test(router): contrast shared vs independent RR counters in select_pd_pair

[OftenDream]

Summary

• Strengthen PD round-robin regressions so tests distinguish a shared counter from independent P/D counters.
• Stage-level tests drive WorkerSelectionStage::select_pd_pair (via get_prefill_policy / get_decode_policy):
  • shared one RoundRobinPolicy Arc → prefill [20,0,20,0], decode [0,20,0,20]
  • independent Arcs → both pools [10,10,10,10]
• Clarify the policy-level algorithm contrast test naming/assertions.

Test plan

• select_pd_pair_shared_round_robin_counter_skips_half_the_workers
• select_pd_pair_independent_round_robin_counters_cover_all_workers
• test_shared_vs_independent_counters_across_two_pools

Made with Cursor

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: a368f758-01d1-4778-8b3b-092b28b66423

📥 Commits

Reviewing files that changed from the base of the PR and between def90a2 and 7d7222b.

📒 Files selected for processing (2)

• model_gateway/src/policies/round_robin.rs
• model_gateway/src/routers/grpc/common/stages/worker_selection.rs

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📝 Walkthrough

Summary by CodeRabbit

• Tests
  • Expanded worker-selection coverage for shared and independent round-robin policies.
  • Added clearer assertions and documentation for selection behavior.
  • Confirmed expected worker distribution and alternation remain unchanged.

Walkthrough

The PR expands round-robin tests to distinguish shared and independent counters. PD worker-selection tests add shared helpers and verify alternating selections for shared policies and even distribution for independent policies.

Changes

Round-robin worker selection

Layer / File(s)	Summary
Round-robin counter assertions model_gateway/src/policies/round_robin.rs	The test name, comments, and assertion messages describe shared-counter alternation and independent-counter distribution.
PD selection scenarios model_gateway/src/routers/grpc/common/stages/worker_selection.rs	Helpers aggregate worker selections. Tests cover alternating workers with shared policies and even worker coverage with independent policies.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• smg-project/smg#2035: Updates the same round-robin and gRPC worker-selection tests.

Suggested labels: tests

Suggested reviewers: catherinesue, key4ng

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: testing shared versus independent round-robin counters in select_pd_pair.
Description check	✅ Passed	The description directly explains the test scenarios, expected distributions, and policy-level test updates.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[OftenDream]

Closing for now; will reopen/create when ready.