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The -G/--cgroups option is to put sender and receiver in different
cgroups in order to measure cgroup context switch overheads.

Users need to make sure the cgroups exist and accessible.  The following
example should the effect of this change.  Please don't forget taskset
before the perf bench to measure cgroup switches properly.  Otherwise
each task would run on a different CPU and generate cgroup switches
regardless of this change.

  # perf stat -e context-switches,cgroup-switches \
  > taskset -c 0 perf bench sched pipe -l 10000 > /dev/null

   Performance counter stats for 'taskset -c 0 perf bench sched pipe -l 10000':

              20,001      context-switches
                   2      cgroup-switches

         0.053449651 seconds time elapsed

         0.011286000 seconds user
         0.041869000 seconds sys

  # perf stat -e context-switches,cgroup-switches \
  > taskset -c 0 perf bench sched pipe -l 10000 -G AAA,BBB > /dev/null

   Performance counter stats for 'taskset -c 0 perf bench sched pipe -l 10000 -G AAA,BBB':

              20,001      context-switches
              20,001      cgroup-switches

         0.052768627 seconds time elapsed

         0.006284000 seconds user
         0.046266000 seconds sys
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Link: https://lore.kernel.org/r/20231017202342.1353124-1-namhyung@kernel.org