Re: [PATCH 1/2] perf/bpf: Allow a bpf program to suppress I/O signals.

[Namhyung Kim]

Hello,

Add Marco Elver to CC.

On Tue, Dec 5, 2023 at 3:16 AM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
>
> On Mon, Dec 04, 2023 at 02:18:49PM -0800, Andrii Nakryiko wrote:
> > On Mon, Dec 4, 2023 at 12:14 PM Kyle Huey <me@xxxxxxxxxxxx> wrote:
> > >
> > > Returning zero from a bpf program attached to a perf event already
> > > suppresses any data output. This allows it to suppress I/O availability
> > > signals too.
> >
> > make sense, just one question below
> >
> > >
> > > Signed-off-by: Kyle Huey <khuey@xxxxxxxxxxxx>
>
> Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
>
> > > ---
> > >  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 perf_event *event,
> > >         rcu_read_unlock();
> > >  out:
> > >         __this_cpu_dec(bpf_prog_active);
> > > -       if (!ret)
> > > +       if (!ret) {
> > > +               event->pending_kill = 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.

Thanks,
Namhyung