linux-kernel - Re: [PATCH v3 1/2] libbpf: Add the ability to suppress perf event enablement

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Message-ID: <CAADnVQ+6WuHrrAg1bQ+-6p1zZAWQVC_EGtt9ocv5aZE9=CxB5g@mail.gmail.com>
Date: Tue, 5 Aug 2025 09:45:57 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Ilya Leoshkevich <iii@...ux.ibm.com>, Eduard <eddyz87@...il.com>
Cc: 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>, bpf <bpf@...r.kernel.org>, 
	"linux-perf-use." <linux-perf-users@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	linux-s390 <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>
Subject: Re: [PATCH v3 1/2] libbpf: Add the ability to suppress perf event enablement

On Tue, Aug 5, 2025 at 6:04 AM Ilya Leoshkevich <iii@...ux.ibm.com> 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
> 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.
>
> Suggested-by: Jiri Olsa <jolsa@...nel.org>
> 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..414c566c4650 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, no_ioctl_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..2d3cc436cdbf 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 no_ioctl_enable;

The patch logic looks fine, but I feel the knob name is too
implementation oriented.
imo "dont_auto_enable" is more descriptive and easier
to reason about.

Let's wait for Eduard/Andrii reviews. This patch has to go
via bpf trees first while the latter via perf.