that callsites rely on — no callsite changes needed.Per AGENTS.md §1: searched open/closed PRs and issues for
request_success finished_reason,RayPrometheusMetric labels,set_default_tags ray,ray metrics labels independent— no duplicate work.Test Plan
New unit tests in
tests/v1/metrics/test_ray_metrics.py(no Ray cluster required — the underlying Ray metric is swapped for aMagicMock):test_ray_counter_labels_returns_independent_children— two.labels(...)calls return distinct objects with independent tag dicts.test_ray_counter_inc_forwards_per_child_tags— each child's tags reach the underlying Ray counter;.inc(0)stays a no-op.test_ray_gauge_labels_returns_independent_children_and_forwards_tags— same forRayGaugeWrapper.set.test_ray_histogram_labels_returns_independent_children_and_forwards_tags— same forRayHistogramWrapper.observe.test_ray_counter_labels_accepts_non_string_label_values— covers thestr(idx)coercion path used by the per-engine split.test_ray_counter_labels_arity_validation— arity check still fires.Local commands run:
The existing
test_engine_log_metrics_raysmoke test requires a built vLLM + GPU environment and is deferred to CI.Test Result
All four pre-commit hooks pass locally:
Production-workload evidence motivating the fix (ground truth via an in-worker
check_stopprobe, compared against the Prometheus scrape on the same run):request.status(ground truth)FINISHED_STOPPEDFINISHED_LENGTH_CAPPEDFINISHED_REPETITIONPre-fix, Prometheus reported 100% of those increments under
vllm:request_success{finished_reason="repetition"}. With the fix, increments land on the matchingFinishReasonstring (stopin this run).AI-assisted (per AGENTS.md §1.3). Every changed line was reviewed by the submitter.