Re: [RFC PATCH v2] perf evsel: Fallback to task-clock when not system wide

[Ian Rogers]

On Mon, Nov 20, 2023 at 4:04 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> When the cycles event isn't available evsel will fallback to the
> cpu-clock software event. task-clock is similar to cpu-clock but only
> runs when the process is running. Falling back to cpu-clock when not
> system wide leads to confusion, by falling back to task-clock it is
> hoped the confusion is less.
>
> Pass the target to determine if task-clock is more appropriate. Update
> a nearby comment and debug string for the change.
>
> ---
> v2. Use target__has_cpu as suggested by Namhyung.
> https://lpc.events/event/17/contributions/1556/
>
> Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>

Ping.

Thanks,
Ian

> ---
>  tools/perf/builtin-record.c |  2 +-
>  tools/perf/builtin-stat.c   |  2 +-
>  tools/perf/builtin-top.c    |  2 +-
>  tools/perf/util/evsel.c     | 18 ++++++++++--------
>  tools/perf/util/evsel.h     |  3 ++-
>  5 files changed, 15 insertions(+), 12 deletions(-)
>
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index 8ec818568662..d8bb59511fdd 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -1350,7 +1350,7 @@ static int record__open(struct record *rec)
>         evlist__for_each_entry(evlist, pos) {
>  try_again:
>                 if (evsel__open(pos, pos->core.cpus, pos->core.threads) < 0) {
> -                       if (evsel__fallback(pos, errno, msg, sizeof(msg))) {
> +                       if (evsel__fallback(pos, &opts->target, errno, msg, sizeof(msg))) {
>                                 if (verbose > 0)
>                                         ui__warning("%s\n", msg);
>                                 goto try_again;
> diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
> index a3af805a1d57..d8e5d6f7a87a 100644
> --- a/tools/perf/builtin-stat.c
> +++ b/tools/perf/builtin-stat.c
> @@ -653,7 +653,7 @@ static enum counter_recovery stat_handle_error(struct evsel *counter)
>                 if ((evsel__leader(counter) != counter) ||
>                     !(counter->core.leader->nr_members > 1))
>                         return COUNTER_SKIP;
> -       } else if (evsel__fallback(counter, errno, msg, sizeof(msg))) {
> +       } else if (evsel__fallback(counter, &target, errno, msg, sizeof(msg))) {
>                 if (verbose > 0)
>                         ui__warning("%s\n", msg);
>                 return COUNTER_RETRY;
> diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
> index ea8c7eca5eee..1e42bd1c7d5a 100644
> --- a/tools/perf/builtin-top.c
> +++ b/tools/perf/builtin-top.c
> @@ -1044,7 +1044,7 @@ static int perf_top__start_counters(struct perf_top *top)
>                             perf_top_overwrite_fallback(top, counter))
>                                 goto try_again;
>
> -                       if (evsel__fallback(counter, errno, msg, sizeof(msg))) {
> +                       if (evsel__fallback(counter, &opts->target, errno, msg, sizeof(msg))) {
>                                 if (verbose > 0)
>                                         ui__warning("%s\n", msg);
>                                 goto try_again;
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index a5da74e3a517..532f34d9fcb5 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -2853,7 +2853,8 @@ u64 evsel__intval_common(struct evsel *evsel, struct perf_sample *sample, const
>
>  #endif
>
> -bool evsel__fallback(struct evsel *evsel, int err, char *msg, size_t msgsize)
> +bool evsel__fallback(struct evsel *evsel, struct target *target, int err,
> +                    char *msg, size_t msgsize)
>  {
>         int paranoid;
>
> @@ -2861,18 +2862,19 @@ bool evsel__fallback(struct evsel *evsel, int err, char *msg, size_t msgsize)
>             evsel->core.attr.type   == PERF_TYPE_HARDWARE &&
>             evsel->core.attr.config == PERF_COUNT_HW_CPU_CYCLES) {
>                 /*
> -                * If it's cycles then fall back to hrtimer based
> -                * cpu-clock-tick sw counter, which is always available even if
> -                * no PMU support.
> +                * If it's cycles then fall back to hrtimer based cpu-clock sw
> +                * counter, which is always available even if no PMU support.
>                  *
>                  * PPC returns ENXIO until 2.6.37 (behavior changed with commit
>                  * b0a873e).
>                  */
> -               scnprintf(msg, msgsize, "%s",
> -"The cycles event is not supported, trying to fall back to cpu-clock-ticks");
> -
>                 evsel->core.attr.type   = PERF_TYPE_SOFTWARE;
> -               evsel->core.attr.config = PERF_COUNT_SW_CPU_CLOCK;
> +               evsel->core.attr.config = target__has_cpu(target)
> +                       ? PERF_COUNT_SW_CPU_CLOCK
> +                       : PERF_COUNT_SW_TASK_CLOCK;
> +               scnprintf(msg, msgsize,
> +                       "The cycles event is not supported, trying to fall back to %s",
> +                       target__has_cpu(target) ? "cpu-clock" : "task-clock");
>
>                 zfree(&evsel->name);
>                 return true;
> diff --git a/tools/perf/util/evsel.h b/tools/perf/util/evsel.h
> index f19ac9f027ef..efbb6e848287 100644
> --- a/tools/perf/util/evsel.h
> +++ b/tools/perf/util/evsel.h
> @@ -460,7 +460,8 @@ static inline bool evsel__is_clock(const struct evsel *evsel)
>                evsel__match(evsel, SOFTWARE, SW_TASK_CLOCK);
>  }
>
> -bool evsel__fallback(struct evsel *evsel, int err, char *msg, size_t msgsize);
> +bool evsel__fallback(struct evsel *evsel, struct target *target, int err,
> +                    char *msg, size_t msgsize);
>  int evsel__open_strerror(struct evsel *evsel, struct target *target,
>                          int err, char *msg, size_t size);
>
> --
> 2.43.0.rc1.413.gea7ed67945-goog
>