perf stat: Reduce min --interval-print to 10ms

perf stat: Reduce min --interval-print to 10ms

The --interval-print parameter was limited to 100ms. However, for
example, 10ms is required to do sophisticated bandwidth analysis using
uncore events.

The test shows that the overhead of the system-wide uncore monitoring
with 10ms interval is only ~2%. So this patch reduces the minimal
interval-print allowd to 10ms.

But 10ms may not work well for all cases. For example, when the
cpus/threads number is very large, for system-wide core event monitoring
the overhead could be high.

To handle this issue, a warning will be displayed when the
interval-print is set between 10ms to 100ms. So users can make a
decision according to their specific cases.

 # perf stat -e uncore_imc_1/cas_count_read/ -a --interval-print 10 -- sleep 1

 print interval < 100ms. The overhead percentage could be high in some
 cases. Please proceed with caution.
 #           time             counts unit events
      0.010200451               0.10 MiB  uncore_imc_1/cas_count_read/
      0.020475117               0.02 MiB  uncore_imc_1/cas_count_read/
      0.030692800               0.01 MiB  uncore_imc_1/cas_count_read/
      0.040948161               0.02 MiB  uncore_imc_1/cas_count_read/
      0.051159564               0.00 MiB  uncore_imc_1/cas_count_read/

Signed-off-by: Kan Liang <kan.liang@intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1443776674-42511-1-git-send-email-kan.liang@intel.com
[ Added warning about overhead when using sub 100ms intervals to the man page ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>