Re: [PATCH 1/2] tracing: Add sysctl to enable/disable tracing on oops

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> On Wed, 2009-09-09 at 10:40 +0800, Li Zefan wrote:
> 
> > > I have another silly question.
> > > Why should we call tracing_off() in oops_enter()?
> > > 
> > 
> > I guess it's because trace outputs generated during oops can
> > overwrite/mess up those generated before oops?
> > 
> > It was added by this commit, but I can't find trace_printk_on_oops.
> 
> That's because Thomas did not bother looking up the actual variable he
> was talking about. s/trace_printk_on_oops/ftrace_dump_on_oops/

After a bit thinking, I think ftrace_dump_on_oops and kernel dump with panic have
very similar requirement.
Then, I think we can reuse this infrastructure for kernel dump.

Requirement
  - Need to know why the problem occur.
  - Need to don't logging oops (panic) internal.

Unfortunately, panic() call panic_notifier after crash_kexec().
it mean tracing_off() was not called when panic on the machine w/ kernel-dump.

	NORET_TYPE void panic(const char * fmt, ...)
	{
	(snip)
	        crash_kexec(NULL);
	(snip)
	        atomic_notifier_call_chain(&panic_notifier_list, 0, buf);

I think we can insert tracing_off() into panic() directly. it only
bit mask operation, it mean it don't have side effect risk.

Perhaps, There are other kernel dump specific requrement. but I guess
it's very small. Maybe it can be handled by trace_die_handler() or something like.

What do you think?


> 
> -- Steve
> 
> > 
> > commit bdff78707f3ce47e891f3201c9666122a70556ce
> > Author: Thomas Gleixner <tglx@linutronix.de>
> > Date:   Fri Jul 24 15:30:45 2009 -0400
> > 
> >     trace: stop tracer in oops_enter()
> > 
> >     If trace_printk_on_oops is set we lose interesting trace information
> >     when the tracer is enabled across oops handling and printing. We want
> >     the trace which might give us information _WHY_ we oopsed.
> > 
>