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

[Jin, Yao]

Hi Jiri, Adrian,

On 7/22/2020 7:08 PM, Jiri Olsa wrote:
> 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@xxxxxxxxxxxxxxx>
> > > > ---
> > > >    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?

I searched the perf source and found the no_aux_samples was only set to true in intel-pt.c. So I 
assume for the non-pt usage, the no_aux_samples is always false.

For non-pt usage,
(opts->sample_intr_regs && !evsel->no_aux_samples && !evsel__is_dummy_event(evsel)) is equal to
(opts->sample_intr_regs && !evsel__is_dummy_event(evsel))

For pt usage, we need to consider the case that evsel__is_dummy_event(evsel) is true or false.

If evsel__is_dummy_event(evsel) is true:
(opts->sample_intr_regs && !evsel->no_aux_samples && !evsel__is_dummy_event(evsel)) is false.
It's expected.

If evsel__is_dummy_event(evsel) is false:
(opts->sample_intr_regs && !evsel->no_aux_samples && !evsel__is_dummy_event(evsel)) is equal to
(opts->sample_intr_regs && !evsel->no_aux_samples)
That's the current code logic.

So I think the condition "(opts->sample_intr_regs && !evsel->no_aux_samples && 
!evsel__is_dummy_event(evsel))" looks reasonable.

Adrian, please correct me if I'm wrong here.

Thanks
Jin Yao

> jirka