linux-kernel - Re: [PATCH v4 8/9] KVM-GST: adjust scheduler cpu power

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Date:	Sun, 03 Jul 2011 16:20:27 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Glauber Costa <glommer@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Rik van Riel <riel@...hat.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Anthony Liguori <aliguori@...ibm.com>,
	Eric B Munson <emunson@...bm.net>, Jeff Law <law@...hat.com>
Subject: Re: [PATCH v4 8/9] KVM-GST: adjust scheduler cpu power

On 07/02/2011 01:24 PM, Peter Zijlstra wrote:
> static void update_rq_clock_task(struct rq *rq, s64 delta)
> {
> 	s64 irq_delta = 0, steal = 0;
>
> 	rq->clock_task += delta;
>
> 	if ((irq_delta + steal)&&  sched_feat(NONTASK_POWER))
> 		sched_rt_avg_update(rq, irq_delta + steal);
> }
>
> And we want it to emit the equivalent of:
>
> static void update_rq_clock_task(struct rq *rq, s64 delta)
> {
> 	rq->clock_task += delta;
> }
>
> Now Glauber is properly paranoid and doesn't trust his compiler (this is
> very hot code in the kernel so any extra code emitted here is sad) and
> chose the heavy handed CPP solution.
>
> Now without checking a all relevant gcc versions on all relevant
> architectures (see you in a few weeks etc..) can we actually rely on gcc
> doing such relatively simple things correct, or should we stick with CPP
> just to make sure?

I'm pretty sure any relevant version of gcc will do the right optimizations.

-- 
error compiling committee.c: too many arguments to function

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