[Bug 234325] pmcstat seems to be broken in sampling mode (at least on amd hardware)

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=234325

            Bug ID: 234325
           Summary: pmcstat seems to be broken in sampling mode (at least
                    on amd hardware)
           Product: Base System
           Version: 12.0-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: misc
          Assignee: bugs at FreeBSD.org
          Reporter: shamaz.mazum at gmail.com

Hello. I use FreeBSD 12.0-RELEASE on two machines (one with Ryzen 5 1600X
processor and the other with FX-6300).

Recently (presumable after upgrading to 12.0-RELEASE) pmcstat stopped working
in sample mode.

I run it as:
`pmcstat -P instructions -O test.out -n 65536 ./noisecpu` on FX-6300
or
`pmcstat -P ex_ret_instr -O test.out -n 65536 ./noisecpu` on Ryzen

and then I run `pmcstat -R test.out -g`

In both cases it creates directories (e.g. ex_ret_instr) which are either empty
or containing only kernel.gmon

`noisecpu` is computation heavy program, calculating value noise on a large
grid and works for ~8 seconds.

Conversion statistics usually looks like following:
#exec/elf 1
#samples/total 169
#samples/unclaimed 165
#callchain/dubious-frames 165

In counting mode (with -p argument) everything seems to be working. Can anybody
confirm this?

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