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

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Date:	Fri, 29 Nov 2013 14:33:40 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Stephane Eranian <eranian@...gle.com>,
	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 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.

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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