Re: [PATCH 3/5] perf cpumap: Add intersect function.

[Ian Rogers <irogers@google.com>]

On Mon, Mar 28, 2022 at 2:00 PM Arnaldo Carvalho de Melo
<arnaldo.melo@gmail.com> wrote:
>
>
>
> On March 28, 2022 5:54:06 PM GMT-03:00, Ian Rogers <irogers@google.com> wrote:
> >On Mon, Mar 28, 2022 at 1:28 PM Arnaldo Carvalho de Melo
> ><acme@kernel.org> wrote:
> >>
> >> Em Sun, Mar 27, 2022 at 11:24:12PM -0700, Ian Rogers escreveu:
> >> > The merge function gives the union of two cpu maps. Add an intersect
> >> > function which will be used in the next change.
> >> >
> >> > Signed-off-by: Ian Rogers <irogers@google.com>
> >> > ---
> >> >  tools/lib/perf/cpumap.c              | 38 ++++++++++++++++++++++++++++
> >> >  tools/lib/perf/include/perf/cpumap.h |  2 ++
> >> >  2 files changed, 40 insertions(+)
> >> >
> >> > diff --git a/tools/lib/perf/cpumap.c b/tools/lib/perf/cpumap.c
> >> > index 953bc50b0e41..56b4d213039f 100644
> >> > --- a/tools/lib/perf/cpumap.c
> >> > +++ b/tools/lib/perf/cpumap.c
> >> > @@ -393,3 +393,41 @@ struct perf_cpu_map *perf_cpu_map__merge(struct perf_cpu_map *orig,
> >> >       perf_cpu_map__put(orig);
> >> >       return merged;
> >> >  }
> >> > +
> >> > +struct perf_cpu_map *perf_cpu_map__intersect(struct perf_cpu_map *orig,
> >> > +                                          struct perf_cpu_map *other)
> >> > +{
> >> > +     struct perf_cpu *tmp_cpus;
> >> > +     int tmp_len;
> >> > +     int i, j, k;
> >> > +     struct perf_cpu_map *merged = NULL;
> >> > +
> >> > +     if (perf_cpu_map__is_subset(other, orig))
> >> > +             return orig;
> >> > +     if (perf_cpu_map__is_subset(orig, other)) {
> >> > +             perf_cpu_map__put(orig);
> >>
> >> Why this put(orig)?
> >
> >As with merge, if orig isn't returned then it is put.
>
> For merge I can see it dropping a reference, i.e. get b and merge it into a, after that b was "consumed"
>
> But for intersect?

The current use case is the intersect of all online CPUs with the
merge of all CPU maps from evsels. So we can generally just reuse
all_cpus, or the common case of both maps contain every CPU. I think
the pattern makes code like:

evlist->cpus = perf_cpu_map__intersect(evlist->cpus, other);

not quite as messy, as without the put you need:

tmp = perf_cpu_map__intersect(evlist->cpus, other);
perf_cpu_map__put(evlist->cpus);
evlist->cpus = tmp;

I'm somewhat agnostic on what the API should be, but it'd be nice if
merge and intersect behaved in a similar way.

Thanks,
Ian

> >
> >> > +             return perf_cpu_map__get(other);
> >>
> >> And why the get here and not on the first if?
> >
> >The first argument orig is either put or returned while the second may
> >be returned only if the reference count is incremented. We could
> >change the API for merge and intersect to put both arguments, or to
> >not put either argument.
> >
> >Thanks,
> >Ian
> >
> >> > +     }
> >> > +
> >> > +     tmp_len = max(orig->nr, other->nr);
> >> > +     tmp_cpus = malloc(tmp_len * sizeof(struct perf_cpu));
> >> > +     if (!tmp_cpus)
> >> > +             return NULL;
> >> > +
> >> > +     i = j = k = 0;
> >> > +     while (i < orig->nr && j < other->nr) {
> >> > +             if (orig->map[i].cpu < other->map[j].cpu)
> >> > +                     i++;
> >> > +             else if (orig->map[i].cpu > other->map[j].cpu)
> >> > +                     j++;
> >> > +             else {
> >> > +                     j++;
> >> > +                     tmp_cpus[k++] = orig->map[i++];
> >> > +             }
> >> > +     }
> >> > +     if (k)
> >> > +             merged = cpu_map__trim_new(k, tmp_cpus);
> >> > +     free(tmp_cpus);
> >> > +     perf_cpu_map__put(orig);
> >> > +     return merged;
> >> > +}
> >> > diff --git a/tools/lib/perf/include/perf/cpumap.h b/tools/lib/perf/include/perf/cpumap.h
> >> > index 4a2edbdb5e2b..a2a7216c0b78 100644
> >> > --- a/tools/lib/perf/include/perf/cpumap.h
> >> > +++ b/tools/lib/perf/include/perf/cpumap.h
> >> > @@ -19,6 +19,8 @@ LIBPERF_API struct perf_cpu_map *perf_cpu_map__read(FILE *file);
> >> >  LIBPERF_API struct perf_cpu_map *perf_cpu_map__get(struct perf_cpu_map *map);
> >> >  LIBPERF_API struct perf_cpu_map *perf_cpu_map__merge(struct perf_cpu_map *orig,
> >> >                                                    struct perf_cpu_map *other);
> >> > +LIBPERF_API struct perf_cpu_map *perf_cpu_map__intersect(struct perf_cpu_map *orig,
> >> > +                                                      struct perf_cpu_map *other);
> >> >  LIBPERF_API void perf_cpu_map__put(struct perf_cpu_map *map);
> >> >  LIBPERF_API struct perf_cpu perf_cpu_map__cpu(const struct perf_cpu_map *cpus, int idx);
> >> >  LIBPERF_API int perf_cpu_map__nr(const struct perf_cpu_map *cpus);
> >> > --
> >> > 2.35.1.1021.g381101b075-goog
> >>
> >> --
> >>
> >> - Arnaldo