Re: [BUG] perf stat: explicit grouping yields unexpected results

[Stephane Eranian]

On Fri, Nov 15, 2013 at 7:34 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Stephane Eranian <eranian@xxxxxxxxxx> wrote:
>
>> 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.
>
> Btw., does the kernel side currently support discovery of such
> impossible group scheduling constraints at group setup time? If not
> then it probably should and it should reject them straight away.
>
The kernel does validate events as they are added to a group.
That's why we have validate_event(), validate_group() and
the fake_cpuc mode.
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