Messages in this thread | | | From | Namhyung Kim <> | Date | Fri, 17 Nov 2023 13:36:06 -0800 | Subject | Re: [RFC PATCH v1] perf evsel: Fallback to task-clock when not system wide |
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Hi Ian,
On Tue, Nov 14, 2023 at 1:13 PM Ian Rogers <irogers@google.com> wrote: > > 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.
I think they are almost the same and no meaningful difference. The cpu-clock event also runs only when the task is running if it's a per-task event.
> > Update a nearby comment and debug string for the change. > > Signed-off-by: Ian Rogers <irogers@google.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
I'm not sure you can use the system_wide flag for this. IIUC it's to override the target setting in some cases (e.g. a dummy event to track sideband events in all CPUs) and I think you need to check target__has_cpu() instead.
But as I said above, it won't make any difference in the output. Conceptually it'd be more natural to use task-clock event for per-task sessions though.
Thanks, Namhyung
> + ? 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 >
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