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.dyl...@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.s...@linux.dev>
Acked-by: Jiri Olsa <jo...@kernel.org>
Signed-off-by: Tao Chen <chen.dyl...@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-9a87-7c1e6f84c...@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all the other stuff in the comment can become outdated way too easily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