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

[Tao Chen]

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

maybe cant_sleep() is better like trace_call_bpf, cant_sleep dose not 
check migration_disabled again, which is done in 
__this_cpu_preempt_check. I will add it in v3.

> >                  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 all the other stuff in the comment can become outdated way too easily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

--
Best Regards
Tao Chen