This patch fixes#1306, by reducing the pinned instances of
PerfCounters.
The issue is caused by creating multiple pinned events in the
same thread, doing so results in the Snapshot(PerfCounterValues* values)
failing, and that's now discoverable.
Creating multile pinned events is an unsupported behavior currently.
The error would be detected at read() time, not
perf_event_open() / iotcl() time.
The unsupported benavior above is confirmed by Stephane Eranian @seranian,
and he also pointed the dectection method.
Finished this patch under the guidance of Mircea Trofin @mtrofin.
* Support optional, user-directed collection of performance counters
The patch allows an engineer wishing to drill into the root causes
of a regression, for example. Currently, only single threaded runs
are supported. The feature is a build-time opt in, and then a runtime
opt in.
The engineer may run the benchmark executable, passing a list of
performance counter names (using libpfm's naming scheme) at the
command line. The counter values will then be collected and reported
back as UserCounters.
This is different from #240 in that it is a benchmark user opt-in, and
the counter collection is transparent to the benchmark.
Currently, this is only supported on platforms where libpfm is
supported.
libpfm: http://perfmon2.sourceforge.net/
* 'Use' values param in Snapshot when BENCHMARK_OS_WINDOWS
This is to avoid unused parameter warning-as-error
* Added missing include for <vector> in perf_counters.cc
* Moved doc to docs
* Added license blurbs
