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

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Message-ID: <f7acfd22-bcf3-4dff-9a87-7c1e6f84ce9c@linux.dev>
Date: Wed, 6 Aug 2025 00:17:43 +0800
From: Tao Chen <chen.dylane@...ux.dev>
To: Jiri Olsa <olsajiri@...il.com>
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

在 2025/8/6 00:04, Jiri Olsa 写道:
> 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/
> 

will fix it, thanks.

> 
>> +	 * 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
>>


-- 
Best Regards
Tao Chen