Threads used for 653 and posix partitions are minimum budget slack consumers that are allowed to run for the duration of the windows they execute in. To achieve this the thread will receive 2 timer interrupts: The first one to transition the minimum fixed budget to slack, at the point all available slack at the thread's rate is granted (i.e. all timeline slack). The second one interrupt occurs when the granted slack is exhausted at which point the thread will receive additional slack which is basically slack generated because system overheads must be conservative. 
Ideally both interrupts could be avoided and these threads would be granted sufficient budget up front that they do not get interrupted until the end of their window.