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

[Ian Rogers <irogers@google.com>]

On Tue, Aug 4, 2020 at 8:50 AM Ian Rogers <irogers@google.com> wrote:
>
> On Tue, Aug 4, 2020 at 7:49 AM Adrian Hunter <adrian.hunter@intel.com> wrote:
> >
> > On 4/08/20 4:33 pm, Ian Rogers wrote:
> > > On Tue, Aug 4, 2020 at 3:08 AM Adrian Hunter <adrian.hunter@intel.com> 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@google.com>
> > >>> ---
> > >>>  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.
> >
> > But perhaps it is premature optimization
>
> The alternative is more loc. I think we can bike shed on this but it's
> not really changing the substance of the change. I'm keen to try to be
> efficient where we can as we see issues at scale.
>
> Thanks,
> Ian

Ping. Do we want to turn this into multiple O(N) searches using a
helper rather than 1 as coded here?

Thanks,
Ian

> > >
> > >>> +             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.
> >
> > Sure
> >
> > >
> > > 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
> > >>>
> > >>
> >