On Wed, 28 Jan 2026 15:24:15 +0800
Yaxiong Tian [off-list ref] wrote:

在 2026/1/28 12:49, Masami Hiramatsu (Google) 写道:

quoted

On Tue, 27 Jan 2026 09:23:12 +0800
Yaxiong Tian [off-list ref] wrote:

quoted

During kernel boot, the setup_boot_kprobe_events() function causes
significant delays, increasing overall startup time.

The root cause is a lock contention chain: its child function
enable_boot_kprobe_events() requires the event_mutex, which is
already held by early_event_add_tracer(). early_event_add_tracer()
itself is blocked waiting for the trace_event_sem  read-write lock,
which is held for an extended period by trace_event_update_all().

To resolve this, we have moved the execution of
setup_boot_kprobe_events() to the trace_init_wq  workqueue, allowing
it to run asynchronously.

Signed-off-by: Yaxiong Tian <redacted>
---
  kernel/trace/trace_kprobe.c | 14 +++++++++++++-
  1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
index 9953506370a5..4c6621f02696 100644
--- a/kernel/trace/trace_kprobe.c
+++ b/kernel/trace/trace_kprobe.c

@@ -2031,6 +2031,13 @@ static __init int init_kprobe_trace_early(void)
  }
  core_initcall(init_kprobe_trace_early);
  
+static struct work_struct kprobe_trace_work __initdata;
+
+static void __init kprobe_trace_works_func(struct work_struct *work)
+{
+	setup_boot_kprobe_events();
+}
+
  /* Make a tracefs interface for controlling probe points */
  static __init int init_kprobe_trace(void)
  {

@@ -2048,7 +2055,12 @@ static __init int init_kprobe_trace(void)
  	trace_create_file("kprobe_profile", TRACE_MODE_READ,
  			  NULL, NULL, &kprobe_profile_ops);
  
-	setup_boot_kprobe_events();
+	if (trace_init_wq) {
+		INIT_WORK(&kprobe_trace_work, kprobe_trace_works_func);
+		queue_work(trace_init_wq, &kprobe_trace_work);

Hmm, this queue_work is not required if kprobe_boot_events_buf[] is
empty. We should check it because most of the time we don't need it.

Yes, I will improve it in the next version.

quoted

Also, deferring initialization makes it indeterminate when this
tracing will begin.

Indeed, While most scenarios don't need boot-time tracing, and users 
prioritize boot speed, we must balance the need for deterministic traces 
with faster startup.

I just wonder why don't you define kprobe events after boot (e.g.
from init script) instead of kernel cmdline. Using cmdline means
it will be used for tracing kernel boot.

- tracing kernel boot -> use kernel cmdline (synchronous)
- tracing user boot -> use tracefs (asynchronous)

quoted

For kprobe event use case, I think setup_boot_kprobe_events() should
check the kprobe_boot_events_buf is empty at first. But I think Yaxiong
use case happens when you are using kprobe events via cmdline, is that
correct?

The issue was identified without enabling kprobe events via the cmdline. 

Interesting. So is it fixed by another patch [1]?

[1] https://lore.kernel.org/all/20260127053848.108473-1-sunliming@linux.dev/ (local)

The core finding is that asynchronous initialization can drastically cut 
boot time under different workloads. As blocking occurs elsewhere in the 
tracing infrastructure beyond just kprobe events, adopting async is a 
broadly applicable strategy for boot time optimization.

Yes, but it is also possible to set it up from user space, because that
user process can work asynchronously.
We can make the ftrace initialization async to accelerate boot time, but
that means it is not useful for tracing kernel boot.

quoted

I think introducing "async" cmdline option is more preferable.

Agreed, this works. Users focused on boot speed over early-boot tracing 
can opt for this parameter to gain a startup performance boost.

Yeah, that is an option. Anyway, basically, users have another option to
setup ftrace after boot user space asynchronously. That is my
recommendation for such use case.

quoted

BTW, I found that the kprobe events from kernel cmdline will be made
after boot-time tracing from bootconfig. Maybe it should be run this
earlier timing too.

Yes. Additionally, this optimization does not conflict with the current 
patch series at all.

I'll submit the updated patch for the next version promptly.

OK.

Thank you,

quoted

Thank you,

quoted

+	} else {
+		setup_boot_kprobe_events();
+	}
  
  	return 0;
  }
-- 
2.25.1

-- 
Masami Hiramatsu (Google) [off-list ref]