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From:   Namhyung Kim <namhyung@kernel.org>
Date:   Wed, 15 Dec 2021 10:15:48 -0800
Message-ID: <CAM9d7cgnQOZVaCs7Yj=siBLkhK7Z5TmmXLBA+VkGs1S3PUOSTw@mail.gmail.com>
Subject: Re: [PATCH 4/5] perf ftrace: Add -b/--use-bpf option for latency subcommand
To:     Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc:     Arnaldo Carvalho de Melo <acme@kernel.org>,
        Jiri Olsa <jolsa@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Andi Kleen <ak@linux.intel.com>,
        Ian Rogers <irogers@google.com>,
        Stephane Eranian <eranian@google.com>,
        Song Liu <songliubraving@fb.com>,
        Changbin Du <changbin.du@gmail.com>
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On Mon, Dec 6, 2021 at 9:16 AM Namhyung Kim <namhyung@kernel.org> wrote:
>
> Hello Athira,
>
> On Sun, Dec 5, 2021 at 6:24 PM Athira Rajeev
> <atrajeev@linux.vnet.ibm.com> wrote:
> >
> >
> >
> > > On 30-Nov-2021, at 4:48 AM, Namhyung Kim <namhyung@kernel.org> wrote:
> > >
> > > The -b/--use-bpf option is to use BPF to get latency info of kernel
> > > functions.  It'd have better performance impact and I observed that
> > > latency of same function is smaller than before when using BPF.
> > >
> > > Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> > > ---
> [SNIP]
> > > @@ -1144,6 +1166,8 @@ int cmd_ftrace(int argc, const char **argv)
> > >       const struct option latency_options[] = {
> > >       OPT_CALLBACK('T', "trace-funcs", &ftrace.filters, "func",
> > >                    "Show latency of given function", parse_filter_func),
> > > +     OPT_BOOLEAN('b', "use-bpf", &ftrace.target.use_bpf,
> > > +                 "Use BPF to measure function latency"),
> >
> >
> > Hi Namhyung,
> >
> > Can this be inside BPF_SKEL check, similar to how we have “bpf-prog” and other options in builtin-stat.c ?
> >
> > #ifdef HAVE_BPF_SKEL
> > <<OPT_BOOLEAN for use-bpf>>
> > #endif
> >
> > Otherwise when using “-b” and if perf is not built with BPF_SKEL, we will just return in perf_ftrace__latency_prepare_bpf without any error messages.
>
> Thanks for reporting this.  Yeah, it should report
> error messages in such conditions.
>
> I think it'd be better to add an error message
> rather than hiding the option.

Well, now we build perf with BPF by default.
So I think it's ok to follow your suggestion.
it'd show the usage and options when the -b option is used
and BPF support is not enabled.

Thanks,
Namhyung