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From:   Marco Elver <elver@google.com>
Date:   Tue, 5 Dec 2023 19:16:38 +0100
Message-ID: <CANpmjNPfoLX=HPy0MhbGqMmGT4jE0Ky29cx5QP_8tJ2u=1ju_Q@mail.gmail.com>
Subject: Re: [PATCH 1/2] perf/bpf: Allow a bpf program to suppress I/O signals.
To:     Namhyung Kim <namhyung@kernel.org>
Cc:     Jiri Olsa <olsajiri@gmail.com>,
        Andrii Nakryiko <andrii.nakryiko@gmail.com>,
        Kyle Huey <me@kylehuey.com>, Kyle Huey <khuey@kylehuey.com>,
        linux-kernel@vger.kernel.org,
        "Robert O'Callahan" <robert@ocallahan.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Ian Rogers <irogers@google.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        linux-perf-users@vger.kernel.org, bpf@vger.kernel.org
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On Tue, 5 Dec 2023 at 19:07, Namhyung Kim <namhyung@kernel.org> wrote:
>
> Hello,
>
> Add Marco Elver to CC.
>
> On Tue, Dec 5, 2023 at 3:16=E2=80=AFAM Jiri Olsa <olsajiri@gmail.com> wro=
te:
> >
> > On Mon, Dec 04, 2023 at 02:18:49PM -0800, Andrii Nakryiko wrote:
> > > On Mon, Dec 4, 2023 at 12:14=E2=80=AFPM Kyle Huey <me@kylehuey.com> w=
rote:
> > > >
> > > > Returning zero from a bpf program attached to a perf event already
> > > > suppresses any data output. This allows it to suppress I/O availabi=
lity
> > > > signals too.
> > >
> > > make sense, just one question below
> > >
> > > >
> > > > Signed-off-by: Kyle Huey <khuey@kylehuey.com>
> >
> > Acked-by: Jiri Olsa <jolsa@kernel.org>
> >
> > > > ---
> > > >  kernel/events/core.c | 4 +++-
> > > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > > > index b704d83a28b2..34d7b19d45eb 100644
> > > > --- a/kernel/events/core.c
> > > > +++ b/kernel/events/core.c
> > > > @@ -10417,8 +10417,10 @@ static void bpf_overflow_handler(struct pe=
rf_event *event,
> > > >         rcu_read_unlock();
> > > >  out:
> > > >         __this_cpu_dec(bpf_prog_active);
> > > > -       if (!ret)
> > > > +       if (!ret) {
> > > > +               event->pending_kill =3D 0;
> > > >                 return;
> > > > +       }
> > >
> > > What's the distinction between event->pending_kill and
> > > event->pending_wakeup? Should we do something about pending_wakeup?
> > > Asking out of complete ignorance of all these perf specifics.
> > >
> >
> > I think zeroing pending_kill is enough.. when it's set the perf code
> > sets pending_wakeup to call perf_event_wakeup in irq code that wakes
> > up event's ring buffer readers and sends sigio if pending_kill is set
>
> Right, IIUC pending_wakeup is set by the ring buffer code when
> a task is waiting for events and it gets enough events (watermark).
> So I think it's good for ring buffer to manage the pending_wakeup.
>
> And pending_kill is set when a task wants a signal delivery even
> without getting enough events.  Clearing pending_kill looks ok
> to suppress normal signals but I'm not sure if it's ok for SIGTRAP.
>
> If we want to handle returning 0 from bpf as if the event didn't
> happen, I think SIGTRAP and event_limit logic should be done
> after the overflow handler depending on pending_kill or something.

I'm not sure which kernel version this is for, but in recent kernels,
the SIGTRAP logic was changed to no longer "abuse" event_limit, and
uses its own "pending_sigtrap" + "pending_work" (on reschedule
transitions).

Thanks,
-- Marco