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[https://nvbugs/6801108][fix] Set NCCL env vars before ncclGetUniqueId in getComm - #19780
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…d in getComm getComm() called setenv() for NCCL_RUNTIME_CONNECT, NCCL_GRAPH_REGISTER and NCCL_NVLS_ENABLE after getUniqueId(). On the group root, ncclGetUniqueId() starts NCCL's bootstrap root thread, and since NCCL 2.30.7 that thread calls getenv() the first time it uses ncclSocketDefaultMagic(). setenv() is not thread-safe: adding a variable can reallocate environ while the bootstrap thread is scanning it. The root rank then occasionally crashed with SIGSEGV and the remaining ranks waited forever in NCCL bootstrap, which showed up as a hang in the TTP cases of test_configurable_moe_multi_gpu. Move the environment updates before getUniqueId(), matching the order already used by NcclCommunicator::createComm(). Signed-off-by: xxi <xxi@nvidia.com> (cherry picked from commit 2013007) Signed-off-by: Simeng Liu <simengl@nvidia.com>
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Priority: ➖ Normal Estimated code review effort: 2 (Simple) | ~10 minutes Change: Bug fix Suggested reviewers: Merge Risk: 🟡 Moderate · up to Concurrent communicator initialization can still fail or hang during NCCL bootstrap when NCCL_GRAPH_REGISTER is unset. Synchronize the process-wide environment setup across both paths before merging. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
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Full details: Description checkExplanation The description clearly explains the race condition and the proposed ordering fix. It omits the required Test Coverage section, the reported lack of runtime validation, and the PR checklist information.
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Review comments at @cpp/tensorrt_llm/common/opUtils.cpp:
- Around line 107-109: Share NCCL environment initialization between getComm and
NcclCommunicator::createComm using one process-wide std::call_once helper or
mutex, and invoke it before either path calls ncclGetUniqueId(). Initialize the
same NCCL variables on both paths while preserving existing no-overwrite
behavior.
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PR_Github #76007 [ run ] triggered by Bot. Commit: |
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FYI, this also resolves https://nvbugspro.nvidia.com/bug/6782224. Thanks! |
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PR_Github #76059 [ run ] triggered by Bot. Commit: |
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PR_Github #76059 [ run ] completed with state |
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Thanks @SimengLiu-nv , @allisonlim-nv ! |
Forward-port of #19596, which was merged into
release/1.3, tomain.getComm()currently updates NCCL environment variables aftergetUniqueId()starts the NCCL bootstrap thread. A concurrentsetenv()can reallocate the environment while that thread reads it, causing an intermittent root-rank crash and leaving the other ranks waiting in bootstrap.Dev Engineer Review
getComm()now applies the existing NCCL environment updates beforegetUniqueId(), which can start NCCL’s bootstrap thread. The Windows and non-Windows settings retain their no-overwrite behavior, and the non-Windows NVLS fallback remains conditional on fabric usability. No public API or communicator initialization changes are reported. C++ build and GPU/NCCL runtime tests were not run for this forward-port.QA Engineer Review
No test changes.
Per-File QA Perspective
cpp/tensorrt_llm/common/opUtils.cpp: Verify that NCCL environment updates occur beforegetUniqueId()and that existing environment values and the conditional NVLS fallback remain effective. Runtime validation was not run for this forward-port.