Re: [PATCH 3/4] perf hist: Do not use event index in hpp__fmt()

[Ian Rogers <irogers@google.com>]

On Wed, Feb 14, 2024 at 9:26 PM Namhyung Kim <namhyung@kernel.org> wrote:
>
> On Wed, Feb 14, 2024 at 4:08 PM Ian Rogers <irogers@google.com> wrote:
> >
> > On Mon, Feb 12, 2024 at 11:52 PM Namhyung Kim <namhyung@kernel.org> wrote:
> > >
> > > The __hpp__fmt() is to print period values in a hist entry.  It handles
> > > event groups using linked pair entries.  Until now, it used event index
> > > to print values of group members.  But we want to make it more robust
> > > and support groups even if some members in the group were removed.
> >
> > I'm unclear how it breaks currently. The evsel idx is set the evlist
> > nr_entries on creation and not updated by a remove. A remove may
> > change a groups leader, should the remove also make the next entry's
> > index idx that of the previous group leader?
>
> The evsel__group_idx() returns evsel->idx - leader->idx.
> If it has a group event {A, B, C} then the index would be 0, 1, 2.
> If it removes B, the group would be {A, C} with index 0 and 2.
> The nr_members is 2 now so it cannot use index 2 for C.
>
> Note that we cannot change the index of C because some information
> like annotation histogram relies on the index.

Ugh, the whole index thing is just messy - perhaps these days we could
have a hashmap with the evsel as a key instead. I remember that I also
forced the idx here:
https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/parse-events.c?h=perf-tools-next#n2049
If it were invariant that the idx were always the position of an event
in the evlist then I think life would be easier, but that won't help
for the arrays of counters that need the index to be constant. I guess
this is why the previous work to do this skipped evsels rather than
removed them.

Thanks,
Ian


> >
> > > Let's use an index table from evsel to value array so that we can skip
> > > dummy events in the output later.
> > >
> > > Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> > > ---
> > >  tools/perf/ui/hist.c | 34 ++++++++++++++++++++++++++++------
> > >  1 file changed, 28 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/tools/perf/ui/hist.c b/tools/perf/ui/hist.c
> > > index 5f4c110d840f..9c4c738edde1 100644
> > > --- a/tools/perf/ui/hist.c
> > > +++ b/tools/perf/ui/hist.c
> > > @@ -48,15 +48,30 @@ static int __hpp__fmt(struct perf_hpp *hpp, struct hist_entry *he,
> > >         if (evsel__is_group_event(evsel)) {
> > >                 int idx;
> > >                 struct hist_entry *pair;
> > > -               int nr_members = evsel->core.nr_members;
> > > +               int nr_members = evsel->core.nr_members - 1;
> >
> > A comment on the -1 would be useful.
>
> The 'nr_members' includes the leader which is already printed.
> In this code, we want to print member events only, hence -1.
> I'll add it to the comment.
>
> Thanks,
> Namhyung
>
> >
> > Thanks,
> > Ian
> >
> >
> > >                 union {
> > >                         u64 period;
> > >                         double percent;
> > >                 } *val;
> > > +               struct evsel *member;
> > > +               struct evsel **idx_table;
> > >
> > >                 val = calloc(nr_members, sizeof(*val));
> > >                 if (val == NULL)
> > > -                       return 0;
> > > +                       goto out;
> > > +
> > > +               idx_table = calloc(nr_members, sizeof(*idx_table));
> > > +               if (idx_table == NULL)
> > > +                       goto out;
> > > +
> > > +               /*
> > > +                * Build an index table for each evsel to the val array.
> > > +                * It cannot use evsel->core.idx because removed events might
> > > +                * create a hole so the index is not consecutive anymore.
> > > +                */
> > > +               idx = 0;
> > > +               for_each_group_member(member, evsel)
> > > +                       idx_table[idx++] = member;
> > >
> > >                 /* collect values in the group members */
> > >                 list_for_each_entry(pair, &he->pairs.head, pairs.node) {
> > > @@ -66,8 +81,15 @@ static int __hpp__fmt(struct perf_hpp *hpp, struct hist_entry *he,
> > >                         if (!total)
> > >                                 continue;
> > >
> > > -                       evsel = hists_to_evsel(pair->hists);
> > > -                       idx = evsel__group_idx(evsel);
> > > +                       member = hists_to_evsel(pair->hists);
> > > +                       for (idx = 0; idx < nr_members; idx++) {
> > > +                               if (idx_table[idx] == member)
> > > +                                       break;
> > > +                       }
> > > +
> > > +                       /* this should not happen */
> > > +                       if (idx == nr_members)
> > > +                               continue;
> > >
> > >                         if (fmt_percent)
> > >                                 val[idx].percent = 100.0 * period / total;
> > > @@ -75,8 +97,7 @@ static int __hpp__fmt(struct perf_hpp *hpp, struct hist_entry *he,
> > >                                 val[idx].period = period;
> > >                 }
> > >
> > > -               /* idx starts from 1 to skip the leader event */
> > > -               for (idx = 1; idx < nr_members; idx++) {
> > > +               for (idx = 0; idx < nr_members; idx++) {
> > >                         if (fmt_percent) {
> > >                                 ret += hpp__call_print_fn(hpp, print_fn,
> > >                                                           fmt, len, val[idx].percent);
> > > @@ -89,6 +110,7 @@ static int __hpp__fmt(struct perf_hpp *hpp, struct hist_entry *he,
> > >                 free(val);
> > >         }
> > >
> > > +out:
> > >         /*
> > >          * Restore original buf and size as it's where caller expects
> > >          * the result will be saved.
> > > --
> > > 2.43.0.687.g38aa6559b0-goog
> > >