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

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Message-ID: <Zrs3ZqRtT3_lNj6a@LQ3V64L9R2.home>
Date: Tue, 13 Aug 2024 11:37:26 +0100
From: Joe Damato <jdamato@...tly.com>
To: Kyle Huey <me@...ehuey.com>
Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>,
	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, 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 Mon, Aug 05, 2024 at 12:55:17PM +0100, Joe Damato wrote:
> On Fri, Jul 26, 2024 at 09:35:43AM -0700, Kyle Huey wrote:
> > Will do.
> > 
> > - Kyle
> > 
> > On Fri, Jul 26, 2024 at 9:34 AM Andrii Nakryiko
> > <andrii.nakryiko@...il.com> wrote:
> > >
> > > On Fri, Jul 26, 2024 at 5:37 AM Kyle Huey <me@...ehuey.com> wrote:
> > > >
> > > > 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.
> > >
> > > I'd appreciate it if you can just incorporate that into your patch and
> > > resend it, thank you!
> > >
> > > >
> > > > - Kyle
> 
> I probably missed the updated patch, but I am happy to test any new
> versions, if needed, to ensure that the bug I hit is fixed.
> 
> Kyle: please let me know if there's a patch you'd like me to test?

Sorry for pinging this thread again; let me know if a fix was merged
and I missed it?

Otherwise, if it'd be helpful, I am happy to modify Kyle's patch to
take Andrii's suggestion and resend, but I don't want to step on
anyone's toes :)

- Joe