linux-kernel - Re: [PATCH bpf-next v2] bpf: Remove migrate_disable in kprobe_multi_link_prog

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Message-ID: <4233d7e9-b563-4c48-beda-b00ac5b4c643@linux.dev>
Date: Wed, 13 Aug 2025 19:54:52 +0800
From: Tao Chen <chen.dylane@...ux.dev>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
 martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org,
 yonghong.song@...ux.dev, john.fastabend@...il.com, kpsingh@...nel.org,
 sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org,
 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 v2] bpf: Remove migrate_disable in
 kprobe_multi_link_prog_run

在 2025/8/13 06:05, Andrii Nakryiko 写道:
> On Tue, Aug 5, 2025 at 9:28 AM Tao Chen <chen.dylane@...ux.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@...ux.dev>
>> Acked-by: Jiri Olsa <jolsa@...nel.org>
>> Signed-off-by: Tao Chen <chen.dylane@...ux.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()
> 

Hi Andrii,

There is __this_cpu_preempt_check in __this_cpu_inc_return, and the 
judgment criteria are similar to cant_migrate, and I'm not sure if it
is enough.

>>                  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 */
will change it in v3.

> 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
>>
-- 
Best Regards
Tao Chen