perf stat: Output JSON MetricExpr metric

perf stat: Output JSON MetricExpr metric

Add generic infrastructure to perf stat to output ratios for
"MetricExpr" entries in the event lists. Many events are more useful as
ratios than in raw form, typically some count in relation to total
ticks.

Transfer the MetricExpr information from the alias to the evsel.

We mark the events that need to be collected for MetricExpr, and also
link the events using them with a pointer. The code is careful to always
prefer the right event in the same group to minimize multiplexing
errors. At the moment only a single relation is supported.

Then add a rblist to the stat shadow code that remembers stats based on
the cpu and context.

Then finally update and retrieve and print these values similarly to the
existing hardcoded perf metrics. We use the simple expression parser
added earlier to evaluate the expression.

Normally we just output the result without further commentary, but for
--metric-only this would lead to empty columns. So for this case use the
original event as description.

There is no attempt to automatically add the MetricExpr event, if it is
missing, however we suggest it to the user, because the user tool
doesn't have enough information to reliably construct a group that is
guaranteed to schedule. So we leave that to the user.

  % perf stat -a -I 1000 -e '{unc_p_clockticks,unc_p_freq_max_os_cycles}'
       1.000147889        800,085,181      unc_p_clockticks
       1.000147889         93,126,241      unc_p_freq_max_os_cycles  #     11.6
       2.000448381        800,218,217      unc_p_clockticks
       2.000448381        142,516,095      unc_p_freq_max_os_cycles  #     17.8
       3.000639852        800,243,057      unc_p_clockticks
       3.000639852        162,292,689      unc_p_freq_max_os_cycles  #     20.3

  % perf stat -a -I 1000 -e '{unc_p_clockticks,unc_p_freq_max_os_cycles}' --metric-only
  #    time         freq_max_os_cycles %
       1.000127077      0.9
       2.000301436      0.7
       3.000456379      0.0

v2: Change from DivideBy to MetricExpr
v3: Use expr__ prefix.  Support more than one other event.
v4: Update description
v5: Only print warning message once for multiple PMUs.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20170320201711.14142-11-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>