linux-kernel - [RFC PATCH v1] perf evsel: Fallback to task-clock when not system wide

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Date:   Tue, 14 Nov 2023 13:12:54 -0800
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>,
        Yang Jihong <yangjihong1@...wei.com>,
        Leo Yan <leo.yan@...aro.org>,
        Sandipan Das <sandipan.das@....com>,
        Ravi Bangoria <ravi.bangoria@....com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ajay Kaher <akaher@...are.com>,
        Alexey Makhalov <amakhalov@...are.com>
Subject: [RFC PATCH v1] perf evsel: Fallback to task-clock when not system wide

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.

Update a nearby comment and debug string for the change.

Signed-off-by: Ian Rogers <irogers@...gle.com>
---
 tools/perf/util/evsel.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index a5da74e3a517..e1175313e4bc 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -2861,18 +2861,19 @@ 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 = evsel->core.system_wide
+			? 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",
+			evsel->core.system_wide ? "cpu-clock" : "task-clock");
 
 		zfree(&evsel->name);
 		return true;
-- 
2.43.0.rc0.421.g78406f8d94-goog