linux-kernel - Re: [PATCH v4 5/5] blktrace: Make init_blk_tracer() asynchronous when trace_async

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Message-ID: <20260129223116.489a01fd@robin>
Date: Thu, 29 Jan 2026 22:31:16 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Yaxiong Tian <tianyaxiong@...inos.cn>
Cc: Jens Axboe <axboe@...nel.dk>, mhiramat@...nel.org,
 mathieu.desnoyers@...icios.com, corbet@....net, skhan@...uxfoundation.org,
 linux-trace-kernel@...r.kernel.org, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v4 5/5] blktrace: Make init_blk_tracer() asynchronous
 when trace_async_init set

On Thu, 29 Jan 2026 22:26:08 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> Use trace_printk() instead. printk now has a "deferred" output. I'm not
> sure if the timestamps of when it prints is when the print took place
> or when it got to the console :-/

I added the below patch and have this result:

   kworker/u33:1-79      [002] .....     1.840855: trace_event_update_all: Start syncing
       swapper/0-1       [005] .....     6.045742: trace_eval_sync: sync maps
   kworker/u33:1-79      [002] .....    12.289296: trace_event_update_all: Finish syncing
       swapper/0-1       [005] .....    12.289387: trace_eval_sync: sync maps complete

Which shows that the final initcall waited for the work queue to complete:

-- Steve

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 396d59202438..33180d5622a8 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -10817,9 +10817,11 @@ subsys_initcall(trace_eval_init);
 
 static int __init trace_eval_sync(void)
 {
+	trace_printk("sync maps\n");
 	/* Make sure the eval map updates are finished */
 	if (eval_map_wq)
 		destroy_workqueue(eval_map_wq);
+	trace_printk("sync maps complete\n");
 	return 0;
 }
 
diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index af6d1fe5cab7..194b344400e9 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -3555,6 +3555,7 @@ void trace_event_update_all(struct trace_eval_map **map, int len)
 	int last_i;
 	int i;
 
+	trace_printk("Start syncing\n");
 	down_write(&trace_event_sem);
 	list_for_each_entry_safe(call, p, &ftrace_events, list) {
 		/* events are usually grouped together with systems */
@@ -3593,6 +3594,8 @@ void trace_event_update_all(struct trace_eval_map **map, int len)
 		cond_resched();
 	}
 	up_write(&trace_event_sem);
+	msleep(10000);
+	trace_printk("Finish syncing\n");
 }
 
 static bool event_in_systems(struct trace_event_call *call,