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

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Date:	Fri, 15 Nov 2013 11:34:05 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Jiri Olsa <jolsa@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...e.hu" <mingo@...e.hu>, David Ahern <dsahern@...il.com>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Namhyung Kim <namhyung.kim@....com>
Subject: Re: [BUG] perf stat: explicit grouping yields unexpected results


* Stephane Eranian <eranian@...gle.com> wrote:

> On Fri, Nov 15, 2013 at 7:34 AM, Ingo Molnar <mingo@...nel.org> wrote:
> >
> > * Stephane Eranian <eranian@...gle.com> 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.

So the problem here isn't really that the kernel doesn't tell 
userspace about it, but that perf stat does not interpret the 
validation result properly.

That brings up an interesting question: what is better for users, if 
we schedule as many as we can and say 'not supported' to the rest 
(current behavior), or if we fail the whole group?

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.

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.

Thanks,

	Ingo
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