Re: [PATCH v2 4/5] perf record: Don't clear event's period if set by a term

[Ian Rogers]

On Tue, Aug 4, 2020 at 3:08 AM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
>
> On 28/07/20 11:57 am, Ian Rogers wrote:
> > If events in a group explicitly set a frequency or period with leader
> > sampling, don't disable the samples on those events.
> >
> > Prior to 5.8:
> > perf record -e '{cycles/period=12345000/,instructions/period=6789000/}:S'
>
> Might be worth explaining this use-case some more.
> Perhaps add it to the leader sampling documentation for perf-list.
>
> > would clear the attributes then apply the config terms. In commit
> > 5f34278867b7 leader sampling configuration was moved to after applying the
> > config terms, in the example, making the instructions' event have its period
> > cleared.
> > This change makes it so that sampling is only disabled if configuration
> > terms aren't present.
> >
> > Fixes: 5f34278867b7 ("perf evlist: Move leader-sampling configuration")
> > Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> > ---
> >  tools/perf/util/record.c | 28 ++++++++++++++++++++--------
> >  1 file changed, 20 insertions(+), 8 deletions(-)
> >
> > diff --git a/tools/perf/util/record.c b/tools/perf/util/record.c
> > index a4cc11592f6b..01d1c6c613f7 100644
> > --- a/tools/perf/util/record.c
> > +++ b/tools/perf/util/record.c
> > @@ -2,6 +2,7 @@
> >  #include "debug.h"
> >  #include "evlist.h"
> >  #include "evsel.h"
> > +#include "evsel_config.h"
> >  #include "parse-events.h"
> >  #include <errno.h>
> >  #include <limits.h>
> > @@ -38,6 +39,9 @@ static void evsel__config_leader_sampling(struct evsel *evsel, struct evlist *ev
> >       struct perf_event_attr *attr = &evsel->core.attr;
> >       struct evsel *leader = evsel->leader;
> >       struct evsel *read_sampler;
> > +     struct evsel_config_term *term;
> > +     struct list_head *config_terms = &evsel->config_terms;
> > +     int term_types, freq_mask;
> >
> >       if (!leader->sample_read)
> >               return;
> > @@ -47,16 +51,24 @@ static void evsel__config_leader_sampling(struct evsel *evsel, struct evlist *ev
> >       if (evsel == read_sampler)
> >               return;
> >
> > +     /* Determine the evsel's config term types. */
> > +     term_types = 0;
> > +     list_for_each_entry(term, config_terms, list) {
> > +             term_types |= 1 << term->type;
> > +     }
> >       /*
> > -      * Disable sampling for all group members other than the leader in
> > -      * case the leader 'leads' the sampling, except when the leader is an
> > -      * AUX area event, in which case the 2nd event in the group is the one
> > -      * that 'leads' the sampling.
> > +      * Disable sampling for all group members except those with explicit
> > +      * config terms or the leader. In the case of an AUX area event, the 2nd
> > +      * event in the group is the one that 'leads' the sampling.
> >        */
> > -     attr->freq           = 0;
> > -     attr->sample_freq    = 0;
> > -     attr->sample_period  = 0;
> > -     attr->write_backward = 0;
> > +     freq_mask = (1 << EVSEL__CONFIG_TERM_FREQ) | (1 << EVSEL__CONFIG_TERM_PERIOD);
> > +     if ((term_types & freq_mask) == 0) {
>
> It would be nicer to have a helper e.g.
>
>         if (!evsel__have_config_term(evsel, FREQ) &&
>             !evsel__have_config_term(evsel, PERIOD)) {

Sure. The point of doing it this way was to avoid repeatedly iterating
over the config term list.

> > +             attr->freq           = 0;
> > +             attr->sample_freq    = 0;
> > +             attr->sample_period  = 0;
>
> If we are not sampling, then maybe we should also put here:
>
>                 attr->write_backward = 0;
>
> > +     }
>
> Then, if we are sampling this evsel shouldn't the backward setting
> match the leader? e.g.
>
>         if (attr->sample_freq)
>                 attr->write_backward = leader->core.attr.write_backward;

Perhaps that should be a follow up change? This change is trying to
make the behavior match the previous behavior.

Thanks,
Ian

> > +     if ((term_types & (1 << EVSEL__CONFIG_TERM_OVERWRITE)) == 0)
> > +             attr->write_backward = 0;
> >
> >       /*
> >        * We don't get a sample for slave events, we make them when delivering
> >
>