linux-kernel - Re: [PATCH] cpuidle: arm: make enter idle operation a bit more efficient

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Date:	Fri, 25 Mar 2016 14:25:13 +0800
From:	Jisheng Zhang <jszhang@...vell.com>
To:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
CC:	<rjw@...ysocki.net>, <daniel.lezcano@...aro.org>,
	<linux-pm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] cpuidle: arm: make enter idle operation a bit more
 efficient

Hi Lorenzo,

On Thu, 24 Mar 2016 16:06:00 +0000 Lorenzo Pieralisi wrote:

> On Thu, Mar 24, 2016 at 01:07:18PM +0800, Jisheng Zhang wrote:
> > Currently, entering idle need to check the idx every time to choose the
> > real entering idle routine. But this check could be avoided by pointing
> > the idle enter function pointer of each idle states to the routines
> > suitable for each states directly.
> > 
> > Signed-off-by: Jisheng Zhang <jszhang@...vell.com>
> > ---
> >  drivers/cpuidle/cpuidle-arm.c | 14 ++++++++------
> >  1 file changed, 8 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/cpuidle/cpuidle-arm.c b/drivers/cpuidle/cpuidle-arm.c
> > index 545069d..48a620f 100644
> > --- a/drivers/cpuidle/cpuidle-arm.c
> > +++ b/drivers/cpuidle/cpuidle-arm.c
> > @@ -23,6 +23,13 @@
> >  
> >  #include "dt_idle_states.h"
> >  
> > +static int arm_enter_wfi_state(struct cpuidle_device *dev,
> > +			       struct cpuidle_driver *drv, int idx)
> > +{
> > +	cpu_do_idle();
> > +	return 0;
> > +}
> > +
> >  /*
> >   * arm_enter_idle_state - Programs CPU to enter the specified state
> >   *
> > @@ -38,11 +45,6 @@ static int arm_enter_idle_state(struct cpuidle_device *dev,
> >  {
> >  	int ret;
> >  
> > -	if (!idx) {
> > -		cpu_do_idle();
> > -		return idx;
> > -	}  
> 
> Mmm...if I wanted to paint your bikeshed I would say idx is in a
> register and you are removing a simple comparison to exchange it
> with a function that adds to code footprint and may even make
> performance worse instead of improving anything.
> 
> I am not sure this patch makes anything more efficient, happy to be
> proven wrong, with significant data.

Thanks for pointing this out. I'll do some measurement and get back to you

Thanks,
Jisheng