linux-kernel - Re: [PATCH] perf/bpf: Don't call bpf_overflow

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Message-ID: <CAP045ArhO4K2vcrhG_GnJNhx=+7v6WLYKsDj4CvqO7HKzBshXg@mail.gmail.com>
Date: Fri, 26 Jul 2024 05:37:40 -0700
From: Kyle Huey <me@...ehuey.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Jiri Olsa <olsajiri@...il.com>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, khuey@...ehuey.com, Ingo Molnar <mingo@...hat.com>, 
	Namhyung Kim <namhyung@...nel.org>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	robert@...llahan.org, Joe Damato <jdamato@...tly.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Ian Rogers <irogers@...gle.com>, 
	Adrian Hunter <adrian.hunter@...el.com>, "Liang, Kan" <kan.liang@...ux.intel.com>, 
	Andrii Nakryiko <andrii@...nel.org>, Song Liu <song@...nel.org>, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH] perf/bpf: Don't call bpf_overflow_handler() for tracing events

On Fri, Jul 19, 2024 at 11:26 AM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Tue, Jul 16, 2024 at 12:25 AM Jiri Olsa <olsajiri@...il.com> wrote:
> >
> > On Mon, Jul 15, 2024 at 09:48:58AM -0700, Kyle Huey wrote:
> > > On Mon, Jul 15, 2024 at 9:30 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > > >
> > > > On Mon, Jul 15, 2024 at 08:19:44AM -0700, Kyle Huey wrote:
> > > >
> > > > > I think this would probably work but stealing the bit seems far more
> > > > > complicated than just gating on perf_event_is_tracing().
> > > >
> > > > perf_event_is_tracing() is something like 3 branches. It is not a simple
> > > > conditional. Combined with that re-load and the wrong return value, this
> > > > all wants a cleanup.
> > > >
> > > > Using that LSB works, it's just that the code aint pretty.
> > >
> > > Maybe we could gate on !event->tp_event instead. Somebody who is more
> > > familiar with this code than me should probably confirm that tp_event
> > > being non-null and perf_event_is_tracing() being true are equivalent
> > > though.
> > >
> >
> > it looks like that's the case, AFAICS tracepoint/kprobe/uprobe events
> > are the only ones having the tp_event pointer set, Masami?
> >
> > fwiw I tried to run bpf selftests with that and it's fine
>
> Why can't we do the most straightforward thing in this case?
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index ab6c4c942f79..cf4645b26c90 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -9707,7 +9707,8 @@ static int __perf_event_overflow(struct perf_event *event,
>
>         ret = __perf_event_account_interrupt(event, throttle);
>
> -       if (event->prog && !bpf_overflow_handler(event, data, regs))
> +       if (event->prog && event->prog->type == BPF_PROG_TYPE_PERF_EVENT &&
> +           !bpf_overflow_handler(event, data, regs))
>                 return ret;
>
>
> >
> > jirka
> >

Yes, that's effectively equivalent to calling perf_event_is_tracing()
and would work too. Do you want to land that patch? It needs to go to
6.10 stable too.

- Kyle