Re: [PATCH 1/2] perf tools: Add reference timestamp to perf header

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Dec 13, 2010 at 02:48:54PM -0200, Arnaldo Carvalho de Melo wrote:
> Em Mon, Dec 13, 2010 at 04:54:56PM +0100, Frederic Weisbecker escreveu:
> > On Mon, Dec 13, 2010 at 07:39:24AM -0700, David S. Ahern wrote:
> > > On 12/12/10 13:16, Frederic Weisbecker wrote:
> > > >>  int perf_session__create_kernel_maps(struct perf_session *self)
> > > >>  {
> > > >>  	int ret = machine__create_kernel_maps(&self->host_machine);
> > > >> @@ -167,6 +190,9 @@ struct perf_session *perf_session__new(const char *filename, int mode, bool forc
> > > >>  		 */
> > > >>  		if (perf_session__create_kernel_maps(self) < 0)
> > > >>  			goto out_delete;
> > > >> +
> > > >> +		if (perf_session__create_ref_time(self) < 0)
> > > >> +			goto out_delete;
> 
> > > > So, it does record it anytime?
> 
> > > Simplest to always add it the header as it does no harm if not used.
> > > To make it conditional means adding an input parameter to
> > > perf_session__new() and then updating all the callers.
> 
> > > Is the preference to make it conditional?
> 
> > No, I suspect the overhead is plain unnoticeable. And if it becomes
> > any problem one day, we can still make that conditional later.
> 
> Right, but it just ocurred to me, can't we encode this in the file stat?
> Also, how do we deal with:
> 
> $ perf record --help
> <SNIP>
>        -A, --append
>            Append to the output file to do incremental profiling.
> <SNIP>
> 
> ?
> 
> I now think this should be implemented as a PERF_RECORD_WALL_CLOCK
> synthesized event, to be inserted just before we start collecting the
> other events.
> 
> That way, when --appending, we just insert another and use that from
> that point on.
> 
> Look at tools/perf/util/event.h, enum perf_user_event_type and look how
> those are synthesized.
> 
> Thanks,
> 
> - Arnaldo

(Adding peterz)

Right, but Peter wanted us to get rid of these user events types.
I guess we can't really do this as new perf tools must be able
to support old perf files.

So this should be the last one to add. Something like PERF_RECORD_GEN_EVT
that has a field in its headers containing a sub-type which can be this
wall clock but can also host about everything in the future.

This way we don't propagate more the possible overlap with the kernel.

Hm?