Re: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event

[Mingwei Zhang]

On Fri, Sep 15, 2023 at 9:10 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> Dummy events are created with an attribute where the period and freq
> are zero. evsel__config will then see the uninitialized values and
> initialize them in evsel__default_freq_period. As fequency mode is
> used by default the dummy event would be set to use frequency
> mode. However, this has no effect on the dummy event but does cause
> unnecessary timers/interrupts. Avoid this overhead by setting the
> period to 1 for dummy events.
>
> evlist__add_aux_dummy calls evlist__add_dummy then sets freq=0 and
> period=1. This isn't necessary after this change and so the setting is
> removed.
>
> From Stephane:
>
> The dummy event is not counting anything. It is used to collect mmap
> records and avoid a race condition during the synthesize mmap phase of
> perf record. As such, it should not cause any overhead during active
> profiling. Yet, it did. Because of a bug the dummy event was
> programmed as a sampling event in frequency mode. Events in that mode
> incur more kernel overheads because on timer tick, the kernel has to
> look at the number of samples for each event and potentially adjust
> the sampling period to achieve the desired frequency. The dummy event
> was therefore adding a frequency event to task and ctx contexts we may
> otherwise not have any, e.g., perf record -a -e
> cpu/event=0x3c,period=10000000/. On each timer tick the
> perf_adjust_freq_unthr_context() is invoked and if ctx->nr_freq is
> non-zero, then the kernel will loop over ALL the events of the context
> looking for frequency mode ones. In doing, so it locks the context,
> and enable/disable the PMU of each hw event. If all the events of the
> context are in period mode, the kernel will have to traverse the list for
> nothing incurring overhead. The overhead is multiplied by a very large
> factor when this happens in a guest kernel. There is no need for the
> dummy event to be in frequency mode, it does not count anything and
> therefore should not cause extra overhead for no reason.
>
> Fixes: 5bae0250237f ("perf evlist: Introduce perf_evlist__new_dummy constructor")
> Reported-by: Stephane Eranian <eranian@xxxxxxxxxx>
> Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> ---
>  tools/perf/util/evlist.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
> index 25c3ebe2c2f5..e36da58522ef 100644
> --- a/tools/perf/util/evlist.c
> +++ b/tools/perf/util/evlist.c
> @@ -251,6 +251,9 @@ static struct evsel *evlist__dummy_event(struct evlist *evlist)
>                 .type   = PERF_TYPE_SOFTWARE,
>                 .config = PERF_COUNT_SW_DUMMY,
>                 .size   = sizeof(attr), /* to capture ABI version */
> +               /* Avoid frequency mode for dummy events to avoid associated timers. */
> +               .freq = 0,
> +               .sample_period = 1,
>         };
>
>         return evsel__new_idx(&attr, evlist->core.nr_entries);
> @@ -277,8 +280,6 @@ struct evsel *evlist__add_aux_dummy(struct evlist *evlist, bool system_wide)
>         evsel->core.attr.exclude_kernel = 1;
>         evsel->core.attr.exclude_guest = 1;
>         evsel->core.attr.exclude_hv = 1;
> -       evsel->core.attr.freq = 0;
> -       evsel->core.attr.sample_period = 1;
>         evsel->core.system_wide = system_wide;
>         evsel->no_aux_samples = true;
>         evsel->name = strdup("dummy:u");
> --
> 2.42.0.459.ge4e396fd5e-goog
>

Thank you very much for the change. I have one quick question about
the PMU unthrottling logic. When I am looking into the function
perf_adjust_freq_unthr_context(), I see the loop with PMU stop and
start in each iteration. Is there a good way to avoid this PMU reset
operation while quickly figuring out the event in frequency mode?

Thanks.
-Mingwei