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

[Ian Rogers]

On Wed, May 4, 2022 at 6:59 AM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> On Wed, May 4, 2022 at 5:54 AM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
> >
> > On 3/05/22 17:03, Ian Rogers wrote:
> > > On Tue, May 3, 2022 at 12:43 AM Adrian Hunter <adrian.hunter@xxxxxxxxx> 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?
> >
> > perf record --per-thread -e intel_pt// uname
> >
> > With patchset "perf intel-pt: Better support for perf record --cpu"
> > the above will have (assuming 8-CPUs):
> >         user_requested_cpus = {-1}
> >         intel_pt evsel->cpus = {-1}
> >         text_poke dummy evsel->cpus = {0-7}
> > which when merged would result in:
> >         before this patch: all_cpus = {-1-7}
> >         after this patch:  all_cpus = {0-7}
> >
> > The absence of -1 will mean that the intel_pt event does not get
> > mmapped.
>
> Thanks, so what's the right fix? To make this work we should:
>  - remove language of dummy being a singular cpu_map
>  - change perf_cpu_map__empty to be something like
> perf_cpu_map__empty_or_just_dummy
>  - change cpu_map__is_dummy to be something llike cpu_map__singular_dummy
>  - add tests on cpu map code for things like the evlist affiinity
> iterator as I'm not clear what will happen when it encounters a -1 CPU
> Note, I'm proposing changing function names rather than implementation
> behavior, as we don't have enough tests to give me confidence that
> changing the existing behavior wouldn't break something. For example,
> --per-thread mode was recently broken:
> https://lore.kernel.org/lkml/e1ce0d93-88cc-af79-e67e-d3c79d166ca6@xxxxxxxxx/
> Do we also need to fix up parse events for software events (e.g.
> faults) where the cpu map is empty but really should be dummy? This
> will likely need a propagate fix as during the parsing propagation
> user_requested_cpus is empty and we want to keep dummy cpu maps, not
> overwrite them with empty.
>
> I see a fair amount of clean up here, which isn't bad. My assumed
> alternative was that the intel_pt code could look for dummy cpu maps
> on the evsels, but then why have dummy cpu maps at all and just use
> empty throughout the code base? We could also add a flag to the evlist
> to say whether any evsel cpu maps contain a dummy/empty map. API wise
> I'm tempted to say that the dummy CPU map should be removed and empty
> just used in its place (less is more, keep it simple).
>
> Something that would help with clarity, I think, would be to land the fix in:
> https://lore.kernel.org/lkml/20220503041757.2365696-3-irogers@xxxxxxxxxx/
> as currently all_cpus contains cpus not in the evsel cpu maps, but are
> residue from when the evsels were parsed.

Just to think out loud on this a bit more. If we imagine the CPU in
the CPU map is going to be consumed by perf_event_open then the
definition of CPU there is:
cpu == -1: Any CPU
cpu >= 0: Specified CPU
A CPU map of {-1-7} would mean any CPU and CPUs 0-7, which for an
evsel would be something of a contradiction. We also have the
contradiction that no CPUs is turned into -1, which means any CPU.
This leads me to think:
 - let's rename dummy to any CPU, this is more intention revealing and
may avoid confusion with the dummy event;
 - let's make empty really mean empty, no specified or any CPU values.
If an evsel is opened, .. with an empty CPU map it is a failure (this
will need parse-event fixes, etc.);
 - let's allow {-1-7} as a CPU map, ie allow any with specified CPUS,
and document that it comes about from merging CPU maps;
 - let's try to push the cleaned up API into libperf and get rid of
leftover bits in tools/perf/util/cpumap.h.

In terms of scheduling I think we can land what's in this series
except this patch and the rename of all_cpus in patch 6/6. This avoids
merge conflicts to more easily land the intel_pt --cpu fixes in:
https://lore.kernel.org/lkml/20220422162402.147958-1-adrian.hunter@xxxxxxxxx/
With that landed then I can do the function rename and cleanup work.
Things like making sure the evlist affinity iterator works an any CPU
in the CPU map.

What do you think?

Thanks,
Ian

> Thanks,
> Ian
>
> > >                                             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@xxxxxxxxxx>
> > >>> ---
> > >>>  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;
> > >>>  }
> > >>>
> > >>
> >