On Tue, Dec 5, 2023 at 10:07 AM 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. Hmm, yes, perhaps. The SIGTRAP thing (which I was previously unaware of) would actually be more useful to us than an I/O signal. I am slightly wary that event_limit appears to have no tests in the kernel tree. - Kyle > Thanks, > Namhyung