Re: [PATCH bpf-next v2] bpf: Remove migrate_disable in kprobe_multi_link_prog_run
From: Andrii Nakryiko <hidden>
Date: 2025-08-12 22:05:56
Also in:
bpf, lkml
On Tue, Aug 5, 2025 at 9:28 AM Tao Chen [off-list ref] wrote:
quoted hunk ↗ jump to hunk
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@linux.dev> Acked-by: Jiri Olsa <jolsa@kernel.org> Signed-off-by: Tao Chen <redacted> --- 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 (local)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()
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/* graph tracer framework ensures we won't migrate */ cant_migrate(); all the other stuff in the comment can become outdated way too easily and/or is sort of general BPF implementation knowledge pw-bot: cr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