linux-kernel - Re: [PATCH 0/5] ftrace: to kill a daemon

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Date:	Fri, 8 Aug 2008 20:36:02 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Roland McGrath <roland@...hat.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	Clark Williams <williams@...hat.com>
Subject: Re: [PATCH 0/5] ftrace: to kill a daemon


On Sat, 9 Aug 2008, Andi Kleen wrote:

> Steven Rostedt <rostedt@...dmis.org> writes:
> 
> > I'm stubborn, I want to get it right _and_ keep it fast.
> 
> For me it would seem better to just not use two part 5 byte nops
> instead of adding such hacks.  I doubt there are that many of them
> anyways. I bet you won't be able to measure any difference between the
> different nop types in any macro benchmark.

I wish we had a true 5 byte nop. The alternative is a jmp 0, which is 
measurable.  This is replacing mcount from a kernel compile with the -pg 
option. With a basic build (not counting modules), I have over 15,000 
locations that are turned into these 5 byte nops.

# objdump -dr vmlinux.o | grep mcount |wc
  15152   45489  764924

If we use the jmp 0, then yes, we will see the overhead. The double nop 
that is used for 5 bytes, is significantly better than the jump.

-- Steve

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