Have you tried adjusting the slack timer so it can avoid some wakeups?
When i say 'idle', i really mean something along the lines of, 'as idle as it is can be'. ie: even in an idle state, their is a lot of processing going on. So it's not so much overhead for RT 'specifically', this is just overhead of running certains programs, that will use cpu regardless of whether or not, you actually can hear what they are doing (or processing).
I haven't played around with /proc/<procid>/task/<task_id>/timer_slack_ns (if that is what you mean)
I've seen that cgroups can manage this stuff, but i haven't investigated it.
the best tunables that i have played with in the past, that are rt-related that yielded good results (but this was latency related) were these three tunables;
i had to run tests over and over until i got the right mix between them. (using standard rt-related tools, like Cyclitest). But these days, things run very smooth and i haven't felt the need to be as crazy about those sorts of tweaks, as my box has been working very well.
That's what I assumed you meant by idle, namely, programs that were still in active memory but not actively processing.
Assuming the programs aren't just terribly written, or need to do synchronous logging even when idle (I think FF still does this), that percentage is likely due to RT_PREEMPT (I'd guess timer setup/breakdown is the main culprit which is why I mentioned slack timer...BTW, cgroup management is going to be easier to do than per proc... https://lkml.org/lkml/2011/10/11/246). However, if your system is working good enough, I sure as hell wouldn't screw with it:) My tuning experience has been very hit or miss. The problem, of course, is defining the problem and creating tests. Unfortunately, for ordinary desktop usage (i.e., encompassing more than irq request delay), it is very hard to measure, and people don't even agree what we should be measuring. If we did, there would be no ambiguity about which scheduler is better for general desktop usage:)
BTW, I don't recall if I told you but I had to uninstall that RT kernel I got from the stanford repos. It was wreaking a wide swathe of destruction all through my desktop.