[https://nvbugs/6601578][fix] Avoid MoE multi-GPU rendezvous port race

[Barry-Delaney]

Motivation

The module-scoped MoE multi-GPU test fixture selected a TCP port by probing and closing a temporary socket, then rebound it only after MPI workers started. On busy CI hosts another process could claim the port in that window, causing EADDRINUSE before any MoE computation. This fixes NVBug 6601578 and its duplicate 6608385; the single-rank sibling was addressed separately by #17497.

Changes

• Have worker rank 0 create TCPStore(port=0) so the OS-selected port is already bound before publication.
• Broadcast the bound port through the MPI communicator and connect nonzero ranks as TCPStore clients.
• Pass the store explicitly to init_process_group, removing the probe-close-rebind path.
• Remove the 6601578 waiver for the real four-GPU MegaMoE CuteDSL test.
• Remove the stale 6572835 single-GPU MegaMoE waiver reintroduced by [None][ci] Waive six pre-existing main-side flaky tests (split from #17792) #17851; the underlying single-rank HashStore fix was already merged in [https://nvbugs/6572835][fix] Use HashStore for single-rank MegaMoE tests #17497.

Testing

• ruff check tests/unittest/_torch/modules/moe/test_moe_module.py
• ruff format --check tests/unittest/_torch/modules/moe/test_moe_module.py
• python3 -m py_compile tests/unittest/_torch/modules/moe/test_moe_module.py
• Pre-commit formatting, YAML, test-list AST, waiver, and lint validations
• pre-commit run --files tests/integration/test_lists/waives.txt
• Four-B200 test_configurable_moe_multi_gpu -k MEGAMOE_CUTEDSL: 4 passed, 2 skipped in 38.25s. Both skips are existing CI-acceleration conditions for the 256-expert parameters; no rendezvous error or hang was observed.

Dev Engineer Review

• Fixed the MoE multi-GPU fixture race.
• Rank 0 now creates a TCPStore with an OS-selected port.
• MPI broadcasts the selected port to all ranks.
• All ranks use the shared store with init_process_group.
• Removed the unused socket helper and fixed-port rendezvous setup.
• Updated obsolete waivers and added waivers in tests/integration/test_lists/waives.txt.
• Updated the NVVM MMA argument schema in dynamic_mainloop.py for block-scaled MMA.
• The changes are consistent with the affected APIs and test-list format.
• Validation included linting, formatting, compilation, pre-commit checks, and four-B200 execution.

QA Engineer Review

• Modified the MegaMoE distributed test fixture in tests/unittest/_torch/modules/moe/test_moe_module.py.
• Removed obsolete waivers and added waivers for the LTX2 example and a LoRA KV-cache scheduler test.
• The modified test remains covered by the integration test list.
• Four-B200 execution passed 4 tests, with 2 existing skips.
• Verdict: sufficient.

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---

Walkthrough

The MegaMoE tests now use an MPI-distributed TCPStore with an OS-selected port. The NVVM MMA mapping uses the newer argument schema. Integration test waivers were updated.

Changes

MegaMoE updates

Layer / File(s)	Summary
Dynamic TCPStore rendezvous tests/unittest/_torch/modules/moe/test_moe_module.py	Rank 0 creates the TCPStore, MPI broadcasts its port, and workers connect to the shared store before NCCL initialization. Unused port-selection code was removed.
NVVM MMA argument mapping tensorrt_llm/_torch/cute_dsl_kernels/mega_moe_nvfp4/dynamic_mainloop.py	The non-legacy mapping now uses mma_kind, d, a, b, and scale_vec_size with Tcgen05MMAScaleVecSize.BLOCK16.
Integration waiver updates tests/integration/test_lists/waives.txt	Obsolete LagunaXS, Qwen3.5, DeepSeek, Llama, and configurable-MoE waivers were removed. LTX2 and LoRA waivers were added.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to e97e7

The PR changes MoE test rendezvous setup and removes stale waivers, with targeted validation reported successful; no actionable merge-blocking risk remains beyond normal checks and review.

Sequence Diagram(s)

sequenceDiagram
  participant Rank0
  participant MPI
  participant Worker
  participant TCPStore
  participant NCCL
  Rank0->>TCPStore: Allocate OS-selected rendezvous port
  Rank0->>MPI: Broadcast port
  MPI->>Worker: Deliver port
  Worker->>TCPStore: Connect to store
  Worker->>NCCL: Initialize process group

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Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the bug fix and accurately summarizes the MoE multi-GPU rendezvous port race change.
Description check	✅ Passed	The description explains the motivation, implementation, waiver updates, and relevant tests; the omitted checklist does not prevent review.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
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Actionable comments posted: 1

🧹 Nitpick comments (1)

tests/unittest/_torch/modules/moe/test_moe_module.py (1)

778-778: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Add parameter and return annotations.

Annotate custom_paths as list[str]. Annotate the procedure return type as None.

As per coding guidelines, “Annotate every function.”

Proposed change

-def _moe_init_worker(custom_paths):
+def _moe_init_worker(custom_paths: list[str]) -> None:

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In `@tests/unittest/_torch/modules/moe/test_moe_module.py` at line 778, Update the
_moe_init_worker function signature to annotate custom_paths as list[str] and
its return type as None.

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Inline comments:
In `@tests/unittest/_torch/modules/moe/test_moe_module.py`:
- Around line 151-184: Add QA test-list entries for
test_configurable_moe_multi_gpu and test_configurable_moe_multi_gpu_eplb,
matching their existing CI coverage in l0_dgx_b200.yml and l0_dgx_b300.yml. Do
not modify the test implementation or add/remove test functions.

---

Nitpick comments:
In `@tests/unittest/_torch/modules/moe/test_moe_module.py`:
- Line 778: Update the _moe_init_worker function signature to annotate
custom_paths as list[str] and its return type as None.

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