linux-kernel - Re: [RESEND PATCH v5 1/4] perf/bpf: Call bpf handler directly, not through overflow machinery

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Message-ID: <ZhYWPGX0RzamxOHx@gmail.com>
Date: Wed, 10 Apr 2024 06:31:56 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Kyle Huey <me@...ehuey.com>
Cc: Kyle Huey <khuey@...ehuey.com>, linux-kernel@...r.kernel.org,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Marco Elver <elver@...gle.com>,
	Yonghong Song <yonghong.song@...ux.dev>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Robert O'Callahan <robert@...llahan.org>,
	Song Liu <song@...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>,
	linux-perf-users@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [RESEND PATCH v5 1/4] perf/bpf: Call bpf handler directly, not
 through overflow machinery


* Kyle Huey <me@...ehuey.com> wrote:

> To ultimately allow bpf programs attached to perf events to completely
> suppress all of the effects of a perf event overflow (rather than just the
> sample output, as they do today), call bpf_overflow_handler() from
> __perf_event_overflow() directly rather than modifying struct perf_event's
> overflow_handler. Return the bpf program's return value from
> bpf_overflow_handler() so that __perf_event_overflow() knows how to
> proceed. Remove the now unnecessary orig_overflow_handler from struct
> perf_event.
> 
> This patch is solely a refactoring and results in no behavior change.
> 
> Signed-off-by: Kyle Huey <khuey@...ehuey.com>
> Suggested-by: Namhyung Kim <namhyung@...nel.org>
> Acked-by: Song Liu <song@...nel.org>
> Acked-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  include/linux/perf_event.h |  6 +-----
>  kernel/events/core.c       | 28 +++++++++++++++-------------
>  2 files changed, 16 insertions(+), 18 deletions(-)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index d2a15c0c6f8a..c7f54fd74d89 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -810,7 +810,6 @@ struct perf_event {
>  	perf_overflow_handler_t		overflow_handler;
>  	void				*overflow_handler_context;
>  #ifdef CONFIG_BPF_SYSCALL
> -	perf_overflow_handler_t		orig_overflow_handler;
>  	struct bpf_prog			*prog;
>  	u64				bpf_cookie;
>  #endif

Could we reduce the #ifdeffery please?

On distros CONFIG_BPF_SYSCALL is almost always enabled, so it's not like 
this truly saves anything on real systems.

I'd suggest making the perf_event::prog and perf_event::bpf_cookie fields 
unconditional.

> +#ifdef CONFIG_BPF_SYSCALL
> +static int bpf_overflow_handler(struct perf_event *event,
> +				struct perf_sample_data *data,
> +				struct pt_regs *regs);
> +#endif

If the function definitions are misordered then first do a patch that moves 
the function earlier in the file, instead of slapping a random prototype 
into a random place.

> -	READ_ONCE(event->overflow_handler)(event, data, regs);
> +#ifdef CONFIG_BPF_SYSCALL
> +	if (!(event->prog && !bpf_overflow_handler(event, data, regs)))
> +#endif
> +		READ_ONCE(event->overflow_handler)(event, data, regs);

This #ifdef would go away too - on !CONFIG_BPF_SYSCALL event->prog should 
always be NULL.

Please keep the #ifdeffery reduction and function-moving patches separate 
from these other changes.

Thanks,

	Ingo