Re: [PATCH v2 1/5] perf record: Set PERF_RECORD_PERIOD if attr->freq is set.

[Ian Rogers]

On Wed, Jul 29, 2020 at 11:52 AM Arnaldo Carvalho de Melo
<acme@xxxxxxxxxx> wrote:
>
> Em Tue, Jul 28, 2020 at 01:57:30AM -0700, Ian Rogers escreveu:
> > From: David Sharp <dhsharp@xxxxxxxxxx>
> >
> > evsel__config() would only set PERF_RECORD_PERIOD if it set attr->freq
>
> There is no such thing as 'PERF_RECORD_PERIOD', its PERF_SAMPLE_PERIOD,
> also...
>
> > from perf record options. When it is set by libpfm events, it would not
> > get set. This changes evsel__config to see if attr->freq is set outside of
> > whether or not it changes attr->freq itself.
> >
> > Signed-off-by: David Sharp <dhsharp@xxxxxxxxxx>
> > Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> > ---
> >  tools/perf/util/evsel.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> > index ef802f6d40c1..811f538f7d77 100644
> > --- a/tools/perf/util/evsel.c
> > +++ b/tools/perf/util/evsel.c
> > @@ -979,13 +979,18 @@ void evsel__config(struct evsel *evsel, struct record_opts *opts,
> >       if (!attr->sample_period || (opts->user_freq != UINT_MAX ||
> >                                    opts->user_interval != ULLONG_MAX)) {
> >               if (opts->freq) {
> > -                     evsel__set_sample_bit(evsel, PERIOD);
> >                       attr->freq              = 1;
> >                       attr->sample_freq       = opts->freq;
> >               } else {
> >                       attr->sample_period = opts->default_interval;
> >               }
> >       }
> > +     /*
> > +      * If attr->freq was set (here or earlier), ask for period
> > +      * to be sampled.
> > +      */
> > +     if (attr->freq)
> > +             evsel__set_sample_bit(evsel, PERIOD);
>
> Why can't the libpfm code set opts?
>
> With this patch we will end up calling evsel__set_sample_bit(evsel,
> PERIOD) twice, which isn't a problem but looks strange.

Thanks Arnaldo! The case I was looking at was something like:
perf record --pfm-events cycles:freq=1000

For regular events this would be:
perf record -e cycles/freq=1000/

With libpfm4 events the perf_event_attr is set up (a public API in
linux/perf_event.h) and then parse_events__add_event is used (an
internal API) to make the evsel and this added to the evlist
(parse_libpfm_events_option). This is similar to the parse_events
function except rather than set up a perf_event_attr the regular
parsing sets up config terms that are then applied to evsel and attr
later in evsel__config, via evsel__apply_config_terms.

I think we can  update this change so that in pfm.c after
parse_events__add_event we do:
if (attr.freq)
  evsel__set_sample_bit(evsel, PERIOD);

This code could also be part of parse_events__add_event. I think the
intent in placing this code here was that it is close to the similar
evsel__apply_config_terms and setting of sample bits in the evsel. The
logic here is already dependent on reading the attr->sample_period.

I'm not sure I follow the double setting case - I think that is only
possible with a config term or with period_set (-P).

Thanks,
Ian


> - Arnaldo
>
> >
> >       if (opts->no_samples)
> >               attr->sample_freq = 0;
> > --
> > 2.28.0.163.g6104cc2f0b6-goog
> >
>
> --
>
> - Arnaldo