Messages in this thread | | | Date | Fri, 29 Nov 2013 14:43:35 +0100 | Subject | Re: [BUG] perf stat: explicit grouping yields unexpected results | From | Stephane Eranian <> |
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On Fri, Nov 29, 2013 at 2:33 PM, Jiri Olsa <jolsa@redhat.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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