Adding 'S' event/group modifier to specify that the event value/s are
read by PERF_SAMPLE_READ sample type processing, instead of the period
value offered by lower layers.

There's additional behaviour change for 'S' modifier being specified on
event group:

Currently all the events within a group makes samples. If user now
specifies 'S' within group modifier, only the leader will trigger
samples. The rest of events in the group will have sampling disabled.

And same as for single events, values of all events within the group
(including leader) are read by PERF_SAMPLE_READ sample type processing.

Following example will create event group with cycles and cache-misses
events, setting the cycles as group leader and the only event to
actually sample. Both cycles and cache-misses event period values are
read by PERF_SAMPLE_READ sample type processing with PERF_FORMAT_GROUP
read format.

Example:

  $ perf record -e '{cycles,cache-misses}:S' ls
  ...
  $ perf report --group --show-total-period --stdio
  ...
  # Samples: 36  of event 'anon group { cycles, cache-misses }'
  # Event count (approx.): 12585593
  #
  #       Overhead          Period  Command      Shared Object                      Symbol
  # ..............  ..............  .......  .................  ..........................
  #
    19.92%   1.20%  2505936     31       ls  [kernel.kallsyms]  [k] mark_held_locks
    13.74%   0.47%  1729327     12       ls  [kernel.kallsyms]  [k] sched_clock_local
    13.64%  23.72%  1716147    612       ls  ld-2.14.90.so      [.] check_match.10805
    13.12%  23.22%  1650778    599       ls  libc-2.14.90.so    [.] _nl_intern_locale_data
    11.24%  29.19%  1414554    753       ls  [kernel.kallsyms]  [k] sched_clock_cpu
     8.50%   0.35%  1070150      9       ls  [kernel.kallsyms]  [k] check_chain_key
  ...
Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-iyoinu3axi11mymwnh2b7fxj@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>