fix(grpc): report vLLM loads per DP rank

[Moersity]

Description

Problem

The vLLM gRPC GetLoads implementation reads a scalar scheduler snapshot and labels it as DP rank 0. For data-parallel engines this can make every virtual gateway worker inherit rank 0 or backend-average telemetry. Missing stats are also reported as an idle rank, which can attract traffic to a rank whose load is actually unknown.

Solution

Read only rank-addressable snapshots from vLLM's logger manager, preserve global DP rank IDs, and use vLLM's profiled per-engine kv_cache_size_tokens for absolute capacity. Missing or invalid telemetry is omitted instead of synthesized as idle. On the gateway side, scope each engine-native load response to the virtual worker's configured DP rank and never fall back to another rank or an aggregate. Fleet rollups returned by an upstream gateway's /loads endpoint retain their worker annotations and are not mistaken for one engine's DP ranks. HTTP probes use the worker's base endpoint, without its virtual @rank identity suffix; Prometheus fallback remains aggregate when native per-rank telemetry is unavailable.

This adapts the original change by haoxli (haoxli0412@gmail.com) and was rebased onto upstream main at de5ff2ce; the commit preserves the original author and DCO sign-off.

Changes

• expose cached vLLM scheduler loads per managed global DP rank
• support the optional dp_rank filter, including explicit rank zero
• reject unmanaged rank filters with INVALID_ARGUMENT
• keep aggregate/scalar stats from leaking into per-rank telemetry
• report profiled per-engine KV capacity and include skipped waiting requests
• project native backend load responses to gateway virtual DP workers
• preserve cached gateway /loads fleet rollups introduced by feat(api): serve engine load from the gateway's cached snapshot #2418
• build native and Prometheus probe URLs from the base worker endpoint
• document rollout and load-field semantics
• add engine-free Python and Rust regression coverage

Test Plan

Executed on ci:~/work/smg before the latest-main rebase; the post-rebase GitHub PR workflow re-runs the complete gate:

• .venv/bin/python -m pytest -q grpc_servicer/tests/test_vllm_loads.py — 34 passed
• .venv/bin/python -m pytest -q -rs grpc_servicer/tests/test_vllm*.py — 70 passed, 0 skipped, using the repository CI environment's pinned vLLM 0.27.1, ZMQ, and msgspec dependencies
• supplemental base-environment pytest -q -rs grpc_servicer/tests — 142 passed; its 5 repository-declared optional-engine skips were not used as vLLM validation
• Python ruff check, ruff format --check, and py_compile on changed Python files
• cargo test -p openai-protocol --test dp_load_projection — 4 passed
• cargo test -p openai-protocol --lib — 111 passed
• cargo test -p smg --lib worker_monitor_tests::backend_load_is_projected_to_virtual_worker_rank — 1 passed
• cargo test -p smg --lib native_loads_tests::native_loads_are_projected_to_the_virtual_worker_rank — 1 passed
• cargo test -q -p smg --lib — 1929 passed, 5 existing ignored tests
• cargo clippy -p openai-protocol -p smg --lib --tests -- -D warnings
• cargo +nightly fmt -p openai-protocol -p smg -- --check
• cargo +nightly fmt --all -- --check

The latest-main conflict resolution adds focused regression cases proving that engine-native responses select one virtual DP rank while annotated gateway fleet rollups are preserved rather than filtered by a non-global rank ID.

Checklist

• cargo +nightly fmt passes
• Scoped production and test targets pass strict Clippy
• Python lint, formatting, focused tests, and complete unit-test suite pass
• Documentation updated
• (Optional) Please join us on Slack #sig-smg to discuss, review, and merge PRs

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Reviewing files that changed from the base of the PR and between a75ffe7 and 96997f6.

📒 Files selected for processing (3)

• crates/protocols/src/worker.rs
• crates/protocols/tests/dp_load_projection.rs
• model_gateway/src/worker/monitor.rs

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📝 Summary

Summary by CodeRabbit

• New Features
  • Added per-rank load reporting for distributed processing workers.
  • Backend load metrics are scoped to the requesting virtual worker, with unavailable ranks reported as unavailable.
  • Added collection and validation of scheduler telemetry from compatible vLLM deployments.
  • Gateway fleet-level load summaries remain available without being treated as individual engine ranks.
• Bug Fixes
  • Invalid, missing, or out-of-range load data is safely omitted or rejected.
  • Empty load responses no longer appear as idle workers.
• Documentation
  • Documented per-rank load reporting, capacity metrics, compatibility behavior, and deployment verification.

Walkthrough

The change adds validated vLLM per-rank load collection, rank-filtered GetLoads responses, protocol-level worker projection, gateway integration, tests, and DP load reporting documentation.

Changes

DP load reporting

Layer / File(s)	Summary
vLLM load collection and validation grpc_servicer/smg_grpc_servicer/vllm/loads.py, grpc_servicer/smg_grpc_servicer/vllm/servicer.py, grpc_servicer/tests/test_vllm_loads.py, grpc_servicer/README.md	The servicer builds per-rank responses from supported logger formats, validates metric ranges, supports rank filtering, and returns INVALID_ARGUMENT for unmanaged ranks. Tests cover rank discovery, missing data, capacity values, invalid metrics, and optional sections. The README documents response fields and DP4 verification steps.
Worker load rank projection crates/protocols/src/worker.rs, crates/protocols/tests/dp_load_projection.rs	WorkerLoadResponse::for_dp_rank distinguishes engine DP ranks from fleet rollups, retains the requested rank, removes aggregate data for virtual workers, updates dp_rank_count, and returns None when the rank is unavailable.
Gateway monitoring integration model_gateway/src/worker/monitor.rs	Backend and native HTTP engine responses are projected onto each virtual worker's DP rank. Gateway fleet rollups remain unprojected. URL construction uses endpoint_url. Tests verify both paths.

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

Merge Risk: ⚪ Minimal · up to 96997

Per-rank load reporting now scopes virtual workers to their configured DP rank and avoids substituting aggregate or missing telemetry. No current merge-blocking risk remains.

Sequence Diagram(s)

sequenceDiagram
  participant GatewayWorker
  participant GetLoads
  participant build_loads_response
  participant SchedulerLogger
  GatewayWorker->>GetLoads: request load data
  GetLoads->>build_loads_response: engine and DP rank request
  build_loads_response->>SchedulerLogger: read managed ranks and snapshots
  SchedulerLogger-->>build_loads_response: per-rank scheduler telemetry
  build_loads_response-->>GetLoads: validated GetLoadsResponse
  GetLoads-->>GatewayWorker: load response
  GatewayWorker->>GatewayWorker: project engine response onto virtual worker rank

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 52.08% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 48 functions across 6 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: fixing gRPC vLLM load reporting so it is reported per DP rank.
Description check	✅ Passed	The description directly explains the DP-rank load-reporting problem, the implementation changes, and the related tests and documentation.
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.

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Actionable comments posted: 1

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Inline comments:
In `@model_gateway/src/worker/monitor.rs`:
- Line 749: Update the NativeLoads::Available branch in the worker load handling
to pass the response through Self::project_backend_load(worker.as_ref(),
response), ensuring native HTTP load data is projected to the virtual worker’s
DP rank before publication or caching. Leave the Prometheus fallback unchanged
and add a regression test covering a multi-rank native HTTP response.

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Configuration used: Organization UI

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Run ID: 745bd56c-d205-4c35-ad9d-b0b005a21938

📥 Commits

Reviewing files that changed from the base of the PR and between d2379b6 and aade05a.

📒 Files selected for processing (7)

• crates/protocols/src/worker.rs
• crates/protocols/tests/dp_load_projection.rs
• grpc_servicer/README.md
• grpc_servicer/smg_grpc_servicer/vllm/loads.py
• grpc_servicer/smg_grpc_servicer/vllm/servicer.py
• grpc_servicer/tests/test_vllm_loads.py
• model_gateway/src/worker/monitor.rs

Included review availability: Your plan provides up to 4 included reviews per hour; 2 remain after this review.