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

[Marco Elver]

On Tue, 5 Dec 2023 at 19:07, Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> 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.

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