linux-kernel - Re: [PATCH v4 1/9] perf evsel: Set BPF output to system-wide

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Message-ID: <CAP-5=fUtcWzGV3n6igzs+eq8dEw2WYsnp8hNeKSUF+6yhZQMcw@mail.gmail.com>
Date: Tue, 24 Sep 2024 19:53:31 -0700
From: Ian Rogers <irogers@...gle.com>
To: Howard Chu <howardchu95@...il.com>
Cc: Namhyung Kim <namhyung@...nel.org>, acme@...nel.org, adrian.hunter@...el.com, 
	jolsa@...nel.org, kan.liang@...ux.intel.com, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/9] perf evsel: Set BPF output to system-wide

On Wed, Aug 7, 2024 at 8:58 PM Howard Chu <howardchu95@...il.com> wrote:
>
> Hello,
>
> The event does open, but bpf_perf_event_output() in BPF will return
> -95(-EOPNOTSUPP), so no output. I think this EOPNOTSUPP is not in
> bpf_trace.c's __bpf_perf_event_output(), but in perf_event's
> perf_event_output() called by BPF.
>
>           <idle>-0       [001] d..4. 154921.079230: bpf_trace_printk: err -95
>
> This is also a bug in perf trace -p <PID>.
>
> Thanks,
> Howard
>
> On Thu, Aug 8, 2024 at 7:21 AM Namhyung Kim <namhyung@...nel.org> wrote:
> >
> > On Wed, Aug 07, 2024 at 11:38:35PM +0800, Howard Chu wrote:
> > > pid = -1 for bpf-output event.
> > >
> > > This makes perf record -p <PID> --off-cpu work. Otherwise bpf-output
> > > cannot be collected.
> >
> > I don't understand why it's necessary.  Why isn't it collected?
> > Is it the kernel to reject the BPF output event to open?
> >
> > Thanks,
> > Namhyung
> >
> > >
> > > Signed-off-by: Howard Chu <howardchu95@...il.com>
> > > ---
> > >  tools/perf/util/evsel.c | 4 ++++
> > >  1 file changed, 4 insertions(+)
> > >
> > > diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> > > index bc603193c477..b961467133cf 100644
> > > --- a/tools/perf/util/evsel.c
> > > +++ b/tools/perf/util/evsel.c
> > > @@ -2282,6 +2282,10 @@ static int evsel__open_cpu(struct evsel *evsel, struct perf_cpu_map *cpus,
> > >
> > >                       test_attr__ready();
> > >
> > > +                     /* BPF output event can only be system-wide */
> > > +                     if (evsel__is_bpf_output(evsel))
> > > +                             pid = -1;

This matches with libbpf:
https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/lib/bpf/libbpf.c#n11967

Thanks,
Ian

> > > +
> > >                       /* Debug message used by test scripts */
> > >                       pr_debug2_peo("sys_perf_event_open: pid %d  cpu %d  group_fd %d  flags %#lx",
> > >                               pid, perf_cpu_map__cpu(cpus, idx).cpu, group_fd, evsel->open_flags);
> > > --
> > > 2.45.2
> > >