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 2 context-switches # 0.001 M/sec 0 cpu-migrations # 0.000 K/sec 266 page-faults # 0.192 M/sec 3515391 cycles # 2.531 GHz 2096636 stalled-cycles-frontend # 59.64% frontend cycles idle <not supported> stalled-cycles-backend 2927468 instructions # 0.83 insns per cycle # 0.72 stalled cycles per insn 615636 branches # 443.328 M/sec 22172 branch-misses # 3.60% of all branches 0.020657192 seconds time elapsed ``` Why is stalled-cycles-backend shown as "not supported"? What kind of CPU, hardware, kernel or user-space software do I need to see this value? Currently tried this on RHEL with Linux 3.12 for x86_64, with matching `perf` version, on different Intel Core i5 and i7 systems (Ivy Bridge type). None of them support stalled-cycles-backend. Some more info: ``` $ perf list | grep stalled stalled-cycles-frontend OR idle-cycles-frontend [Hardware event] stalled-cycles-frontend…

Accepted Answer

Looks like `perf` has not been updated to understand all the performance monitoring events that Ivy Bridge supports. Fortunately there is a generic, albeit cumbersome, interface that allows you to access the full list of performance monitoring events. I didn't see `stalled-cycles-backend` in the list when I gave it a quick look, but maybe I missed, or maybe they have broken it down by all the different events that could stall the backend.

We start with

```
perf list --help
```

...shows the following NOTE

```
    1. Intel(R) 64 and IA-32 Architectures Software Developer's Manual
       Volume 3B: System Programming Guide
       http://www.intel.com/Assets/PDF/manual/253669.pdf
```

...armed with that URL you end up in

```
http://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-vol-3b-part-2-manual.pdf
```

...you want section 19.3

19.3 PERFORMANCE MONITORING EVENTS FOR 3RD GENERATION INTEL® CORE™ PROCESSORS 3rd generation Intel® Core™ processors and Intel Xeon processor E3-1200 v2 product family are based on Intel microarchitecture code name Ivy Bridge. They support architectural performance-monitoring events listed in Table 19-1. Non-architectural performance-monitoring events in the processor core are listed in Table 19-5. The events in Table 19-5 apply to processors with CPUID signature of DisplayFamily_DisplayModel encoding with the following values: 06_3AH.

...so for `architectural` events you need Table 19-1

19.1 ARCHITECTURAL PERFORMANCE-MONITORING EVENTS Architectural performance events are introduced in Intel Core Solo and Intel Core Duo processors. They are also supported on processors based on Intel Core microarchitecture. Table 19-1 lists pre-defined architectural performance events that can be configured using general-purpose performance counters and associated event-select registers.

**Table 19-1. Architectural Performance Events

... now comes the tricky part, you take the `UMask Value` as the upper 2 hex digits and the `Event Num` is the lower 2 hex digits of a 4 hex digit hardware register number to be given to `perf stat`.

```
perf stat --help
```

```
   -e, --event=
       Select the PMU event. Selection can be a symbolic event name (use
       perf list to list all events) or a raw PMU event (eventsel+umask) in
       the form of rNNN where NNN is a hexadecimal event descriptor.
```

... it says `NNN` but you can give it `NNNN`. Let's verify that this works, let's ask `perf stat` for cache-misses both as a symbolic event name and as a hex number from table 19-1. We'll invoke the `date` command for simplicity.

```
$ perf stat -e r412e -e cache-misses date

Fri Mar 28 09:28:52 CDT 2014

Performance counter stats for 'date':

2292 r412e                                                       
          2292 cache-misses

0.003322663 seconds time elapsed

$ 
```

As you can see both reported the same number, so far so good. Now we go to Table 19-5 for the non-architectural hardware registers, of which there are too many too list here, but I'll list a few:

why does perf stat show "stalled-cycles-backend" as <not supported>?

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