linux-kernel - Re: [PATCH v4 1/2] libbpf: Add the ability to suppress perf event enablement

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Message-ID: <d6cf6faf-f6bf-41f6-b2fd-2694bc62753e@linux.dev>
Date: Wed, 6 Aug 2025 08:25:01 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: Ilya Leoshkevich <iii@...ux.ibm.com>, Alexei Starovoitov
 <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>
Cc: bpf@...r.kernel.org, linux-perf-users@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
 Thomas Richter <tmricht@...ux.ibm.com>, Jiri Olsa <jolsa@...nel.org>,
 Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Alexander Gordeev <agordeev@...ux.ibm.com>,
 Eduard Zingerman <eddyz87@...il.com>
Subject: Re: [PATCH v4 1/2] libbpf: Add the ability to suppress perf event
 enablement



On 8/6/25 4:40 AM, Ilya Leoshkevich wrote:
> Automatically enabling a perf event after attaching a BPF prog to it is
> not always desirable.
>
> Add a new no_ioctl_enable field to struct bpf_perf_event_opts. While

no_ioctl_enable =>  dont_enable

> introducing ioctl_enable instead would be nicer in that it would avoid
> a double negation in the implementation, it would make
> DECLARE_LIBBPF_OPTS() less efficient.
>
> Acked-by: Eduard Zingerman <eddyz87@...il.com>
> Suggested-by: Jiri Olsa <jolsa@...nel.org>
> Tested-by: Thomas Richter <tmricht@...ux.ibm.com>
> Co-developed-by: Thomas Richter <tmricht@...ux.ibm.com>
> Signed-off-by: Thomas Richter <tmricht@...ux.ibm.com>
> Signed-off-by: Ilya Leoshkevich <iii@...ux.ibm.com>
> ---
>   tools/lib/bpf/libbpf.c | 13 ++++++++-----
>   tools/lib/bpf/libbpf.h |  4 +++-
>   2 files changed, 11 insertions(+), 6 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index fb4d92c5c339..8f5a81b672e1 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -10965,11 +10965,14 @@ struct bpf_link *bpf_program__attach_perf_event_opts(const struct bpf_program *p
>   		}
>   		link->link.fd = pfd;
>   	}
> -	if (ioctl(pfd, PERF_EVENT_IOC_ENABLE, 0) < 0) {
> -		err = -errno;
> -		pr_warn("prog '%s': failed to enable perf_event FD %d: %s\n",
> -			prog->name, pfd, errstr(err));
> -		goto err_out;
> +
> +	if (!OPTS_GET(opts, dont_enable, false)) {
> +		if (ioctl(pfd, PERF_EVENT_IOC_ENABLE, 0) < 0) {
> +			err = -errno;
> +			pr_warn("prog '%s': failed to enable perf_event FD %d: %s\n",
> +				prog->name, pfd, errstr(err));
> +			goto err_out;
> +		}
>   	}
>   
>   	return &link->link;
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index d1cf813a057b..455a957cb702 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -499,9 +499,11 @@ struct bpf_perf_event_opts {
>   	__u64 bpf_cookie;
>   	/* don't use BPF link when attach BPF program */
>   	bool force_ioctl_attach;
> +	/* don't automatically enable the event */
> +	bool dont_enable;
>   	size_t :0;
>   };
> -#define bpf_perf_event_opts__last_field force_ioctl_attach
> +#define bpf_perf_event_opts__last_field dont_enable
>   
>   LIBBPF_API struct bpf_link *
>   bpf_program__attach_perf_event(const struct bpf_program *prog, int pfd);