[PATCH] tracing: Register static events output early

[PATCH] tracing: Register static events output early

[Frederic Weisbecker]

Early uses of trace_printk() or trace_dump_stack() won't be
displayed correctly if they are printed before device initcalls
through explicit calls to ftrace_dump() or if there is an early
BUG() and ftrace_dump_on_oops is set. This is because we only
initialize these static events output callbacks at that initcall
level.

Initialize these events output at the earliest initcall state
to make boot debugging possible sooner.

Before:

[    1.936168] Dumping ftrace buffer:
[    1.936168] ---------------------------------
[    1.936168]   <idle>-0       3d.s. 2299861us : Unknown type 4
[    1.936168] ---------------------------------

After:

[    1.946182] Dumping ftrace buffer:
[    1.946182] ---------------------------------
[    1.946182]   <idle>-0       3d.s. 2309844us : <stack trace>
[    1.946182]  => tick_setup_sched_timer
[    1.946182]  => hrtimer_run_pending
[    1.946182]  => run_timer_softirq
[    1.946182]  => __do_softirq
[    1.946182]  => call_softirq
[    1.946182]  => do_softirq
[    1.946182]  => irq_exit
[    1.946182]  => smp_apic_timer_interrupt
[    1.946182] ---------------------------------

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
---
 kernel/trace/trace_output.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
index cf535cc..ad92d9c 100644
--- a/kernel/trace/trace_output.c
+++ b/kernel/trace/trace_output.c
@@ -1278,4 +1278,4 @@ __init static int init_events(void)
 
 	return 0;
 }
-device_initcall(init_events);
+early_initcall(init_events);
-- 
1.7.3.2

Re: [PATCH] tracing: Register static events output early

[Steven Rostedt]

On Thu, 2011-05-19 at 01:29 +0200, Frederic Weisbecker wrote:

> diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
> index cf535cc..ad92d9c 100644
> --- a/kernel/trace/trace_output.c
> +++ b/kernel/trace/trace_output.c
> @@ -1278,4 +1278,4 @@ __init static int init_events(void)
>  
>  	return 0;
>  }
> -device_initcall(init_events);
> +early_initcall(init_events);

Hmm, careful here. The ring buffers are also allocated at early_init().

But you may be OK, as things like trace_printk() don't do anything if
"tracing_disabled" is set, and it's set on bootup and cleared after we
allocate the buffers.

-- Steve

Re: [PATCH] tracing: Register static events output early

[Frederic Weisbecker]

On Wed, May 18, 2011 at 07:37:19PM -0400, Steven Rostedt wrote:
> On Thu, 2011-05-19 at 01:29 +0200, Frederic Weisbecker wrote:
> 
> > diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
> > index cf535cc..ad92d9c 100644
> > --- a/kernel/trace/trace_output.c
> > +++ b/kernel/trace/trace_output.c
> > @@ -1278,4 +1278,4 @@ __init static int init_events(void)
> >  
> >  	return 0;
> >  }
> > -device_initcall(init_events);
> > +early_initcall(init_events);
> 
> Hmm, careful here. The ring buffers are also allocated at early_init().
> 
> But you may be OK, as things like trace_printk() don't do anything if
> "tracing_disabled" is set, and it's set on bootup and cleared after we
> allocate the buffers.

Yep, and the output callback only impact the read side anyway, once the trace
has been already pulled out of the buffer. I hope I'm not missing something in the
way...