opreport error: No sample file found

Hi,

I try to use opreport but it fails because no sample file is created
when i run the operf command. i know operf works because the
default, cpu_clk_unhalted, works.

[root@localhost ~]# operf --event br_inst_retired:1000000  ./a.out
Hello World
operf: Profiler started

Profiling done.
[root@localhost ~]# opreport
Using /root/oprofile_data/samples/ for samples directory.
opreport error: No sample file found: If using opcontrol for profiling,
try running 'opcontrol --dump'; otherwise, specify a session containing
sample files.

[root@localhost ~]# operf   ./a.out
Hello World
operf: Profiler started

Profiling done.
[root@localhost ~]# opreport
Using /root/oprofile_data/samples/ for samples directory.
CPU: Intel Skylake microarchitecture, speed 3e+06 MHz (estimated)
Counted cpu_clk_unhalted events () with a unit mask of 0x00 (Core
cycles when at least one thread on the physical core is not in halt
state) count 100000
cpu_clk_unhalt...|
  samples|      %|
------------------
        4 100.000 a.out
    cpu_clk_unhalt...|
      samples|      %|
    ------------------
            3 75.0000 no-vmlinux
            1 25.0000 ld-2.17.so
[root@localhost ~]#

could you give me some pointers? thanks

yah