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

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Date:   Tue, 06 Sep 2022 17:56:37 -0300
From:   Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To:     Marco Elver <elver@...gle.com>, 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 September 6, 2022 5:50:05 PM GMT-03:00, Marco Elver <elver@...gle.com> wrote:
>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.

perf could go fancy and try to add a probe using a source file+line where older kernels would fail 8-)

Anyway, perf already does all sorts of kernel capability checks, perhaps this is one of can for sure detect it's something available :-/

One new way could be to look at BTF?

>
>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.