perf cgroup: Simplify arguments when tracking multiple events

perf cgroup: Simplify arguments when tracking multiple events

When using -G with one cgroup and -e with multiple events, only the
first event gets the correct cgroup setting, all events from the second
onwards will track system-wide events.

If the user wants to track multiple events for a specific cgroup, the
user must give parameters like the following:

  $ perf stat -e e1 -e e2 -e e3 -G test,test,test

This patch simplify this case, just type one cgroup:

  $ perf stat -e e1 -e e2 -e e3 -G test

  $ mkdir -p /sys/fs/cgroup/perf_event/empty_cgroup
  $ perf stat -e cycles -e cache-misses -a -I 1000 -G empty_cgroup

Before:

     1.001007226   <not counted>      cycles	   empty_cgroup
     1.001007226           7,506      cache-misses

After:

     1.000834097   <not counted>      cycles	   empty_cgroup
     1.000834097   <not counted>      cache-misses empty_cgroup

Signed-off-by: weiping zhang <zhangweiping@didichuxing.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180129154805.GA6284@localhost.didichuxing.com
[ Improved the doc text a bit, providing an example for cgroup + system wide counting ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>