Re: incorrect perf counters

[riya khanna <riyakhanna1983 at gmail dot com>]

Thanks! changing attr in runtime/linux/perf.c fixes the problem.

On Mon, Jul 25, 2016 at 5:08 PM, Josh Stone <jistone@redhat.com> wrote:
> On 07/25/2016 01:55 PM, riya khanna wrote:
>> Thanks!
>>
>> Is there a quick way to do that? Perhaps modify tapset? I make
>> necessary changes and test.
>
> For a quick hack to see if it gets values you like, you could tweak
> stp->attr in runtime/linux/perf.c, just before the calls to
> perf_event_create_kernel_counter().  You don't need to recompile
> systemtap for this, as runtime code is freshly compiled into the module
> generated for your script.
>
> A more permanent solution would affect how the .attr is written in
> tapset-perfmon.cxx, perf_derived_probe_group::emit_module_decls().
>
>> On Mon, Jul 25, 2016 at 3:45 PM, Josh Stone <jistone@redhat.com> wrote:
>>> On 07/25/2016 12:15 PM, riya khanna wrote:
>>>> Hi,
>>>>
>>>> I'm trying to collect perf counters using systemtap. I followed
>>>> systemtap/EXAMPLES/profiling/perf.stp to create a script to monitor a
>>>> user process, enable counters on function entry, and spew out counters
>>>> on function return. Please find the script (perf.stp) attached.
>>>>
>>>> I use the stap script described above to monitor function func()'s
>>>> entry and exit.
>>>>
>>>> void func(void)
>>>> {
>>>>     printf("Measuring instruction count for this printf\n");
>>>> }
>>>>
>>>> Just to verify systemtap reported counters, I created a program
>>>> (test.c) that uses perf_event_open() syscall to monitor func()'s
>>>> entry/exit and spew out counters. Please find it attached as well.
>>>>
>>>> However, the counters reported by perf.stp are very different from
>>>> what is reported by test.c. For example, instruction count reported by
>>>> test.c and perf.stp are 593 and 10610-10690.
>>>>
>>>> This is how I launch perf.stp to monitor func() in test.c:
>>>>
>>>> sudo /usr/local/bin/stap -s1 perf.stp test func
>>>>
>>>> Why such a difference? Am I missing something?
>>>
>>> I suspect it's because your calls set exclude_kernel and exclude_hv, and
>>> systemtap doesn't do this.
>>>
>>> Perhaps we should do this for the .process variant, or at least make it
>>> an option to count only userspace.
>>>
>