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

[Jin, Yao]

Hi Adrian,

Could you help to check if following condition will break PT?

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

Thanks
Jin Yao

On 7/23/2020 9:01 AM, Jin, Yao wrote:
> 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:
> > > > > ÂÂÂ # ./perf record -e cycles:p -IXMM0 -a -- sleep 1
> > > > > ÂÂÂ [ perf record: Woken up 1 times to write data ]
> > > > > ÂÂÂ [ perf record: Captured and wrote 0.413 MB perf.data (45 samples) ]
> > > > >
> > > > > ÂÂ v2:
> > > > > ÂÂ ---
> > > > > ÂÂ Rebase to perf/core
> > > > >
> > > > > Fixes: 0a892c1c9472 ("perf record: Add dummy event during system wide synthesis")
> > > > > Signed-off-by: Jin Yao <yao.jin@xxxxxxxxxxxxxxx>
> > > > > ---
> > > > > ÂÂ tools/perf/util/evsel.c | 6 ++++--
> > > > > ÂÂ 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,
> > > > > ÂÂÂÂÂÂ if (callchain && callchain->enabled && !evsel->no_aux_samples)
> > > > > ÂÂÂÂÂÂÂÂÂÂ evsel__config_callchain(evsel, opts, callchain);
> > > > > -ÂÂÂ if (opts->sample_intr_regs && !evsel->no_aux_samples) {
> > > > > +ÂÂÂ if (opts->sample_intr_regs && !evsel->no_aux_samples &&
> > > > > +ÂÂÂÂÂÂÂ !evsel__is_dummy_event(evsel)) {
> > > >
> > > > hum, I thought it'd look something like this:
> > > >
> > > > ÂÂÂ 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)) {
> > > ÂÂÂÂattr->sample_regs_intr = opts->sample_intr_regs;
> > > ÂÂÂÂevsel__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:
> >
> > ÂÂÂ evsel->no_aux_samples == true and evsel__is_dummy_event(evsel) = false
> >
> > then the original condition will be false for non dummy event
> >
> > ÂÂ (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