linux-kernel - Re: [PATCH 1/6] ftrace: move function tracer functions out of trace.c

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Message-ID: <20090116111243.GA20082@elte.hu>
Date:	Fri, 16 Jan 2009 12:12:43 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 1/6] ftrace: move function tracer functions out of
	trace.c


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Thu, 15 Jan 2009 23:53:43 -0500 Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> >  static void
> > +function_trace_call_preempt_only(unsigned long ip, unsigned long parent_ip)
> > +{
> > +	struct trace_array *tr = func_trace;
> > +	struct trace_array_cpu *data;
> > +	unsigned long flags;
> > +	long disabled;
> > +	int cpu, resched;
> > +	int pc;
> > +
> > +	if (unlikely(!ftrace_function_enabled))
> > +		return;
> 
> We're optimizing for the tracing-is-enabled case.  What's the thinking 
> here?

There's two levels here: first the patched in callsites. Those are NOPs in 
the usual case - there's no overhead for the default 'function tracing is 
built in but not enabled' case.

There's a second level: a /debug/tracing/tracing_enabled lightweight 
dynamic flag to flip tracing on/off while the tracer is enabled. _That_ 
one, if it ever matters to a codepath, is default-enabled.

I.e. the above code sequence is correct.

Thanks,

	Ingo
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