If the user doesn't specify any event, try the most precise "cycles"
available, i.e. start by "cycles:ppp" and go on removing "p" till it
works.

E.g.

  $ perf record usleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.017 MB perf.data (11 samples) ]
  $ perf evlist
  cycles:pp
  $ perf evlist -v
  cycles:pp: size: 112, { sample_period, sample_freq }: 4000, sample_type:
  IP|TID|TIME|PERIOD, disabled: 1, inherit: 1, mmap: 1, comm: 1, freq: 1,
  enable_on_exec: 1, task: 1, precise_ip: 2, sample_id_all: 1,
  exclude_guest: 1, mmap2: 1, comm_exec: 1
  $ grep 'model name' /proc/cpuinfo | head -1
  model name	: Intel(R) Core(TM) i7-3667U CPU @ 2.00GHz
  $

When 'cycles' appears explicitely is specified this will not be tried,
i.e. the user has full control of the level of precision to be used:

  $ perf record -e cycles usleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.016 MB perf.data (9 samples) ]
  $ perf evlist
  cycles
  $ perf evlist -v
  cycles: size: 112, { sample_period, sample_freq }: 4000, sample_type:
  IP|TID|TIME|PERIOD, disabled: 1, inherit: 1, mmap: 1, comm: 1, freq: 1,
  enable_on_exec: 1, task: 1, sample_id_all: 1, exclude_guest: 1, mmap2:
  1, comm_exec: 1
  $

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Chandler Carruth <chandlerc@gmail.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://www.youtube.com/watch?v=nXaxk27zwlk
Link: http://lkml.kernel.org/n/tip-b1ywebmt22pi78vjxau01wth@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>