linux-kernel - Re: RFC: How to handle function tracing, frame pointers and -mfentry?

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Date:	Fri, 27 Apr 2012 15:43:37 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	linux-kbuild <linux-kbuild@...r.kernel.org>,
	Michal Marek <mmarek@...e.cz>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: RFC: How to handle function tracing, frame pointers and
 -mfentry?

On Fri, 2012-04-27 at 14:46 -0400, Steven Rostedt wrote:

> Currently, function tracing selects CONFIG_FRAME_POINTER for various
> archs (including x86), as the kernel will not compile without it, if
> function tracing is enabled. But if -mfentry is available with the
> compiler, it does not have this dependency. The kernel will compile fine
> with -pg -mfentry and without frame pointers.

Just to show the difference. I ran several iterations of hackbench, on
an Intel Quad Core2 2.6GHz.

Here's the fentry code with frame pointers (tracing disabled):

Time: 2.006
Time: 2.028
Time: 2.028
Time: 1.999
Time: 2.035
Time: 2.037
Time: 2.006
Time: 1.996
Time: 2.049
Time: 1.991
Time: 2.038
Time: 2.047
Time: 2.039
Time: 2.000
Time: 2.021
Time: 2.011
Time: 2.007
Time: 2.024
Time: 2.033
Time: 2.027
Time: 2.044


And the fentry code with frame pointers disabled:

Time: 1.870
Time: 1.861
Time: 1.865
Time: 1.884
Time: 1.867
Time: 1.867
Time: 1.875
Time: 1.883
Time: 1.863
Time: 1.877
Time: 1.865
Time: 1.885
Time: 1.842
Time: 1.863
Time: 1.899
Time: 1.877
Time: 1.837
Time: 1.900
Time: 1.897
Time: 1.877
Time: 1.853
Time: 1.856

That's about a 8% difference.

-- Steve


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