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Date: Wed, 27 Oct 2021 10:58:36 +0200
From: Jiri Olsa <jolsa@...hat.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
LKML <linux-kernel@...r.kernel.org>,
Andi Kleen <ak@...ux.intel.com>,
Ian Rogers <irogers@...gle.com>,
Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v2] perf evsel: Fix missing exclude_{host,guest} setting
On Tue, Oct 26, 2021 at 12:30:42PM -0700, Namhyung Kim wrote:
> Hi Jiri,
>
> On Tue, Oct 26, 2021 at 11:32 AM Jiri Olsa <jolsa@...hat.com> wrote:
> >
> > On Mon, Oct 25, 2021 at 09:33:51PM -0700, Namhyung Kim wrote:
> > > The current logic for the perf missing feature has a bug that it can
> > > wrongly clear some modifiers like G or H. Actually some PMUs don't
> > > support any filtering or exclusion while others do. But we check it
> > > as a global feature.
> > >
> > > For example, the cycles event can have 'G' modifier to enable it only
> > > in the guest mode on x86. When you don't run any VMs it'll return 0.
> > >
> > > # perf stat -a -e cycles:G sleep 1
> > >
> > > Performance counter stats for 'system wide':
> > >
> > > 0 cycles:G
> > >
> > > 1.000721670 seconds time elapsed
> > >
> > > But when it's used with other pmu events that don't support G modifier,
> > > it'll be reset and return non-zero values.
> > >
> > > # perf stat -a -e cycles:G,msr/tsc/ sleep 1
> > >
> > > Performance counter stats for 'system wide':
> > >
> > > 538,029,960 cycles:G
> > > 16,924,010,738 msr/tsc/
> > >
> > > 1.001815327 seconds time elapsed
> > >
> > > This is because of the missing feature detection logic being global.
> > > Add a hashmap to set pmu-specific exclude_host/guest features.
> > >
> > > Reported-by: Stephane Eranian <eranian@...gle.com>
> > > Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> > > ---
> > > v2 changes)
> > > * change to enum perf_missing_pmu_features
> > > * pass NULL to hashmap__find() to skip checking
> > > * add a blank line after declaration
> > >
> > > tools/perf/util/evsel.c | 47 +++++++++++++++++++++++++++++++++++------
> > > tools/perf/util/evsel.h | 7 ++++++
> > > 2 files changed, 48 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> > > index dbfeceb2546c..f33801b4615d 100644
> > > --- a/tools/perf/util/evsel.c
> > > +++ b/tools/perf/util/evsel.c
> > > @@ -1434,6 +1434,10 @@ void evsel__delete(struct evsel *evsel)
> > > {
> > > evsel__exit(evsel);
> > > free(evsel);
> > > +
> > > + /* just free it for the first evsel */
> > > + hashmap__free(perf_missing_features.pmu);
> > > + perf_missing_features.pmu = NULL;
> >
> > should this just remove evsel from perf_missing_features.pmu?
>
> Nop, it'll release all the hash nodes (pmu). Each evsel can come
> from a same PMU.
>
> >
> > I'm not sure when we should actually release this.. what's your rationale
> > for releasing it in here?
>
> Yeah, that's a problem when we should release it. Maybe we can
> add a function and call it in the exit path but I didn't want to change
> all the callsites just for this.
yea, I think we probably need it in other places as well,
but that should be separate patchset I think
>
> As evsel__delete() is called in the exit path, I thought it's a
> place where we can free the hashmap. But I'm not very happy
> with that either.. Do you have a better idea?
nope ;-)
>
> If another evsel is open after this function, I think it should
> be fine since it will create a new hash map and check the
> missing feature again.
I think let's keep it as it is for now and we'll see
thanks,
jirka
>
>
> >
> > > }
> > >
> > > void evsel__compute_deltas(struct evsel *evsel, int cpu, int thread,
> > > @@ -1791,6 +1795,23 @@ static int __evsel__prepare_open(struct evsel *evsel, struct perf_cpu_map *cpus,
> > > return 0;
> > > }
> > >
> > > +#define PMU_HASH_BITS 4
> > > +
> > > +static size_t pmu_hash(const void *key, void *ctx __maybe_unused)
> > > +{
> > > + const struct evsel *evsel = key;
> > > +
> > > + return hash_bits(evsel->core.attr.type, PMU_HASH_BITS);
> > > +}
> > > +
> > > +static bool pmu_equal(const void *key1, const void *key2, void *ctx __maybe_unused)
> > > +{
> > > + const struct evsel *a = key1;
> > > + const struct evsel *b = key2;
> > > +
> > > + return a->core.attr.type == b->core.attr.type;
> > > +}
> > > +
> > > static void evsel__disable_missing_features(struct evsel *evsel)
> > > {
> > > if (perf_missing_features.weight_struct) {
> > > @@ -1807,8 +1828,13 @@ static void evsel__disable_missing_features(struct evsel *evsel)
> > > evsel->open_flags &= ~(unsigned long)PERF_FLAG_FD_CLOEXEC;
> > > if (perf_missing_features.mmap2)
> > > evsel->core.attr.mmap2 = 0;
> > > - if (perf_missing_features.exclude_guest)
> > > - evsel->core.attr.exclude_guest = evsel->core.attr.exclude_host = 0;
> > > + if (perf_missing_features.exclude_guest) {
> > > + /* we only have EXCLUDE_GUEST bit, let's skip checking */
> > > + if (hashmap__find(perf_missing_features.pmu, evsel, NULL)) {
> > > + evsel->core.attr.exclude_guest = 0;
> > > + evsel->core.attr.exclude_host = 0;
> > > + }
> > > + }
> > > if (perf_missing_features.lbr_flags)
> > > evsel->core.attr.branch_sample_type &= ~(PERF_SAMPLE_BRANCH_NO_FLAGS |
> > > PERF_SAMPLE_BRANCH_NO_CYCLES);
> > > @@ -1840,6 +1866,9 @@ int evsel__prepare_open(struct evsel *evsel, struct perf_cpu_map *cpus,
> > >
> > > bool evsel__detect_missing_features(struct evsel *evsel)
> > > {
> > > + if (perf_missing_features.pmu == NULL)
> > > + perf_missing_features.pmu = hashmap__new(pmu_hash, pmu_equal, NULL);
> >
> > missing allocation fail check
>
> Ok, will change.
>
> Thanks for your review,
> Namhyung
>
>
> >
> > > +
> > > /*
> > > * Must probe features in the order they were added to the
> > > * perf_event_attr interface.
> > > @@ -1900,10 +1929,16 @@ bool evsel__detect_missing_features(struct evsel *evsel)
> > > perf_missing_features.mmap2 = true;
> > > pr_debug2_peo("switching off mmap2\n");
> > > return true;
> > > - } else if (!perf_missing_features.exclude_guest &&
> > > - (evsel->core.attr.exclude_guest || evsel->core.attr.exclude_host)) {
> > > - perf_missing_features.exclude_guest = true;
> > > - pr_debug2_peo("switching off exclude_guest, exclude_host\n");
> > > + } else if ((evsel->core.attr.exclude_guest || evsel->core.attr.exclude_host) &&
> > > + !hashmap__find(perf_missing_features.pmu, evsel, NULL)) {
> > > + uintptr_t pmu_features = PERF_MISSING_PMU_EXCLUDE_GUEST;
> > > +
> > > + hashmap__add(perf_missing_features.pmu, evsel, (void *)pmu_features);
> > > +
> > > + if (!perf_missing_features.exclude_guest) {
> > > + perf_missing_features.exclude_guest = true;
> > > + pr_debug2_peo("switching off exclude_guest, exclude_host\n");
> > > + }
> > > return true;
> > > } else if (!perf_missing_features.sample_id_all) {
> > > perf_missing_features.sample_id_all = true;
> > > diff --git a/tools/perf/util/evsel.h b/tools/perf/util/evsel.h
> > > index 1f7edfa8568a..11b5ece19f0e 100644
> > > --- a/tools/perf/util/evsel.h
> > > +++ b/tools/perf/util/evsel.h
> > > @@ -172,6 +172,13 @@ struct perf_missing_features {
> > > bool data_page_size;
> > > bool code_page_size;
> > > bool weight_struct;
> > > +
> > > + /* contains enum perf_missing_pmu_features below */
> > > + struct hashmap *pmu;
> > > +};
> > > +
> > > +enum perf_missing_pmu_features {
> > > + PERF_MISSING_PMU_EXCLUDE_GUEST = 1UL << 0,
> > > };
> > >
> > > extern struct perf_missing_features perf_missing_features;
> > > --
> > > 2.33.0.1079.g6e70778dc9-goog
> > >
> >
>
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