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

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Date:	Fri, 29 Nov 2013 14:52:31 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	Stephane Eranian <eranian@...gle.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 02:43:35PM +0100, Stephane Eranian wrote:
> 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

how does perf know it's grouping and not something else?

> error message. Why do you need kernel for this?

like how would you differentiate EINVAL from validate_group or say
from set_ext_hw_attr (got by using unsupported cache event) ?

jirka
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