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

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Date:	Fri, 29 Nov 2013 14:43:35 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	Andi Kleen <ak@...ux.intel.com>, Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...e.hu" <mingo@...e.hu>, David Ahern <dsahern@...il.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Namhyung Kim <namhyung.kim@....com>
Subject: Re: [BUG] perf stat: explicit grouping yields unexpected results

On Fri, Nov 29, 2013 at 2:33 PM, Jiri Olsa <jolsa@...hat.com> wrote:
> On Sat, Nov 16, 2013 at 07:41:34PM -0800, Andi Kleen wrote:
>> > I'd say that the default behavior should be what Jiri implemented: get
>> > the most out of the situation and inform. But you are right in that
>> > 'forcing' all elements of a group to be valid should be possible as
>> > well - if a special perf stat option or event format is used.
>>
>> When something is multiplexed it can have a very
>> large measurement error. For workloads that fluctuate quite a bit, and the
>> fluctuations do not line up well with the multiplexing interval,
>> the default scaling does not give good results.
>>
>> So you expect to get good data, but you get very bad data.
>>
>> When collecting data for a large number of events it is important
>> to group them correctly, so that events that are directly dependent
>> on each other in equations are properly grouped.
>>
>> When explicit groups were added the user likely considered this
>> problem, so it's not good to silently override the choices.
>>
>> If a user doesn't care they can always not use groups.
>>
>> > Even in that second case it shouldn't say <unsupported> for everything
>> > in the result, but should deny the run immediately and return with an
>> > error, and should tell the user how many events in the group fit and
>> > which ones didn't.
>>
>> Returning this information would be great, but it would really
>> need an extended errno, or just a error string reported out.
>
> (sry for late reply, I was still ooo, and missed this conversation)
>
> I agree, when the last event fails sys_perf_event_open
> due to the validate_group check, we will get just EINVAL
>
> Was there any discussion about the error (or erorr string)
> propagation from sys_perf_event_open?
>
> Something like below? user space supply buffer for error string.
>
No. Why do you need kernel changes for that.
Perf gets the error, knows it is grouping and prints an appropriate
error message. Why do you need kernel for this?

> jirka
>
>
> ---
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index e1802d6..a827870 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -331,8 +331,8 @@ struct perf_event_attr {
>          */
>         __u32   sample_stack_user;
>
> -       /* Align to u64. */
> -       __u32   __reserved_2;
> +       __u32   errstr_size;
> +       char    *errstr;
>  };
>
>  #define perf_flags(attr)       (*(&(attr)->read_format + 1))
>
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