linux-kernel - Re: [PATCH 2/3] trace: fix default boot up tracer

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Message-ID: <alpine.DEB.1.10.0902022327080.2192@gandalf.stny.rr.com>
Date:	Mon, 2 Feb 2009 23:33:52 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 2/3] trace: fix default boot up tracer


On Mon, 2 Feb 2009, Andrew Morton wrote:
> > 
> > The lock_kernel addition was added when the BKL became a spinlock again. 
> > The selftests needed to be able to sleep, and this caused issues.
> 
> Sleeping inside lock_kernel() is quite OK.  Confused.

I did not explain that quite well. I need to focus on the emails 
that I write, and not do it half concentrating on code that I'm
also writing :-/

The preempt tracer expects preemption enabled when the self test is 
executed. Because the self test for preempt tracer is basically:

start_trace();
preempt_disable();
udelay(x);
preempt_enable();
stop_trace();

make sure we have a delay.

This failed, because lock_kernel now disables preemption. So that 
preempt_disable() never triggers the trace, and the test sees that nothing 
was recorded. This causes a failure to be flagged, and we disable the 
preempt tracer.

> 
> What is the call path to this function?  Does it all happen under
> ftrace_init()?  If not, do we risk breaking start_kernel()'s
> thou-shalt-not-enable-interrupts-early rule which powerpc (at least)
> imposes?

This function is always called via the initcall functions.

> 
> > The register_tracer was initial written to be pluggable at any time. 
> > Perhaps in the future to allow modules. But this does not seem to have 
> > panned out.
> > 
> > Since we have the lock_kernel there anyway, if we ever need to handle 
> > modules, that will need a different interface anyway. I guess I can nuke 
> > the unregister tracer.
> 
> And add some __init/__initdatas?

I'll take some time to analyze what can be annotated.

-- Steve
> 
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