Re: [RFC PATCH v1] perf pmu: Assume sysfs events are always lowercase

[Ian Rogers]

On Thu, Apr 18, 2024 at 8:35 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
>
>
>
> On 2024-04-13 12:08 a.m., Ian Rogers wrote:
> > Perf event names aren't case sensitive. For sysfs events the entire
> > directory of events is read then iterated comparing names in a case
> > insensitive way, most often to see if an event is present.
> >
> > Consider:
> > $ perf stat -e inst_retired.any true
> >
> > The event inst_retired.any may be present in any PMU, so every PMU's
> > sysfs events are loaded and then searched with strcasecmp to see if
> > any match. This event is only present on the cpu PMU as a json event
> > so a lot of events were loaded from sysfs unnecessarily just to prove
> > an event didn't exist there.
> >
> > This change avoids loading all the events by assuming sysfs event
> > names are always lowercase.
>
> From what I searched in the kernel, it looks like all the sysfs event
> names (from different ARCHs and devices) are lowercase. It should not
> break the existing usages.
> I don't see a reason why a event name must be uppercase.
> However, I think we need to add something to guarantee the assumption.
>
> Could you please update the doc to describe the assumption?
> Documentation/ABI/testing/sysfs-bus-event_source-devices-events
> So everybody can follow the rule.
>
> I think a perf test is required as well.
> Maybe we can extend the test__pmu_events() to do the name check.

Great suggestions, I'll do a v2 with this. I've been thinking for a
while we need a PMU driver test. A particular problem is for weak
groups, where broken drivers don't return perf event open failures for
groups that exceed the number of counters. There's the enum value
MetricNoGroupEvents to workaround this problem in metrics, but it
would still be nice to get the drivers fixed. So rather than keep
growing the event parsing tests, I think adding it to the pmu test and
making the pmu test a fuller suite checking properties of PMUs would
be good.

Thanks,
Ian

> Thanks,
> Kan
>
> > It then uses file exists and only loads
> > the events when the desired event is present.
> >
> > For the example above, the number of openat calls measured by perf
> > trace on a tigerlake laptop goes from 325 down to 255. The reduction
> > will be larger for machines with many PMUs, particularly replicated
> > uncore PMUs.
> >
> > Make pmu_aliases_parse early return when aliases are loaded, ensure
> > the function is called before all uses of the aliases list.
> >
> > Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> > ---
> >  tools/perf/util/pmu.c | 20 +++++++++++++++-----
> >  1 file changed, 15 insertions(+), 5 deletions(-)
> >
> > diff --git a/tools/perf/util/pmu.c b/tools/perf/util/pmu.c
> > index ab30f22eaf10..ce72c99e4f61 100644
> > --- a/tools/perf/util/pmu.c
> > +++ b/tools/perf/util/pmu.c
> > @@ -425,9 +425,16 @@ static struct perf_pmu_alias *perf_pmu__find_alias(struct perf_pmu *pmu,
> >  {
> >       struct perf_pmu_alias *alias;
> >
> > -     if (load && !pmu->sysfs_aliases_loaded)
> > -             pmu_aliases_parse(pmu);
> > +     if (load && !pmu->sysfs_aliases_loaded) {
> > +             char event_file_name[FILENAME_MAX + 8];
> >
> > +             scnprintf(event_file_name, sizeof(event_file_name), "events/%s", name);
> > +             for (size_t i = 7, n = 7 + strlen(name); i < n; i++)
> > +                     event_file_name[i] = tolower(event_file_name[i]);
> > +
> > +             if (perf_pmu__file_exists(pmu, event_file_name))
> > +                     pmu_aliases_parse(pmu);
> > +     }
> >       list_for_each_entry(alias, &pmu->aliases, list) {
> >               if (!strcasecmp(alias->name, name))
> >                       return alias;
> > @@ -605,6 +612,9 @@ static int pmu_aliases_parse(struct perf_pmu *pmu)
> >       size_t len;
> >       int fd, dir_fd;
> >
> > +     if (pmu->sysfs_aliases_loaded)
> > +             return 0;
> > +
> >       len = perf_pmu__event_source_devices_scnprintf(path, sizeof(path));
> >       if (!len)
> >               return 0;
> > @@ -1689,9 +1699,7 @@ size_t perf_pmu__num_events(struct perf_pmu *pmu)
> >  {
> >       size_t nr;
> >
> > -     if (!pmu->sysfs_aliases_loaded)
> > -             pmu_aliases_parse(pmu);
> > -
> > +     pmu_aliases_parse(pmu);
> >       nr = pmu->sysfs_aliases;
> >
> >       if (pmu->cpu_aliases_added)
> > @@ -1750,6 +1758,7 @@ int perf_pmu__for_each_event(struct perf_pmu *pmu, bool skip_duplicate_pmus,
> >       struct strbuf sb;
> >
> >       strbuf_init(&sb, /*hint=*/ 0);
> > +     pmu_aliases_parse(pmu);
> >       pmu_add_cpu_aliases(pmu);
> >       list_for_each_entry(event, &pmu->aliases, list) {
> >               size_t buf_used;
> > @@ -2154,6 +2163,7 @@ const char *perf_pmu__name_from_config(struct perf_pmu *pmu, u64 config)
> >       if (!pmu)
> >               return NULL;
> >
> > +     pmu_aliases_parse(pmu);
> >       pmu_add_cpu_aliases(pmu);
> >       list_for_each_entry(event, &pmu->aliases, list) {
> >               struct perf_event_attr attr = {.config = 0,};