cp: fix: Fix crash when using activation_checkpointing (1676) into r0.5.0

[chtruong814]

beep boop [🤖]: Hi @yfw 👋,

we've cherry picked #1676 into  for you! 🚀

Please review and approve this cherry pick by your convenience!

Summary by CodeRabbit

• Bug Fixes
  • Resolved metadata mismatch errors in attention mechanisms during activation checkpointing.
  • Improved activation checkpointing support for distributed tensor training configurations.

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[github-actions]

⚠️ File Consistency Check

Check based on commit: eb24a98 (PR #1682 from cherry-pick-1676-r0.5.0)

⚠️ DTensor Policy Worker Synchronization Warning

The file nemo_rl/models/policy/workers/dtensor_policy_worker_v2.py was modified in this PR, but nemo_rl/models/policy/workers/dtensor_policy_worker.py was not updated.

Why this matters:
These files contain related DTensor policy worker implementations that should be kept synchronized to ensure consistency across different versions.

Action required:

• Please review if the changes in nemo_rl/models/policy/workers/dtensor_policy_worker_v2.py should also be applied to nemo_rl/models/policy/workers/dtensor_policy_worker.py
• Update nemo_rl/models/policy/workers/dtensor_policy_worker.py if necessary to maintain consistency
• If the files are intentionally different, please add a comment in the PR explaining why

Files to check:

• Modified: nemo_rl/models/policy/workers/dtensor_policy_worker_v2.py
• Not modified: nemo_rl/models/policy/workers/dtensor_policy_worker.py

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_{This check ensures that related file implementations remain synchronized across the codebase. If you believe this warning is incorrect or the files should intentionally differ, please add a comment explaining the reasoning.}

[coderabbitai]

📝 Walkthrough

Walkthrough

Modified the condition for SDPA method construction in DTensorPolicyWorkerV2's __init__ to also trigger when activation checkpointing is enabled, alongside the existing cp_size > 1 check. Added clarification that CUDNN_ATTENTION must be excluded when activation checkpointing is active to prevent forward-backward metadata mismatch errors.

Changes

Cohort / File(s)	Summary
SDPA Backend Selection Logic nemo_rl/models/policy/workers/dtensor_policy_worker_v2.py	Expanded condition for SDPA method construction from cp_size > 1 to also include activation_checkpointing flag; added commentary on CUDNN_ATTENTION exclusion requirement; updated SDPBackend import usage context

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

• Verify the expanded conditional logic is semantically correct and aligns with intended behavior when both cp_size > 1 and activation_checkpointing are evaluated
• Confirm CUDNN_ATTENTION exclusion is properly enforced under the new condition to prevent the documented metadata mismatch error

Possibly related PRs

• PR #1676 — Directly modifies the same DTensorPolicyWorkerV2 __init__ logic to select SDPA backend when activation_checkpointing is enabled with CUDNN_ATTENTION exclusion
• PR #1665 — Modifies the same file to refactor SDPA backend (sdpa_method/SDPBackend) determination and propagation into model construction

Suggested labels

r0.5.0, CI:L1

Suggested reviewers

• yfw
• terrykong

Pre-merge checks and finishing touches

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately reflects the main change: fixing a crash when using activation_checkpointing, which aligns with the code modifications in dtensor_policy_worker_v2.py.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Test Results For Major Changes	✅ Passed	PR #1682 is a minor bug fix cherry-pick that expands a condition to handle activation_checkpointing and excludes CUDNN_ATTENTION to prevent crashes. The code modification is limited and backward compatible.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch cherry-pick-1676-r0.5.0

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🔇 Additional comments (1)

nemo_rl/models/policy/workers/dtensor_policy_worker_v2.py (1)

285-300: Fix correctly excludes CUDNN_ATTENTION backend when activation checkpointing is enabled, preventing metadata mismatch errors.

The expanded condition appropriately triggers SDPA backend selection when either context parallelism is enabled or activation checkpointing is active. By restricting to FLASH_ATTENTION and EFFICIENT_ATTENTION backends in these cases, the fix resolves compatibility issues where CUDNN_ATTENTION causes recomputed tensor metadata mismatches during activation checkpointing. The implementation is clean, the comment explains the technical rationale, and the import is properly scoped. Code follows all guidelines including copyright header presence and config attribute access patterns.

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