Re: [PATCH v5 4/6] perf cpumap: Handle dummy maps as empty in subset

[Ian Rogers <irogers@google.com>]

On Tue, May 3, 2022 at 12:43 AM Adrian Hunter <adrian.hunter@intel.com> wrote:
>
> On 3/05/22 07:17, Ian Rogers wrote:
> > perf_cpu_map__empty is true for empty and dummy maps. Make is_subset
> > respect that.
>
> As I wrote before, I am not keen on this because it prevents -1, as a
> valid 3rd parameter to perf_event_open(), from being represented
> in merged evsel cpu maps.
>
> Why do you want this?

Thanks Adrian, could you give me a test case (command line) where the
differing dummy and empty behavior matters? Normally cpus/own_cpus are
set to null during parsing. They may get replaced with
user_requested_cpus:
https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/lib/perf/evlist.c?h=perf/core#n44
https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/lib/perf/evlist.c?h=perf/core#n45
(should it be on line 45 that !empty is expected?)

During merge the null/empty all_cpus drops this value, which doesn't
matter as the behavior with empty is the same as dummy:
https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/lib/perf/evsel.c?h=perf/core#n119

What's concerning me is the definition of empty:
https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/lib/perf/cpumap.c?h=perf/core#n279
```
return map ? map->map[0].cpu == -1 : true;
```
If the first entry can be -1 and there can be other CPUs merged after
then that cpu map will be empty by the definition above. Perhaps it
should be:
```
return map ? (map->nr == 1 && map->map[0].cpu == -1) : true;
```
but it seems you prefer:
```
return (map == NULL) ? true : false;
```

You'd asked what the behavior with a dummy is and clearly it is
somewhat muddy. That is what this patch and unit test is trying to
clean up.

Thanks,
Ian

> >
> > Signed-off-by: Ian Rogers <irogers@google.com>
> > ---
> >  tools/lib/perf/cpumap.c   |  4 ++--
> >  tools/perf/tests/cpumap.c | 10 +++++++++-
> >  2 files changed, 11 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/lib/perf/cpumap.c b/tools/lib/perf/cpumap.c
> > index 384d5e076ee4..9c83675788c2 100644
> > --- a/tools/lib/perf/cpumap.c
> > +++ b/tools/lib/perf/cpumap.c
> > @@ -322,9 +322,9 @@ struct perf_cpu perf_cpu_map__max(struct perf_cpu_map *map)
> >  /** Is 'b' a subset of 'a'. */
> >  bool perf_cpu_map__is_subset(const struct perf_cpu_map *a, const struct perf_cpu_map *b)
> >  {
> > -     if (a == b || !b)
> > +     if (a == b || perf_cpu_map__empty(b))
> >               return true;
> > -     if (!a || b->nr > a->nr)
> > +     if (perf_cpu_map__empty(a) || b->nr > a->nr)
> >               return false;
> >
> >       for (int i = 0, j = 0; i < a->nr; i++) {
> > diff --git a/tools/perf/tests/cpumap.c b/tools/perf/tests/cpumap.c
> > index f94929ebb54b..d52b58395385 100644
> > --- a/tools/perf/tests/cpumap.c
> > +++ b/tools/perf/tests/cpumap.c
> > @@ -128,13 +128,21 @@ static int test__cpu_map_merge(struct test_suite *test __maybe_unused, int subte
> >       struct perf_cpu_map *a = perf_cpu_map__new("4,2,1");
> >       struct perf_cpu_map *b = perf_cpu_map__new("4,5,7");
> >       struct perf_cpu_map *c = perf_cpu_map__merge(a, b);
> > +     struct perf_cpu_map *d = perf_cpu_map__dummy_new();
> > +     struct perf_cpu_map *e = perf_cpu_map__merge(b, d);
> >       char buf[100];
> >
> >       TEST_ASSERT_VAL("failed to merge map: bad nr", perf_cpu_map__nr(c) == 5);
> >       cpu_map__snprint(c, buf, sizeof(buf));
> >       TEST_ASSERT_VAL("failed to merge map: bad result", !strcmp(buf, "1-2,4-5,7"));
> > -     perf_cpu_map__put(b);
> > +
> > +     TEST_ASSERT_VAL("failed to merge map: bad nr", perf_cpu_map__nr(e) == 3);
> > +     cpu_map__snprint(e, buf, sizeof(buf));
> > +     TEST_ASSERT_VAL("failed to merge map: bad result", !strcmp(buf, "4-5,7"));
> > +
> >       perf_cpu_map__put(c);
> > +     perf_cpu_map__put(d);
> > +     perf_cpu_map__put(e);
> >       return 0;
> >  }
> >
>