latency histogram with BPF

[Daniel Wagner]

Hi Steven and Tom,

I was playing a bit with BPF to see if it would be possible to
get a preempt off latency histogram. On paper this looks like
doable but adding kprobes on trace_preempt_[on|off] is not a
clever idea. Who would have thought? :)

In both cases BPF or based on Tom's 'hist' triggers' patches, there is
some trickery necessary to get it working. While the first approach
has more flexibility what you want to measure or how you want to
present it, I suspect it will be harder to get it working/accepted. 

Anyway, here is some code to laugh at.