repeat warning on OpenMPI oversubscribe

Author: boegel

docs/installing-on-top.md

@@ -355,7 +355,9 @@ Our output is full of errors like
 ```
 What went wrong?
 
-??? warning "`There are not enough slots available in the system to satisfy...`"
+### Problem with MPI in Slurm job context
+
+!!! warning "`There are not enough slots available in the system to satisfy...`"
 
     If you see a different error which starts with `There are not enough slots available in the system to satisfy...`
     this is because you are trying to run an MPI job while already in a Slurm job context (perhaps an interactive
@@ -652,6 +654,20 @@ Total Test time (real) =   2.03 sec
 have explicitly verified that our package is using EESSI for all it's dependencies and is using the runtime loader
 provided by EESSI.
 
+!!! Warning "Problem with MPI in Slurm job context"
+
+    If the tests are still failing, it's probably due to errors that include:
+    ```
+    There are not enough slots available in the system to satisfy...
+    ```
+
+    OpenMPI needs to be configured to allow oversubscription, by running:
+    ``` { .bash .copy }
+    export PRTE_MCA_rmaps_default_mapping_policy=:oversubscribe
+    ```
+
+    See also this [subsection](#problem-with-mpi-in-slurm-job-context).
+
 ## Extending EESSI using `EESSI-extend` 
 
 While the `buildenv` may allow many applications to be built directly on top of EESSI, it cannot work miracles