[GitHub] [incubator-mxnet] DickJC123 opened a new pull request #16391: cuDNN non-persistant bidirectional RNN dgrad sync fix

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DickJC123 opened a new pull request #16391: cuDNN non-persistant bidirectional RNN dgrad sync fix
URL: https://github.com/apache/incubator-mxnet/pull/16391
 
 
   ## Description ##
   Background: A non-deterministic failure of test_operator_gpu.py:test_lstm_bidirectional that was observed just once in our own CI was found to be due to cuDNN's RNN dgrad implementation.  cuDNN launches many kernels as part of the RNN dgrad into various auxiliary streams that are different than the primary user-settable stream of the cuDNN handle.  A final kernel launched by cuDNN into one of these aux streams was not being synchronized (via events) back to the handle's stream.  When MXNet's RNN::Backward() returns, a gradient summation kernel can be launched by the GPU worker into its main stream and this kernel's execution could potentially overlap or even precede that of the final cuDNN RNN dgrad kernel.  MXNet's calling of cuDNN's wgrad immediately after dgrad makes this data-race failure exceedingly rare, and in fact we discovered the problem by code inspection, not by being able to reproduce the original CI failure.
   
   The first commit of this PR will demonstrate the failure.  The approach is to have MXNet now skip the RNN wgrad operation if grad_req = {'data':'add', 'parameters':'none'}, plus expand the test_lstm_bidirectional test to invoke this case.  Skipping wgrad aggravates the data race by allowing MXNet to launch the gradient summation kernel sooner after the RNN dgrad kernels are launched with no intervening wgrad gpu activity.
   
   Once the failure is solidly demonstrated, a follow-up commit will supply the fix.  The fix will use cuda events directed at the legacy default stream to ensure all dgrad GPU activity is complete before the wgrad or other kernels begin.  Unlike a cudaDeviceSynchronize(), the fix will not block the CPU.  nvprof-based timing analysis of the fix shows no measurable difference in timing for the single-RNN case analyzed.
   
   The fix is needed for all versions of cuDNN supported by MXNet (so up to the current v7.6.4) and is needed only for non-persistent bidirectional RNNs.
   
   ## Checklist ##
   ### Essentials ###
   Please feel free to remove inapplicable items for your PR.
   - [ ] The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant [JIRA issue](https://issues.apache.org/jira/projects/MXNET/issues) created (except PRs with tiny changes)
   - [ ] Changes are complete (i.e. I finished coding on this PR)
   - [X] All changes have test coverage:
   - Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
   - Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
   - Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
   - [X] Code is well-documented: 
   - For user-facing API changes, API doc string has been updated. 
   - For new C++ functions in header files, their functionalities and arguments are documented. 
   - For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
   - Check the API doc at https://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
   - [X] To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change
   
   ### Changes ###
   - [ ] Feature1, tests, (and when applicable, API doc)
   - [ ] Feature2, tests, (and when applicable, API doc)
   
   ## Comments ##
   - If this change is a backward incompatible change, why must this change be made.
   - Interesting edge cases to note here
   

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