Where are the Spark logs on EMR?

If you SSH into the master node of your cluster then you should be able to find the stdout, stderr, syslog and controller logs under:

/mnt/var/log/hadoop/steps/<stepname>

The event logs, the ones required for the spark-history-server can be found at :

hdfs:///var/log/spark/apps

I also spent a lot of time figuring this out. Found logs in the following location: EMR UI Console -> Summary -> Log URI -> Containers -> application_xxx_xxx -> container_yyy_yy_yy -> stdout.gz.

With the deploy mode of cluster on yarn the Spark driver and hence the user code executed will be within the Application Master container. It sounds like you had EMR debugging enabled on the cluster so logs should have also pushed to S3. In the S3 location look at task-attempts/<applicationid>/<firstcontainer>/*.