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

Fixes: 5bae0250237f ("perf evlist: Introduce perf_evlist__new_dummy constructor")
Reported-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Ian Rogers <irogers@google.com>
---
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
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