Re: [PATCH v2 5/7] perf evlist: Skip dummy event sample_type check for evlist_config

[Yang Jihong]

Hello,

On 2023/7/17 22:41, Adrian Hunter wrote:
> On 15/07/23 06:29, Yang Jihong wrote:
> > The dummp event does not contain sampls data. Therefore, sample_type does
> > not need to be checked.
> >
> > Currently, the sample id format of the actual sampling event may be changed
> > after the dummy event is added.
> >
> > Signed-off-by: Yang Jihong <yangjihong1@xxxxxxxxxx>
> > ---
> >   tools/perf/util/record.c | 7 +++++++
> >   1 file changed, 7 insertions(+)
> >
> > diff --git a/tools/perf/util/record.c b/tools/perf/util/record.c
> > index 9eb5c6a08999..0240be3b340f 100644
> > --- a/tools/perf/util/record.c
> > +++ b/tools/perf/util/record.c
> > @@ -128,6 +128,13 @@ void evlist__config(struct evlist *evlist, struct record_opts *opts, struct call
> >   		evlist__for_each_entry(evlist, evsel) {
> >   			if (evsel->core.attr.sample_type == first->core.attr.sample_type)
> >   				continue;
> > +
> > +			/*
> > +			 * Skip the sample_type check for the dummy event
> > +			 * because it does not have any samples anyway.
> > +			 */
> > +			if (evsel__is_dummy_event(evsel))
> > +				continue;
>
> Sideband event records have "ID samples" so the sample type still matters.
Okay, will remove this patch in next version.

Can I ask a little more about this?

Use PERF_SAMPLE_IDENTIFICATION instead of PERF_SAMPLE_ID because for 
samples of type PERF_RECORD_SAMPLE, there may be different record 
formats due to different *sample_type* settings, so the fixed SAMPLE_ID 
 location mode PERF_SAMPLE_NAME is required here.

However, for the sideband event, the samples of the PERF_RECORD_SAMPLE 
type is not recorded (only PERF_RECORD_MMAP, PERF_RECORD_COMM, and so 
on). Therefore, the "use sample identifier "check can be skipped here.

That's my understanding of PERF_SAMPLE_IDENTIFICATION . If there is any 
error, please help to correct it.

*Sideband event records have "ID samples" so the sample type still matters.*

Does this mean that sideband will also record samples of type 
PERF_RECORD_SAMPLE? What exactly is the sampling data?

Thanks,
Yang