linux-kernel - Re: [PATCH] perf test: Skip sigtrap test on old kernels

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Date:   Tue, 6 Sep 2022 22:50:05 +0200
From:   Marco Elver <elver@...gle.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ian Rogers <irogers@...gle.com>,
        linux-perf-users <linux-perf-users@...r.kernel.org>
Subject: Re: [PATCH] perf test: Skip sigtrap test on old kernels

On Tue, 6 Sept 2022 at 20:31, Namhyung Kim <namhyung@...nel.org> wrote:
>
> On Tue, Sep 6, 2022 at 5:45 AM Arnaldo Carvalho de Melo <acme@...nel.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@...nel.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@...gle.com>
> > > > Signed-off-by: Namhyung Kim <namhyung@...nel.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?

Yes, that's what I meant, that it could point out an issue with
sigtrap perf_event_open().

If there's no clear way to determine if it's just not supported or a
bug, it'd be better to leave it as-is.

Some other options:

1. Provide a way to disable certain tests, if it's known they will
fail for otherwise benign reasons i.e. no support.

2. Provide a command line option to skip instead of fail tests where
perf_event_open() returns some particular errnos. The default will be
fail, but you can then choose to trust that failure of
perf_event_open() means no support, and pass the option.