[BUG] perf stat: explicit grouping yields unexpected results

[Stephane Eranian]

Jiri,

I was trying the grouping support in perf stat and I was surprised
to see that if I create a group that is too big to be scheduled, and
where only N out of P events can fit, perf stat still yields counts
for the N events. I was expecting 0 counts or <not supported>.

The kernel semantic is to schedule all the events in a group or none.
Perf does something different and this is confusing. If you use explicit
grouping then I think you want to group to fail if not all the events can
be scheduled:

On an IvyBridge:
$ perf stat --g -e
'{cycles,instructions,branches,branches,branches,branches,branches}'
noploop 1
     3 229 417 079 cycles
     3 223 919 023 instructions              #    1,00  insns per cycle
     3 220 868 098 branches
     3 220 868 098 branches
     3 220 868 098 branches
     3 220 868 098 branches
   <not supported> branches

I think it should be: <not supported> for all events.
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