Re: [PATCH v2] perf evsel: Don't set sample_regs_intr/sample_regs_user for dummy event

[Jiri Olsa <jolsa@redhat.com>]

On Wed, Jul 22, 2020 at 01:00:03PM +0800, Jin, Yao wrote:

SNIP

> > > 
> > > If we use -IXMM0, the attr>sample_regs_intr will be set with
> > > PERF_REG_EXTENDED_MASK bit.
> > > 
> > > It doesn't make sense to set attr->sample_regs_intr for a
> > > software dummy event.
> > > 
> > > This patch adds dummy event checking before setting
> > > attr->sample_regs_intr and attr->sample_regs_user.
> > > 
> > > After:
> > >    # ./perf record -e cycles:p -IXMM0 -a -- sleep 1
> > >    [ perf record: Woken up 1 times to write data ]
> > >    [ perf record: Captured and wrote 0.413 MB perf.data (45 samples) ]
> > > 
> > >   v2:
> > >   ---
> > >   Rebase to perf/core
> > > 
> > > Fixes: 0a892c1c9472 ("perf record: Add dummy event during system wide synthesis")
> > > Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
> > > ---
> > >   tools/perf/util/evsel.c | 6 ++++--
> > >   1 file changed, 4 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> > > index 9aa51a65593d..11794d3b7879 100644
> > > --- a/tools/perf/util/evsel.c
> > > +++ b/tools/perf/util/evsel.c
> > > @@ -1014,12 +1014,14 @@ void evsel__config(struct evsel *evsel, struct record_opts *opts,
> > >   	if (callchain && callchain->enabled && !evsel->no_aux_samples)
> > >   		evsel__config_callchain(evsel, opts, callchain);
> > > -	if (opts->sample_intr_regs && !evsel->no_aux_samples) {
> > > +	if (opts->sample_intr_regs && !evsel->no_aux_samples &&
> > > +	    !evsel__is_dummy_event(evsel)) {
> > 
> > hum, I thought it'd look something like this:
> > 
> >    if (opts->sample_intr_regs && (!evsel->no_aux_samples || !evsel__is_dummy_event(evsel))
> > 
> > but I'm not sure how no_aux_samples flag works exactly.. so it might be
> > correct.. just making sure ;-)
> > 
> > cc-ing Adrian
> > 
> > jirka
> > 
> > 
> 
> no_aux_samples is set to false by default and it's only set to true by pt, right?
> 
> So most of the time, !evsel->no_aux_samples is always true.
> 
> if (opts->sample_intr_regs && (!evsel->no_aux_samples || !evsel__is_dummy_event(evsel)) {
> 	attr->sample_regs_intr = opts->sample_intr_regs;
> 	evsel__set_sample_bit(evsel, REGS_INTR);
> }
> 
> So even if the evsel is dummy event, the condition check is true. :(
> 
> Or maybe I misunderstand anything?

I was just curious, because I did not follow the no_aux_samples
usage in detail.. so how about a case where:

   evsel->no_aux_samples == true and evsel__is_dummy_event(evsel) = false

then the original condition will be false for non dummy event

  (opts->sample_intr_regs && !evsel->no_aux_samples && !evsel__is_dummy_event(evsel))

is that ok?

jirka