linux-kernel - Re: [RFC/PATCH] Prevent wasting time to find out get_parent

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Message-ID: <20120424234513.GA11841@leaf>
Date:	Tue, 24 Apr 2012 16:45:14 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Minho Ban <mhban@...sung.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Paul Turner <pjt@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC/PATCH] Prevent wasting time to find out get_parent_ip

On Wed, Apr 25, 2012 at 08:31:24AM +0900, Minho Ban wrote:
> On 04/24/2012 09:53 PM, Peter Zijlstra wrote:
> > On Tue, 2012-04-24 at 21:36 +0900, Minho Ban wrote:
> >> trace_preempt_on/off looks empty if PREEMPT_TRACER is off. But actually it is
> >> spending time to find out get_parent_ip(even CALLER_ADDR for some ARCH) which is
> >> in argument. This seems not fair for those who expect to do nothing but increase
> >> or decrease count.
> > 
> > You can do the same by making them CPP macros and adding a comment as to
> > why they're macros instead of inlines..
> > 
> 
> Thank you for pointing this out, certainly macros look better. I'll amend this.

As an alternative, how about making get_parent_ip and its called
functions static inlines?  Then the compiler can eliminate them via dead
code elimination.

Or, how about declaring get_parent_ip with the GCC "pure" attribute?
That would tell GCC that it can safely eliminate calls to the function.

- Josh Triplett
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