This commit adds support for a new modifier "D", which requests that the
event, or group of events, be pinned to the PMU.

The "p" modifier is already taken for precise, and "P" may be used in
future to mean "fully precise".

So we use "D", which stands for pinneD - and looks like a padlock, or if
you're using the ":D" syntax perf smiles at you.

This is an oft-requested feature from our HW folks, who want to be able
to run a large number of events, but also want 100% accurate results for
instructions per cycle.

Comparison of results with and without pinning:

$ perf stat -e '{cycles,instructions}:D' -e cycles,instructions,...

  79,590,480,683 cycles         #  0.000 GHz
 166,123,716,524 instructions   #  2.09  insns per cycle
                                #  0.11  stalled cycles per insn

  79,352,134,463 cycles         #  0.000 GHz                     [11.11%]
 165,178,301,818 instructions   #  2.08  insns per cycle
                                #  0.11  stalled cycles per insn [11.13%]

As you can see although perf does a very good job of scaling the values
in the non-pinned case, there is some small discrepancy.

The patch is fairly straight forward, the one detail is that we need to
make sure we only request pinning for the group leader when we have a
group.
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Tested-by: Jiri Olsa <jolsa@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1375795686-4226-1-git-send-email-michael@ellerman.id.au
[ Use perf_evsel__is_group_leader instead of open coded equivalent, as
  suggested by Jiri Olsa ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>