perf stat: Add --for-each-cgroup option

perf stat: Add --for-each-cgroup option

The --for-each-cgroup option is a syntax sugar to monitor large number
of cgroups easily.  Current command line requires to list all the events
and cgroups even if users want to monitor same events for each cgroup.
This patch addresses that usage by copying given events for each cgroup
on user's behalf.

For instance, if they want to monitor 6 events for 200 cgroups each they
should write 1200 event names (with -e) AND 1200 cgroup names (with -G)
on the command line.  But with this change, they can just specify 6
events and 200 cgroups with a new option.

A simpler example below: It wants to measure 3 events for 2 cgroups ('A'
and 'B').  The result is that total 6 events are counted like below.

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

   Performance counter stats for 'system wide':

              988.18 msec cpu-clock                 A #    0.987 CPUs utilized
       3,153,761,702      cycles                    A #    3.200 GHz                      (100.00%)
       8,067,769,847      instructions              A #    2.57  insn per cycle           (100.00%)
              982.71 msec cpu-clock                 B #    0.982 CPUs utilized
       3,136,093,298      cycles                    B #    3.182 GHz                      (99.99%)
       8,109,619,327      instructions              B #    2.58  insn per cycle           (99.99%)

         1.001228054 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: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20200924124455.336326-3-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>