Comment 9 for bug 764796

If perf is to work properly with the PMU counters we have to unrevert the revert of the PMU interrupt code commit. I have a commit to do this (to put the slightly refactored code back actually) queued up for the tilt repo.

This code was originally reverted due to some people seeing a hang on boot. If we are to turn this code back on we need to watch for this, but this needs to be done if perf is to be very useful.

Note that none of this addresses the oprofile counter issue (recorded elsewhere), which is ultimately unfixable in software.