measuring and/or dealing with "idleness" and variable frequency

[Rick Jones <rick.jones2@hp.com>]

I am coming to "perf" (record -e cycles and report) from an old-time 
background where one could see the "idle routine" in a profile and know 
how "idle" (in terms of classic reporting a la top) a CPU was.  Back 
then the frequency was fixed, the idle loop was always "running" when 
the CPU was idle and there were no hardware threads.  Life was simple 
and good.

Now I have to realign to the present and am wondering where/how to get 
started.  I've looked at https://perf.wiki.kernel.org/index.php/Tutorial

which talks about:

> By default, perf record uses the cycles event as the sampling event.
> This is a generic hardware event that is mapped to a
> hardware-specific PMU event by the kernel. For Intel, it is mapped to
> UNHALTED_CORE_CYCLES. This event does not maintain a constant
> correlation to time in the presence of CPU frequency scaling. Intel
> provides another event, called UNHALTED_REFERENCE_CYCLES but this
> event is NOT currently available with perf_events.
>
> On AMD systems, the event is mapped to CPU_CLK_UNHALTED and this
> event is also subject to frequency scaling. On any Intel or AMD
> processor, the cycle event does not count when the processor is idle,
> i.e., when it calls mwait().

And I am wondering if those are all still true.

I am running a 3.5.0 kernel in an Ubuntu environment (3.5.0-23-generic 
#35~precise1-Ubuntu SMP), and among the things I wish to do is sanity 
check the Idle % being reported by top on one system which is passing 
traffic back and forth between two others.  Compared to the CPU 
utilization reported by top on those two systems, the reports for this 
one look extraordinarily low.

The CPU involved here is:

Intel(R) Xeon(R) CPU E5-2670 0 @ 2.60GHz

I am also interested in seeing how the CPU consumption changes as I 
tweak the setup of the middle box, does it go more or less idle which 
routines have their consumption change etc etc.  and I am wondering if I 
can get that all with just perf or if I have to correlate things between 
multiple tools.

thanks, and happy benchmarking,

rick jones