As of now it doesn't consider cgroups when collecting shadow stats and
metrics so counter values from different cgroups will be saved in a same
slot.  This resulted in incorrect numbers when those cgroups have
different workloads.

For example, let's look at the scenario below: cgroups A and C runs same
workload which burns a cpu while cgroup B runs a light workload.

  $ perf stat -a -e cycles,instructions --for-each-cgroup A,B,C  sleep 1

   Performance counter stats for 'system wide':

     3,958,116,522      cycles                A
     6,722,650,929      instructions          A #    2.53  insn per cycle
         1,132,741      cycles                B
           571,743      instructions          B #    0.00  insn per cycle
     4,007,799,935      cycles                C
     6,793,181,523      instructions          C #    2.56  insn per cycle

       1.001050869 seconds time elapsed

When I run 'perf stat' with single workload, it usually shows IPC around
1.7.  We can verify it (6,722,650,929.0 / 3,958,116,522 = 1.698) for cgroup A.

But in this case, since cgroups are ignored, cycles are averaged so it
used the lower value for IPC calculation and resulted in around 2.5.

  avg cycle: (3958116522 + 1132741 + 4007799935) / 3 = 2655683066
  IPC (A)  :  6722650929 / 2655683066 = 2.531
  IPC (B)  :      571743 / 2655683066 = 0.0002
  IPC (C)  :  6793181523 / 2655683066 = 2.557

We can simply compare cgroup pointers in the evsel and it'll be NULL
when cgroups are not specified.  With this patch, I can see correct
numbers like below:

  $ perf stat -a -e cycles,instructions --for-each-cgroup A,B,C  sleep 1

  Performance counter stats for 'system wide':

     4,171,051,687      cycles                A
     7,219,793,922      instructions          A #    1.73  insn per cycle
         1,051,189      cycles                B
           583,102      instructions          B #    0.55  insn per cycle
     4,171,124,710      cycles                C
     7,192,944,580      instructions          C #    1.72  insn per cycle

       1.007909814 seconds time elapsed

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20210115071139.257042-2-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>