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Date: Fri, 25 Aug 2023 10:20:54 -0300
From: Arnaldo Carvalho de Melo <acme@...nel.org>
To: Ian Rogers <irogers@...gle.com>
Cc: "liwei (GF)" <liwei391@...wei.com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...nel.org>,
Namhyung Kim <namhyung@...nel.org>,
Adrian Hunter <adrian.hunter@...el.com>,
Kan Liang <kan.liang@...ux.intel.com>,
Sean Christopherson <seanjc@...gle.com>,
K Prateek Nayak <kprateek.nayak@....com>,
linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] perf header: Fix missing PMU caps
Em Thu, Aug 24, 2023 at 03:54:00PM -0700, Ian Rogers escreveu:
> On Mon, Aug 21, 2023 at 6:16 AM Arnaldo Carvalho de Melo
> <acme@...nel.org> wrote:
> >
> > Em Sat, Aug 19, 2023 at 12:16:09PM +0800, liwei (GF) escreveu:
> > > Hi Ian:
> > >
> > > On 2023/8/19 1:19, Ian Rogers wrote:
> > > > PMU caps are written as HEADER_PMU_CAPS or for the special case of the
> > > > PMU "cpu" as HEADER_CPU_PMU_CAPS. As the PMU "cpu" is special, and not
> > > > any "core" PMU, the logic had become broken and core PMUs not called
> > > > "cpu" were not having their caps written. This affects ARM and s390
> > > > non-hybrid PMUs.
> > > >
> > > > Simplify the PMU caps writing logic to scan one fewer time and to be
> > > > more explicit in its behavior.
> > > >
> > > > Reported-by: Wei Li <liwei391@...wei.com>
> > > > Fixes: 178ddf3bad98 ("perf header: Avoid hybrid PMU list in write_pmu_caps")
> > > > Signed-off-by: Ian Rogers <irogers@...gle.com>
> > > > ---
> > > > tools/perf/util/header.c | 31 ++++++++++++++++---------------
> > > > 1 file changed, 16 insertions(+), 15 deletions(-)
> > > >
> > > > diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
> > > > index 52fbf526fe74..13c71d28e0eb 100644
> > > > --- a/tools/perf/util/header.c
> > > > +++ b/tools/perf/util/header.c
> > > > @@ -1605,8 +1605,15 @@ static int write_pmu_caps(struct feat_fd *ff,
> > > > int ret;
> > > >
> > > > while ((pmu = perf_pmus__scan(pmu))) {
> > > > - if (!pmu->name || !strcmp(pmu->name, "cpu") ||
> > > > - perf_pmu__caps_parse(pmu) <= 0)
> > > > + if (!strcmp(pmu->name, "cpu")) {
> > >
> > > So you removed the check of 'pmu->name', does this check really redundant? since
> > > we can find such checks in many places in the perf code. If not, i think it is
> > > necessary for strcmp().
> >
> > Indeed, when sorting in tools/perf/util/pmus.c in cmp_sevent() we have:
> >
> > /* Order by PMU name. */
> > if (as->pmu != bs->pmu) {
> > a_pmu_name = a_pmu_name ?: (as->pmu->name ?: "");
> > b_pmu_name = b_pmu_name ?: (bs->pmu->name ?: "");
> > ret = strcmp(a_pmu_name, b_pmu_name);
> > if (ret)
> > return ret;
> > }
> >
> >
> > And even if in this specific case, for some reason, we could guarantee
> > that pmu->name isn't NULL, then removing that check should be best left
> > for a separate patch with an explanation as to why that is safe.
> >
> > Having it as:
> >
> > while ((pmu = perf_pmus__scan(pmu))) {
> > - if (!pmu->name || !strcmp(pmu->name, "cpu") ||
> > - perf_pmu__caps_parse(pmu) <= 0)
> > + if (!pmu->name || !strcmp(pmu->name, "cpu")) {
> >
> > even eases a bit reviewing, as we see we're just removing that
> > perf_pmu__caps_parse(pmu) line.
> >
> > Ian?
>
> The pmu name is initialized with:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/pmu.c?h=perf-tools-next#n1001
> pmu->name = strdup(name);
> if (!pmu->name)
> goto err;
>
> so name can't be NULL, strdup of NULL is segv, as if it were pmu would
> be NULL. I'll clean this up in an additional patch on top of this one.
Ok, perhaps at some point we can introduce a perf_pmu__name_is(pmu,
"cpu") and then have an assert for it not being NULL, as for some time
those tests were performed.
But thanks for checking, I'll apply the patches.
- Arnaldo
> Thanks,
> Ian
>
> > - Arnaldo
> >
> >
> > > > + /*
> > > > + * The "cpu" PMU is special and covered by
> > > > + * HEADER_CPU_PMU_CAPS. Note, core PMUs are
> > > > + * counted/written here for ARM, s390 and Intel hybrid.
> > > > + */
> > > > + continue;
> > > > + }
> > > > + if (perf_pmu__caps_parse(pmu) <= 0)
> > > > continue;
> > > > nr_pmu++;
> > > > }
> > > > @@ -1619,23 +1626,17 @@ static int write_pmu_caps(struct feat_fd *ff,
> > > > return 0;
> > > >
> > > > /*
> > > > - * Write hybrid pmu caps first to maintain compatibility with
> > > > - * older perf tool.
> > > > + * Note older perf tools assume core PMUs come first, this is a property
> > > > + * of perf_pmus__scan.
> > > > */
> > > > - if (perf_pmus__num_core_pmus() > 1) {
> > > > - pmu = NULL;
> > > > - while ((pmu = perf_pmus__scan_core(pmu))) {
> > > > - ret = __write_pmu_caps(ff, pmu, true);
> > > > - if (ret < 0)
> > > > - return ret;
> > > > - }
> > > > - }
> > > > -
> > > > pmu = NULL;
> > > > while ((pmu = perf_pmus__scan(pmu))) {
> > > > - if (pmu->is_core || !pmu->nr_caps)
> > > > + if (!strcmp(pmu->name, "cpu")) {
> > >
> > > same here
> > >
> > > Thanks,
> > > Wei
> > >
> > > > + /* Skip as above. */
> > > > + continue;
> > > > + }
> > > > + if (perf_pmu__caps_parse(pmu) <= 0)
> > > > continue;
> > > > -
> > > > ret = __write_pmu_caps(ff, pmu, true);
> > > > if (ret < 0)
> > > > return ret;
> >
> > --
> >
> > - Arnaldo
--
- Arnaldo
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