Interrupt latency problems

[Bakul Shah]

Is it possible to cheaply check how long a mutex was held?
If it crosses some sysctl var set threshold you can increment
some count and/or drop into the debugger.  That is one way to
find all mutex slowpokes.  You can even find a distribution
by incrementing an array element.  Array indexed by heldtime
rounded down to multiples of 10ms or so.  Just an idea --
ignore it if it doesn't make sense!