[https://nvbugs/6732110][fix] Cache the split communicator in a new `MNNVLAllReduce.allreduce_mnnvl_pending_c… by trtllm-agent · Pull Request #19101 · NVIDIA/TensorRT-LLM

trtllm-agent · 2026-09-12T11:29:19Z

Summary

Root cause: The MNNVL workspace is published only after fabric allocation succeeds, so a failed McastGPUBuffer construction leaves the cache empty and every later AllReduce module issues another world-wide mpi_comm().Split(), leaking one communicator per layer until MPI communicator resources are exhausted and ranks die asymmetrically.
Fix: Cache the split communicator in a new MNNVLAllReduce.allreduce_mnnvl_pending_comms ClassVar, reuse it on retry, and hand ownership to the workspace at publish time so the checkpoint hooks stay byte-identical to upstream; construction retries are preserved (9→9).
Original test: python .repair-bot/repro.py
Automated fix generated by repair-bot

Test plan

Verify fix on the same GPU type as the original failure
Check for regressions in related tests

Links

Bug: https://nvbugs/6732110

Reproduction comparison

Failed commit: repro_on_failed_commit at c21a908
Signature: CUDA_ERROR_UNKNOWN, as on the GB300 run).
ToT: repro_on_tot at b349408
Signature: CUDA_ERROR_UNKNOWN, as on the GB300 run).
Signature relation: needs_agent

Dev Engineer Review

MNNVLAllReduce now caches pending split communicators and serializes workspace initialization per mapping. Failed McastGPUBuffer construction can reuse the communicator. Successful construction transfers ownership to the published workspace. Verify cache cleanup and communicator ownership under retries and concurrent calls. No current review findings or test execution results were supplied.

QA Engineer Review

Modified tests/unittest/_torch/distributed/test_mnnvl_allreduce_lifecycle.py. The tests cover pending-communicator reuse, serialized concurrent construction, and checkpoint-restore failure paths, including protocol and communicator-duplication failures. The relevant tests are not shown in the integration CI or manual-QA test lists. Coverage verdict: needs follow-up because execution results and test-list registration are not confirmed.

Per-File QA Perspective

tensorrt_llm/_torch/distributed/ops.py: Verify pending-communicator reuse after construction failure, single-workspace publication under concurrency, and ownership transfer after successful allocation.
tests/unittest/_torch/distributed/test_mnnvl_allreduce_lifecycle.py: Verify test_workspace_retry_reuses_pending_communicator, test_concurrent_workspace_construction_is_serialized, and the checkpoint-restore failure tests. No integration test-list entry is shown.

…ss failed workspace attempts get_or_scale_allreduce_mnnvl_workspace publishes into allreduce_mnnvl_workspaces[mapping] only after the fabric allocation succeeds. When McastGPUBuffer construction fails, nothing is published, so every later AllReduce module re-enters the split branch and issues another world-wide mpi_comm().Split() -- one leaked communicator per construction site (~200 on a 94-layer MoE at TP16). AllReduce.__init__ swallows the MNNVL exception, which lets the retry storm run to completion; exhausting MPI communicator resources then kills ranks asymmetrically, and survivors report the opaque NCCL 'remote process exited or there was a network error' from getComm. Track communicators split for a mapping whose workspace is not yet published in MNNVLAllReduce.allreduce_mnnvl_pending_comms and reuse them on retry. Ownership is handed to the workspace at publish time, so a communicator reachable from a workspace is never also in the cache and the checkpoint hooks need no change. Reuse after a failed attempt is safe because McastDeviceMemory borrows the communicator (freeComm=false) and its destructor never frees it. Construction retries are not suppressed. Signed-off-by: trtllm-agent <296075020+trtllm-agent@users.noreply.github.com>

coderabbitai · 2026-09-12T11:36:11Z

Walkthrough

The MNNVL all-reduce path now serializes workspace construction per mapping. It reuses pending communicators after failed construction and removes them after successful publication. Tests cover retry and concurrent construction behavior.

Changes

MNNVL workspace lifecycle

Layer / File(s)	Summary
Workspace construction synchronization `tensorrt_llm/_torch/distributed/ops.py`, `tests/unittest/_torch/distributed/test_mnnvl_allreduce_lifecycle.py`	Per-mapping locks serialize workspace lookup and construction. Concurrent callers receive the same workspace.
Pending communicator lifecycle `tensorrt_llm/_torch/distributed/ops.py`, `tests/unittest/_torch/distributed/test_mnnvl_allreduce_lifecycle.py`	The implementation caches split communicators during construction retries and removes them after successful workspace initialization.
Lifecycle test support `tests/unittest/_torch/distributed/test_mnnvl_allreduce_lifecycle.py`	Fake MNNVL components and tests cover communicator reuse and single-construction concurrency.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~15 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant MNNVLAllReduce
  participant Communicator
  participant Workspace
  Caller->>MNNVLAllReduce: request workspace
  MNNVLAllReduce->>MNNVLAllReduce: acquire mapping lock
  MNNVLAllReduce->>Communicator: reuse or create split communicator
  MNNVLAllReduce->>Workspace: construct workspace
  Workspace-->>MNNVLAllReduce: publish workspace
  MNNVLAllReduce-->>Caller: return workspace

Suggested reviewers: bowenfu

Merge Risk: 🔵 Low · up to c3e4f

The communicator retry behavior is covered, but the concurrency regression test is scheduler-dependent and may not catch a future loss of serialization. Make its synchronization deterministic before relying on it for this regression.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 6.25% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 16 functions across 2 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 identifies the NVBugs issue, the fix type, and the main change: caching the split communicator for MNNVL all-reduce workspace initialization.
Description check	✅ Passed	The description explains the root cause, the proposed fix, test coverage, reproduction results, and bug link. It does not use the template's exact Description, Test Coverage, and PR Checklist headings…
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: 2

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Inline comments:
In `@tensorrt_llm/_torch/distributed/ops.py`:
- Around line 201-209: Add a distributed MNNVL regression test in the existing
MNNVL test module covering pending communicator reuse: make McastGPUBuffer fail
once, retry construction with the same Mapping, assert mpi_comm().Split is
called only once and the pending communicator is reused, then verify successful
publication removes the pending entry.
- Around line 204-209: Serialize MNNVL initialization per mapping in the
AllReduce/MNNVLAllReduce setup path: protect the complete pending_comms lookup,
MPI communicator Split, workspace construction, and ownership transfer with
shared per-mapping synchronization. Ensure concurrent constructors reuse the
single published communicator and workspace, preventing duplicate creation,
overwrite, and leaks.

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Inline comments:
In `@tests/unittest/_torch/distributed/test_mnnvl_allreduce_lifecycle.py`:
- Around line 160-162: Update the concurrent construction test around
construction_barrier.wait in the mocked McastGPUBuffer flow to coordinate
executor entry deterministically using events or an instrumented lock. Remove
the threading.BrokenBarrierError handler and do not allow a timeout to be
swallowed; ensure both calls contend on the per-mapping lock before construction
completes so construction_count == 1 verifies genuine concurrency.

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tensorrt_llm/_torch/distributed/ops.py
tests/unittest/_torch/distributed/test_mnnvl_allreduce_lifecycle.py

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cascade812 · 2026-09-18T16:30:36Z

/bot run

cascade812

Address the race condition issue and approve.

github-actions · 2026-09-18T16:31:50Z

Automatically added "ci: full pre-merge approved" because this PR has satisfied the required GitHub review approvals. Unresolved review conversations and other required checks remain independent merge requirements.

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trtllm-agent requested a review from a team as a code owner September 12, 2026 11:29

trtllm-agent requested review from YihuiLu512 and lori-ren September 12, 2026 11:29

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Comment thread tensorrt_llm/_torch/distributed/ops.py

Comment thread tensorrt_llm/_torch/distributed/ops.py

[https://nvbugs/6732110][fix] Serialize MNNVL workspace initialization

c3e4fb6

Signed-off-by: Guiju Zhang <guijuz@nvidia.com>

coderabbitai Bot reviewed Sep 18, 2026

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Comment thread tests/unittest/_torch/distributed/test_mnnvl_allreduce_lifecycle.py

cascade812 approved these changes Sep 18, 2026

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trtllm-agent added the ci: full pre-merge approved label Sep 18, 2026

cascade812 enabled auto-merge (squash) September 19, 2026 04:49

cascade812 merged commit afb6d11 into NVIDIA:main Sep 22, 2026
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cascade812 deleted the repair-bot-bug6732110 branch September 23, 2026 20:24

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Walkthrough

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