Do you know the exachk score and if there are unresolved issues flagged in Exachk.
Generally Exadata is pretty well tuned and kernel level tuning is not recommended.
For performance problems if you have diagnostics and tuning pack then study the AWR reports. You may implement IORM and other resource management options to prioritise critical processes. In most cases with Exadata its a case of bad code running just fast enough to reach production that would not have been able to run at all in a conventional system. To isolate the bad performers go for AWR or if you don't have the requisite licenses then statspack.