Thanks. That kernel is new enough to have the patch for the infamous Linux kernel futex bug detailed here: https://groups.google.com/d/topic/mechanical-sympathy/QbmpZxp6C64
To answer your questions above:
What you're seeing is likely just normal behavior for Cassandra and is an artifact of its staged event driven architecture (SEDA). You can read more about that if you follow the links in the post above. There is work to move from SEDA to a thread-per-core (TPC) architecture, which you can read about in https://issues.apache.org/jira/browse/CASSANDRA-10989
There are a number of tuning parameters you can tune to adjust the number of threads working on a few of the various stages within Cassandra (e.g. memtable_flush_writers, native_transport_max_threads, and max_hints_delivery_threads).
There will of course be performance impacts for tuning these parameters and the right values will depend on your data model, hardware, and workload (among other things).