linux-kernel - Re: [PATCH -v4 9/9] tracing: add function graph tracer support for MIPS

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Message-Id: <1256150092.18347.3294.camel@gandalf.stny.rr.com>
Date:	Wed, 21 Oct 2009 14:34:52 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Nicholas Mc Guire <der.herr@...r.at>
Cc:	David Daney <ddaney@...iumnetworks.com>,
	Wu Zhangjin <wuzhangjin@...il.com>,
	linux-kernel@...r.kernel.org, linux-mips@...ux-mips.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [PATCH -v4 9/9] tracing: add function graph tracer support for
 MIPS

On Wed, 2009-10-21 at 20:17 +0200, Nicholas Mc Guire wrote:
> > 
> > 
> > We're not doing back traces. We need to modify the return of the
> > function being called. Note, the above functions that end with ";" are
> > leaf functions. Non leaf functions show "{" and end with "}".
> > 
> > The trick here is to find a reliable way to modify the return address.
> >
> would it not more or less be the same thing if you used -finstrument-functions
> then and provide a stub __cyg_profile_func_enter/exit initialized to an empty
> function until you replace it during tracing. This does give you an overhead
> when you are not tracing - but it would make the tracer implementation quite
> generic.

-finstrument-functions adds a substantial overhead when not tracing, and
there's no easy way to remove it. The beauty with this approach is that
-pg only adds a couple of instructions (one on x86). When tracing is
disabled, that one line is converted to a nop.

On x86, hackbench reports no difference between running with dynamic
ftrace configured but disabled (includes function graph configured) and
without it configured.

This allows function tracing to be configured in on production
environments.

-- Steve


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