perf test fail :: "perf stat --bpf-counters --for-each-cgroup test"

[Michael Petlan <mpetlan@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 1069 bytes --]

Hello Namhyung,

we were investigating some test failures of the testcase mentioned
in $subj. We have narrowed it down to:

    # perf stat -C 0,1 --for-each-cgroup system.slice,user.slice -e cycles -- taskset -c 1 perf test -w thloop
    
    Performance counter stats for 'CPU(s) 0,1':
         <not counted>      cycles                           system.slice
         3,020,401,084      cycles                           user.slice                       
    
         1.009787097 seconds time elapsed

As seen, the system.slice is not counted properly in our case. It
happens even without bpf-counters being involved.

There were rumours that it might be caused due to too small system
load, but it apparently happens even when the load was replaced by
"thloop" workload from perf-test's workload library. However, even
so, if the load was insufficient, we'd see a value – 0 instead of
"not counted". The "<not counted>" result is printed if the counter
wasn't properly enabled and running.

Have you encountered this problem? What could cause it?

Thanks.
Michael