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

[Namhyung Kim <namhyung@kernel.org>]

Hello,

On Fri, Jul 19, 2024 at 2:50 AM Michael Petlan <mpetlan@redhat.com> wrote:
>
> 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?

Sometimes, it depends on the system activity.
I assumed there would be some system operation running
on the given CPUs at any time.  If not, it can fail..

Thanks,
Namhyung