linux-kernel - Re: [PATCH bpf-next] bpf: Remove migrate_disable in kprobe_multi_link_prog

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Message-ID: <aJIrkAWK4ob5rCZ5@krava>
Date: Tue, 5 Aug 2025 18:04:32 +0200
From: Jiri Olsa <olsajiri@...il.com>
To: Tao Chen <chen.dylane@...ux.dev>
Cc: song@...nel.org, ast@...nel.org, daniel@...earbox.net,
	andrii@...nel.org, martin.lau@...ux.dev, eddyz87@...il.com,
	yonghong.song@...ux.dev, john.fastabend@...il.com,
	kpsingh@...nel.org, sdf@...ichev.me, haoluo@...gle.com,
	mattbobrowski@...gle.com, rostedt@...dmis.org, mhiramat@...nel.org,
	mathieu.desnoyers@...icios.com, bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next] bpf: Remove migrate_disable in
 kprobe_multi_link_prog_run

On Tue, Aug 05, 2025 at 08:23:12PM +0800, Tao Chen wrote:
> bpf program should run under migration disabled, kprobe_multi_link_prog_run
> called 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")
> Signed-off-by: Tao Chen <chen.dylane@...ux.dev>

Acked-by: Jiri Olsa <jolsa@...nel.org>

thanks,
jirka


> ---
>  kernel/trace/bpf_trace.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 3ae52978cae..1993fc62539 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,
>  		goto out;
>  	}
>  
> -	migrate_disable();
> +	/*
> +	 * bpf program should run under migration disabled, kprobe_multi_link_prog_run
> +	 * called the way from graph tracer, which disables preemption in

nit, s/called the way/called all the way/


> +	 * 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.
> +	 */
>  	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
>