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From:   Namhyung Kim <namhyung@kernel.org>
Date:   Tue, 6 Sep 2022 11:31:04 -0700
Message-ID: <CAM9d7ci2ZujdY75DUtZA+f=fru7yh8VJrj8-r2RgZetu57u61A@mail.gmail.com>
Subject: Re: [PATCH] perf test: Skip sigtrap test on old kernels
To:     Arnaldo Carvalho de Melo <acme@kernel.org>
Cc:     Marco Elver <elver@google.com>, Jiri Olsa <jolsa@kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ian Rogers <irogers@google.com>,
        linux-perf-users <linux-perf-users@vger.kernel.org>
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On Tue, Sep 6, 2022 at 5:45 AM Arnaldo Carvalho de Melo <acme@kernel.org> wrote:
>
> Em Sat, Sep 03, 2022 at 08:52:01AM +0200, Marco Elver escreveu:
> > On Sat, 3 Sept 2022 at 02:02, Namhyung Kim <namhyung@kernel.org> wrote:
> > >
> > > If it runs on an old kernel, perf_event_open would fail because of the
> > > new fields sigtrap and sig_data.  Just skip the test if it failed.
> > >
> > > Cc: Marco Elver <elver@google.com>
> > > Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> > > ---
> > >  tools/perf/tests/sigtrap.c | 1 +
> > >  1 file changed, 1 insertion(+)
> > >
> > > diff --git a/tools/perf/tests/sigtrap.c b/tools/perf/tests/sigtrap.c
> > > index e32ece90e164..7057566e6ae4 100644
> > > --- a/tools/perf/tests/sigtrap.c
> > > +++ b/tools/perf/tests/sigtrap.c
> > > @@ -140,6 +140,7 @@ static int test__sigtrap(struct test_suite *test __maybe_unused, int subtest __m
> > >         fd = sys_perf_event_open(&attr, 0, -1, -1, perf_event_open_cloexec_flag());
> > >         if (fd < 0) {
> > >                 pr_debug("FAILED sys_perf_event_open(): %s\n", str_error_r(errno, sbuf, sizeof(sbuf)));
> > > +               ret = TEST_SKIP;
> >
> > Wouldn't we be interested if perf_event_open() fails because it could
> > actually be a bug? By skipping we'll be more likely to miss the fact
> > there's a real problem.
> >
> > That's my naive thinking at least - what do other perf tests usually
> > do in this case?
>
> Yeah, I was going to try and check if this is the only way that, with
> the given arguments, perf_event_open would fail, but its better to at
> least check errno against -EINVAL or something?

EINVAL would be too generic and the kernel returns it in many places.
I really wish we could have a better error reporting mechanism.

Maybe we could retry perf_event_open with sigtrap and sig_data cleared.
If it succeeded, then we can skip the test.  If it still failed, then report
the error.  But it still couldn't find a bug in the sigtrap code.
What do you think?

Thanks,
Namhyung