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

[Athira Rajeev <atrajeev@linux.vnet.ibm.com>]

> On 21-Nov-2023, at 5:34 AM, Ian Rogers <irogers@google.com> 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@google.com>
> ---
> 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))) {

Hi Ian

Tested this with perf record and I could find the code fallback to using task-clock

./perf record -v ls
Warning:
The cycles event is not supported, trying to fall back to task-clock

But in case of “perf stat”, in my environment, found that the code path won't invoke “evsel__fallback”.

Snippet for builtin-stat.c
            if (errno == EINVAL || errno == ENOSYS ||
            errno == ENOENT || errno == EOPNOTSUPP ||
            errno == ENXIO) {
                if (verbose > 0)
                        ui__warning("%s event is not supported by the kernel.\n",
                                    evsel__name(counter));
                counter->supported = false;
                /*
                 * errored is a sticky flag that means one of the counter's
                 * cpu event had a problem and needs to be reexamined.
                 */
                counter->errored = true;

                if ((evsel__leader(counter) != counter) ||
                    !(counter->core.leader->nr_members > 1))
                        return COUNTER_SKIP;
        } else if (evsel__fallback(counter, &target, errno, msg, sizeof(msg))) {
                if (verbose > 0)
                        ui__warning("%s\n", msg);
                return COUNTER_RETRY;

So if the perf_event_open returns ENOENT, we won’t do a fallback in builtin-stat.c
Should we address cycles differently here ? Any comments ?

Thanks
Athira
>   
> 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
>