linux-kernel - Re: [PATCH] tracing: Make sure trace_printk() can output as soon as it can be used

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Date:   Thu, 5 Jan 2023 01:06:18 +0000
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH] tracing: Make sure trace_printk() can output as soon as
 it can be used

On Wed, Jan 04, 2023 at 04:14:12PM -0500, Steven Rostedt wrote:
> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
> 
> Currently trace_printk() can be used as soon as early_trace_init() is
> called from start_kernel(). But if a crash happens, and
> "ftrace_dump_on_oops" is set on the kernel command line, all you get will
> be:
> 
>   [    0.456075]   <idle>-0         0dN.2. 347519us : Unknown type 6
>   [    0.456075]   <idle>-0         0dN.2. 353141us : Unknown type 6
>   [    0.456075]   <idle>-0         0dN.2. 358684us : Unknown type 6
> 
> This is because the trace_printk() event (type 6) hasn't been registered
> yet. That gets done via an early_initcall(), which may be early, but not
> early enough.
> 
> Instead of registering the trace_printk() event (and other ftrace events,
> which are not trace events) via an early_initcall(), have them registered at
> the same time that trace_printk() can be used. This way, if there is a
> crash before early_initcall(), then the trace_printk()s will actually be
> useful.
> 
> Cc: stable@...r.kernel.org
> Reported-by: "Joel Fernandes (Google)" <joel@...lfernandes.org>
> Fixes: e725c731e3bb1 ("tracing: Split tracing initialization into two for early initialization")
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>

Tested-by: Joel Fernandes (Google) <joel@...lfernandes.org>

thanks,

 - Joel


> ---
>  kernel/trace/trace.c        | 2 ++
>  kernel/trace/trace.h        | 1 +
>  kernel/trace/trace_output.c | 3 +--
>  3 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index d3005279165d..80de338f1277 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -10196,6 +10196,8 @@ void __init early_trace_init(void)
>  			static_key_enable(&tracepoint_printk_key.key);
>  	}
>  	tracer_alloc_buffers();
> +
> +	init_events();
>  }
>  
>  void __init trace_init(void)
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 900e75d96c84..11c4918ff202 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -1504,6 +1504,7 @@ extern void trace_event_enable_cmd_record(bool enable);
>  extern void trace_event_enable_tgid_record(bool enable);
>  
>  extern int event_trace_init(void);
> +extern int init_events(void);
>  extern int event_trace_add_tracer(struct dentry *parent, struct trace_array *tr);
>  extern int event_trace_del_tracer(struct trace_array *tr);
>  extern void __trace_early_add_events(struct trace_array *tr);
> diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
> index 67f47ea27921..5cd4fb656306 100644
> --- a/kernel/trace/trace_output.c
> +++ b/kernel/trace/trace_output.c
> @@ -1568,7 +1568,7 @@ static struct trace_event *events[] __initdata = {
>  	NULL
>  };
>  
> -__init static int init_events(void)
> +__init int init_events(void)
>  {
>  	struct trace_event *event;
>  	int i, ret;
> @@ -1581,4 +1581,3 @@ __init static int init_events(void)
>  
>  	return 0;
>  }
> -early_initcall(init_events);
> -- 
> 2.35.1
>