Re: incorrect perf counters

[riya khanna <riyakhanna1983 at gmail dot com>]

In tapsets.cxx, emit_perf_read_handler() emits
"_stp_perf_read(smp_processor_id(), i)" to read perf counters values.
However, for non-systemwide perf counters (i.e.  with stp->system_wide
= 0), "_stp_perf_init()" calls perf_event_create_kernel_counter() with
cpu = -1.

Does this mean that probe perf.xx.process("y").counter("z") will only
be read on current CPU? Because I see 0s being reported for hw
counters.

On Wed, Jul 27, 2016 at 7:10 PM, riya khanna <riyakhanna1983@gmail.com> wrote:
> Thanks!
>
> This is how I'm measuring perf counters delta (attached perf.stp).
> However, this may add unnecessary overhead.
>
> Additionally, perf.stp seems to work correctly when I attach to a
> process name with single PID. However, if I attach to a process name
> (e.g. apache httpd) that has multiple PIDs (e.g. apache workers, some
> spawned at runtime to serve requests), then counters reported are
> incorrect (sometimes 0). "perf stat" tool reports correct counters.
>
> I was wondering if "probe perf" collects per-cpu counters or across
> all CPUs? Is the perf state transferred/saved upon cpu/context switch?
> How can I accomplish this similar to what "perf stat" command line
> tool does?
>
> -Riya
>
> On Wed, Jul 27, 2016 at 11:59 AM, Frank Ch. Eigler <fche@redhat.com> wrote:
>>
>> riyakhanna1983 wrote:
>>
>>> Is is possible to enable/disable counters on a function entry/exit?
>>
>> Enabling/disabling is a relatively large amount of work.  If you just
>> want a *delta* between function entry/exit, you could accumulate that
>> thusly:
>>
>> stap -e '
>> global counts
>> probe perf.sw.cpu_clock.counter("foo") {}
>> probe process.function("*").return { counts <<< @perf("foo") - @entry(@perf("foo")) }
>> ' -c /bin/ls
>>
>> ... but actually it seems we're not implementing @entry(@perf(...))
>> correctly just now, so you may need to do this instead:
>>
>> global counts, zoo%
>> probe perf.sw.cpu_clock.counter("foo") {}
>> probe process.function("main").call { zoo[tid()] = @perf("foo") }
>> probe process.function("main").return { counts <<< @perf("foo") - zoo[tid()] }
>> ' -c /bin/ls
>>
>> - FChE