linux-kernel - Re: [PATCH V2 1/2] sched: idle: Encapsulate the code to compile it out

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Date:	Fri, 02 May 2014 14:09:11 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	peterz@...radead.org, mingo@...e.hu, amit.kucheria@...aro.org,
	linaro-kernel@...ts.linaro.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Nicolas Pitre <nicolas.pitre@...aro.org>
Subject: Re: [PATCH V2 1/2] sched: idle: Encapsulate the code to compile it out

On Friday, May 02, 2014 10:59:11 AM Daniel Lezcano wrote:
> On 05/01/2014 12:56 AM, Rafael J. Wysocki wrote:
> > On Thursday, May 01, 2014 12:47:25 AM Rafael J. Wysocki wrote:
> >> On Wednesday, April 30, 2014 02:01:02 PM Daniel Lezcano wrote:
> >>> Encapsulate the large portion of cpuidle_idle_call inside another
> >>> function so when CONFIG_CPU_IDLE=n, the code will be compiled out.
> >>> Also that is benefitial for the clarity of the code as it removes
> >>> a nested indentation level.
> >>>
> >>> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> >>
> >> Well, this conflicts with
> >>
> >> https://patchwork.kernel.org/patch/4071541/
> >>
> >> which you haven't commented on and I still want cpuidle_select() to be able to
> >> return negative values because of
> >>
> >> https://patchwork.kernel.org/patch/4089631/
> >>
> >> (and I have one more patch on top of these two that requires this).
> >
> > Moreover (along the lines of Nico said) after https://patchwork.kernel.org/patch/4071541/
> > we actually don't need the #ifdef CONFIG_CPU_IDLE in your patch, because cpuidle_select()
> > for CONFIG_CPU_IDLE unset is a static inline returning a negative number and the compiler
> > should optimize out the blocks that depend on it being non-negative.
> 
> Thanks for the head up.
> 
> Actually that was to solve a compilation issue with the next patch when 
> adding the cpuidle state in the struct rq.
> 
> When the option CPU_IDLE is not set, the code assinging the cpu idle 
> state in the rq is still there while in the struct rq the field is 
> compiled out with the ifdef macro. If I rely on the compiler 
> optimization, the compilation error will happen.

I see.

If you don't put the new idle_state field in struct_rq under the #ifdef,
you won't need to worry about the build problem.

Alternatively, you can define

#ifdef CONFIG_CPU_IDLE
static inline void rq_set_idle_state(struct rq *rq, struct cpuidle_state *state)
{
	rq->idle_state = state;
}
#else
static inline void rq_set_idle_state(struct rq *rq, struct cpuidle_state *state) {}
#endif

and use rq_set_idle_state() to set that field.

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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