linux-kernel - Re: [PATCH v2 05/10] perf evsel: Allow const evsel for certain accesses

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Date:   Thu, 2 Mar 2023 17:40:30 -0800
From:   Ian Rogers <irogers@...gle.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Zhengjun Xing <zhengjun.xing@...ux.intel.com>,
        Ravi Bangoria <ravi.bangoria@....com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        "Steinar H. Gunderson" <sesse@...gle.com>,
        Kim Phillips <kim.phillips@....com>,
        Florian Fischer <florian.fischer@...q.space>,
        James Clark <james.clark@....com>,
        Suzuki Poulouse <suzuki.poulose@....com>,
        Sean Christopherson <seanjc@...gle.com>,
        Leo Yan <leo.yan@...aro.org>,
        John Garry <john.g.garry@...cle.com>,
        Kajol Jain <kjain@...ux.ibm.com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v2 05/10] perf evsel: Allow const evsel for certain accesses

On Thu, Mar 2, 2023 at 4:14 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> Hi Ian,
>
> On Thu, Mar 2, 2023 at 1:26 PM Ian Rogers <irogers@...gle.com> wrote:
> >
> > List sorting, added later to evlist, passes const elements requiring
> > helper functions to also be const. Make the argument to
> > evsel__find_pmu, evsel__is_aux_event and evsel__leader const.
> >
> > Signed-off-by: Ian Rogers <irogers@...gle.com>
> > ---
> >  tools/perf/util/evsel.c  | 2 +-
> >  tools/perf/util/evsel.h  | 6 +++---
> >  tools/perf/util/pmu.c    | 6 +++---
> >  tools/perf/util/python.c | 2 +-
> >  4 files changed, 8 insertions(+), 8 deletions(-)
> >
> > diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> > index 51e8ce6edddc..2dc2c24252bb 100644
> > --- a/tools/perf/util/evsel.c
> > +++ b/tools/perf/util/evsel.c
> > @@ -3139,7 +3139,7 @@ bool evsel__is_hybrid(const struct evsel *evsel)
> >         return evsel->pmu_name && perf_pmu__is_hybrid(evsel->pmu_name);
> >  }
> >
> > -struct evsel *evsel__leader(struct evsel *evsel)
> > +struct evsel *evsel__leader(const struct evsel *evsel)
>
> It seems like it should return const struct evsel *, right?

Yes, but that breaks things :-)

> >  {
> >         return container_of(evsel->core.leader, struct evsel, core);
> >  }
> > diff --git a/tools/perf/util/evsel.h b/tools/perf/util/evsel.h
> > index 814a49ebb7e3..676c499323e9 100644
> > --- a/tools/perf/util/evsel.h
> > +++ b/tools/perf/util/evsel.h
> > @@ -212,8 +212,8 @@ int evsel__object_config(size_t object_size,
> >                          int (*init)(struct evsel *evsel),
> >                          void (*fini)(struct evsel *evsel));
> >
> > -struct perf_pmu *evsel__find_pmu(struct evsel *evsel);
> > -bool evsel__is_aux_event(struct evsel *evsel);
> > +struct perf_pmu *evsel__find_pmu(const struct evsel *evsel);
> > +bool evsel__is_aux_event(const struct evsel *evsel);
> >
> >  struct evsel *evsel__new_idx(struct perf_event_attr *attr, int idx);
> >
> > @@ -505,7 +505,7 @@ int evsel__store_ids(struct evsel *evsel, struct evlist *evlist);
> >
> >  void evsel__zero_per_pkg(struct evsel *evsel);
> >  bool evsel__is_hybrid(const struct evsel *evsel);
> > -struct evsel *evsel__leader(struct evsel *evsel);
> > +struct evsel *evsel__leader(const struct evsel *evsel);
> >  bool evsel__has_leader(struct evsel *evsel, struct evsel *leader);
> >  bool evsel__is_leader(struct evsel *evsel);
> >  void evsel__set_leader(struct evsel *evsel, struct evsel *leader);
> > diff --git a/tools/perf/util/pmu.c b/tools/perf/util/pmu.c
> > index 43b6182d96b7..45d9b8e28e16 100644
> > --- a/tools/perf/util/pmu.c
> > +++ b/tools/perf/util/pmu.c
> > @@ -988,7 +988,7 @@ struct perf_pmu *perf_pmu__scan(struct perf_pmu *pmu)
> >         return NULL;
> >  }
> >
> > -struct perf_pmu *evsel__find_pmu(struct evsel *evsel)
> > +struct perf_pmu *evsel__find_pmu(const struct evsel *evsel)
> >  {
> >         struct perf_pmu *pmu = NULL;
> >
> > @@ -1000,11 +1000,11 @@ struct perf_pmu *evsel__find_pmu(struct evsel *evsel)
> >                         break;
> >         }
> >
> > -       evsel->pmu = pmu;
> > +       ((struct evsel *)evsel)->pmu = pmu;
>
> Maybe we can move it out of the function and make callers set it.

Right, I considered this a case where in C++ we'd use mutable so
casting away the const wasn't the worst offense.

Thanks,
Ian

> Thanks,
> Namhyung
>
>
> >         return pmu;
> >  }
> >
> > -bool evsel__is_aux_event(struct evsel *evsel)
> > +bool evsel__is_aux_event(const struct evsel *evsel)
> >  {
> >         struct perf_pmu *pmu = evsel__find_pmu(evsel);
> >
> > diff --git a/tools/perf/util/python.c b/tools/perf/util/python.c
> > index 42e8b813d010..ab48ffbb6448 100644
> > --- a/tools/perf/util/python.c
> > +++ b/tools/perf/util/python.c
> > @@ -83,7 +83,7 @@ const char *perf_env__arch(struct perf_env *env __maybe_unused)
> >   * far, for the perf python binding known usecases, revisit if this become
> >   * necessary.
> >   */
> > -struct perf_pmu *evsel__find_pmu(struct evsel *evsel __maybe_unused)
> > +struct perf_pmu *evsel__find_pmu(const struct evsel *evsel __maybe_unused)
> >  {
> >         return NULL;
> >  }
> > --
> > 2.40.0.rc0.216.gc4246ad0f0-goog
> >