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

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Message-ID: <ZW8Gi2QI5ceAJfab@krava>
Date:   Tue, 5 Dec 2023 12:16:27 +0100
From:   Jiri Olsa <olsajiri@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Kyle Huey <me@...ehuey.com>, Kyle Huey <khuey@...ehuey.com>,
        linux-kernel@...r.kernel.org,
        Robert O'Callahan <robert@...llahan.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        linux-perf-users@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH 1/2] perf/bpf: Allow a bpf program to suppress I/O
 signals.

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@...ehuey.com> 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@...ehuey.com>

Acked-by: Jiri Olsa <jolsa@...nel.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 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

jirka

> 
> >
> >         event->orig_overflow_handler(event, data, regs);
> >  }
> > --
> > 2.34.1
> >
> >