[Question] perf header: Why the HEADER_EVENT_DESC is needed?

[Question] perf header: Why the HEADER_EVENT_DESC is needed?

[Namhyung Kim]

Hi Stephane,

I wonder why we need to save the event descriptions in the feature area
of file header. Since the perf_event_attr, its ids and the event name
are already saved in the header, it seems a duplicate work to do that.
For printing the information, we might reuse those info or evlist
directly IMHO.

Am I missing something?

Thanks,
Namhyung

Re: [Question] perf header: Why the HEADER_EVENT_DESC is needed?

[Stephane Eranian]

On Sat, Jul 21, 2012 at 1:11 PM, Namhyung Kim <namhyung@kernel.org> wrote:
> Hi Stephane,
>
> I wonder why we need to save the event descriptions in the feature area
> of file header. Since the perf_event_attr, its ids and the event name
> are already saved in the header, it seems a duplicate work to do that.
> For printing the information, we might reuse those info or evlist
> directly IMHO.
>
> Am I missing something?

Unfortunately, I think you are.

There event names are not saved in header.c except indirectly by write_cmdline.
But more importantly, event_desc makes it possible to connect an event name as
passed on the cmdline with the corresponding attr struct.

Furthermore, it is also present in pipe mode, with the patch I posted
months ago.

Finally, you cannot arbitrarily remove features like this. We do have
tools that do
rely on the presence of EVENT_DESC.