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SubjectRe: [PATCH 01/47] perf: Add PERF_EVENT_IOC_ID ioctl to return event ID
On Fri, Aug 09, 2013 at 10:50:27AM -0400, Vince Weaver wrote:
> On Wed, 7 Aug 2013, Arnaldo Carvalho de Melo wrote:
>
> > From: Jiri Olsa <jolsa@redhat.com>
> >
> > The only way to get the event ID is by reading the event fd,
> > followed by parsing the ID value out of the returned data.
> >
> > While this is ok for current read format used by perf tool,
> > it is not ok when we use PERF_FORMAT_GROUP format.
> >
> > With this format the data are returned for the whole group
> > and there's no way to find out what ID belongs to our fd
> > (if we are not group leader event).
>
> I'm a little confused by this.
>
> If you have an event group (with PERF_FORMAT_GROUP) and you specify
> PERF_FORMAT_ID in read_format, then when you read you get the id
> values for each sibling event along with their values, right?
>
> This is how PAPI does things. So I wouldn't say it's impossible to get
> the event IDs with PERF_FORMAT_GROUP events.
>
> Though maybe you're saying it's hard to map sibling fds to id values if
> you don't have the list of all fds in an event group in the order they
> were added. Which I guess may be true, but I'd think your analysis tool
> would have other problems if you didn't have that mapping.

right, the idea behind is not to be dependent
on how kernel chains events in the list

jirka


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