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Date: Tue, 16 Dec 2008 22:59:41 +0100
From: Ingo Molnar <mingo@elte.hu>
To: =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
        Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] tracing/ftrace: use preempt_enable_no_resched_notrace
	in ring_buffer_time_stamp()
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* Frédéric Weisbecker <fweisbec@gmail.com> wrote:

> 2008/12/16 Steven Rostedt <rostedt@goodmis.org>:
> >
> >
> > On Tue, 16 Dec 2008, Fr?d?ric Weisbecker wrote:
> >>
> >> The hard reboots I've seen are related to x86-64 while
> >> disabling/reenabling a CPU through /sys/device/system/cpu
> >> No tracer was enabled at these times (the problem still remains with
> >> latest updates on -tip for half an hour).
> >
> > Do you have STACK_TRACER enabled?
> >
> > -- Steve
> >
> 
> You were right. I've just built a kernel without STACK_TRACER and the 
> problem disappears...

i noticed high stack-tracer overhead too. Which is understandable i guess: 
the stack tracer keeps the mcount callbacks running all the time and can 
save the stack backtrace of the highest-stack-usage point in time that the 
kernel ever has hit in the past. That is a pretty powerful debug 
capability, with appropriate costs.

	Ingo