Messages in this thread Patch in this message | | | Date | Fri, 29 Nov 2013 14:33:40 +0100 | From | Jiri Olsa <> | Subject | Re: [BUG] perf stat: explicit grouping yields unexpected results |
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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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