Re: [PATCH bpf-next v2] bpf: Remove migrate_disable in kprobe_multi_link_prog_run

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Aug 5, 2025 at 9:28 AM Tao Chen <chen.dylane@linux.dev> wrote:
>
> bpf program should run under migration disabled, kprobe_multi_link_prog_run
> called all the way from graph tracer, which disables preemption in
> function_graph_enter_regs, as Jiri and Yonghong suggested, there is no
> need to use migrate_disable. As a result, some overhead maybe will be
> reduced.
>
> Fixes: 0dcac2725406 ("bpf: Add multi kprobe link")
> Acked-by: Yonghong Song <yonghong.song@linux.dev>
> Acked-by: Jiri Olsa <jolsa@kernel.org>
> Signed-off-by: Tao Chen <chen.dylane@linux.dev>
> ---
>  kernel/trace/bpf_trace.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> Change list:
>  v1 -> v2:
>   - s/called the way/called all the way/.(Jiri)
>  v1: https://lore.kernel.org/bpf/f7acfd22-bcf3-4dff-9a87-7c1e6f84ce9c@linux.dev
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 3ae52978cae..5701791e3cb 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2734,14 +2734,19 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,

even though bpf_prog_run() eventually calls cant_migrate(), we should
add it before that __this_cpu_inc_return() call as well, because that
one is relying on that non-migration independently from bpf_prog_run()

>                 goto out;
>         }
>
> -       migrate_disable();
> +       /*
> +        * bpf program should run under migration disabled, kprobe_multi_link_prog_run
> +        * called all the way from graph tracer, which disables preemption in
> +        * function_graph_enter_regs, so there is no need to use migrate_disable.
> +        * Accessing the above percpu data bpf_prog_active is also safe for the same
> +        * reason.
> +        */

let's shorten this a bit to something like:

/* graph tracer framework ensures we won't migrate */
cant_migrate();

all the other stuff in the comment can become outdated way too easily
and/or is sort of general BPF implementation knowledge

pw-bot: cr


>         rcu_read_lock();
>         regs = ftrace_partial_regs(fregs, bpf_kprobe_multi_pt_regs_ptr());
>         old_run_ctx = bpf_set_run_ctx(&run_ctx.session_ctx.run_ctx);
>         err = bpf_prog_run(link->link.prog, regs);
>         bpf_reset_run_ctx(old_run_ctx);
>         rcu_read_unlock();
> -       migrate_enable();
>
>   out:
>         __this_cpu_dec(bpf_prog_active);
> --
> 2.48.1
>