Our test is basically running nodetool repair on specific keyspaces (such as keyspace1) and the test is also triggering nodetool compact keyspace1 standard1 in the background.
And so it looks like running major compactions & repairs lead to that issue when using LCS.
Below is an excerpt from the thread dump (the rest is attached)
CPU usage is at 100%
ttop shows that the compaction thread consumes all the CPU