why does perf stat show “stalled-cycles-backend” as ?

Question

Running perf stat ls shows this:

Performance counter stats for \'ls\':

          1.388670 task-clock                #    0.067 CPUs utilized                


        

Answer 1

Just found Re: perf, x86: Add parts of the remaining haswell PMU functionality:

> AFAICS backend stall cycles are documented to work on Ivy Bridge.

I'm not aware of any documentation that presents these events
as accurate frontend/backend stalls without using the full
TopDown methology (Optimization manual B.3.2)

So IIUC stalled-cycles-backend counters are too unreliable on Ivy Bridge, and that's why the kernel devs have decided to not support them.

And sure enough, Linux' perf_event_intel.c supports PERF_COUNT_HW_STALLED_CYCLES_BACKEND for Nehalem, Xeon E7 and SandyBridge, but not for IvyBridge. PERF_COUNT_HW_STALLED_CYCLES_FRONTEND is supported for IvyBridge, though.

So I guess there won't be a way to get this counter on my current CPU - either switch CPUs or use the full top-down methodology mentioned in the mail (and described here and here)