Messages in this thread | | | Date | Fri, 29 Nov 2013 14:52:31 +0100 | From | Jiri Olsa <> | Subject | Re: [BUG] perf stat: explicit grouping yields unexpected results |
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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@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
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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