Add ray stack tool for debugging (#3213)

doc/source/rllib.rst

@@ -99,3 +99,6 @@ If you encounter errors like
 `blas_thread_init: pthread_create: Resource temporarily unavailable` when using many workers,
 try setting ``OMP_NUM_THREADS=1``. Similarly, check configured system limits with
 `ulimit -a` for other resource limit errors.
+
+For debugging unexpected hangs or performance problems, you can run ``ray stack`` to dump
+the stack traces of all Ray workers on the current node. This requires py-spy to be installed.

doc/source/troubleshooting.rst

@@ -92,6 +92,11 @@ of the following reasons.
 Hanging
 -------
 
+.. tip::
+    
+    You can run ``ray stack`` to dump the stack traces of all Ray workers on
+    the current node. This requires py-spy to be installed.
+
 If a workload is hanging and not progressing, the problem may be one of the
 following.