Re: [PATCH] perf record: Add a dummy event for a hybrid system

[Namhyung Kim]

On Thu, Jul 8, 2021 at 9:05 AM <kan.liang@xxxxxxxxx> wrote:
>
> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
>
> Some symbols may not be resolved if a user only monitor one type of PMU.
>
>  $ sudo perf record -e cpu_atom/branch-instructions/ ./big_small_workload
>  $ sudo perf report –stdio
>  # Overhead  Command    Shared Object      Symbol
>  # ........  .........  .................
>  # ......................................
>  #
>     28.02%  perf-exec  [unknown]          [.] 0x0000000000401cf6
>     11.32%  perf-exec  [unknown]          [.] 0x0000000000401d04
>     10.90%  perf-exec  [unknown]          [.] 0x0000000000401d11
>     10.61%  perf-exec  [unknown]          [.] 0x0000000000401cfc
>
> To parse symbols, the side-band events, e.g., COMM, which are generated
> by the kernel are required. To decide whether to generate the side-band
> event, the kernel relies on the event_filter_match() to filter the
> unrelated events. On a hybrid system, the event_filter_match() further
> checks the CPU mask of the current enabled PMU. If an event is collected
> on the CPU which doesn't have an enabled PMU, it's treated as an
> unrelated event.
>
> The "big_small_workload" is created in a big core, but runs on a small
> core. The side-band events are filtered, because the user only monitors
> the PMU of the small core. The big core PMU is not enabled.
>
> For a hybrid system, a dummy event is required to generate the complete
> side-band events.
>
> Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>

Acked-by: Namhyung Kim <namhyung@xxxxxxxxxx>

Thanks,
Namhyung


> ---
>  tools/perf/builtin-record.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index 3337b5f..99607b9 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -891,11 +891,12 @@ static int record__open(struct record *rec)
>         int rc = 0;
>
>         /*
> -        * For initial_delay or system wide, we need to add a dummy event so
> -        * that we can track PERF_RECORD_MMAP to cover the delay of waiting or
> -        * event synthesis.
> +        * For initial_delay or system wide or a hybrid system, we need to
> +        * add a dummy event so that we can track PERF_RECORD_MMAP to cover
> +        * the delay of waiting or event synthesis.
>          */
> -       if (opts->initial_delay || target__has_cpu(&opts->target)) {
> +       if (opts->initial_delay || target__has_cpu(&opts->target) ||
> +           perf_pmu__has_hybrid()) {
>                 pos = evlist__get_tracking_event(evlist);
>                 if (!evsel__is_dummy_event(pos)) {
>                         /* Set up dummy event. */
> --
> 2.7.4
>