linux-kernel - [PATCH v1] perf evlist: Avoid frequency mode for the dummy event

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Date:   Fri, 15 Sep 2023 20:56:40 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Yang Jihong <yangjihong1@...wei.com>,
        Stephane Eranian <eranian@...gle.com>
Subject: [PATCH v1] perf evlist: Avoid frequency mode for the dummy event

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@...gle.com>
Signed-off-by: Ian Rogers <irogers@...gle.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