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

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Date:	Thu, 24 Mar 2016 16:06:00 +0000
From:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:	Jisheng Zhang <jszhang@...vell.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

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,
Lorenzo

> -
>  	ret = cpu_pm_enter();
>  	if (!ret) {
>  		/*
> @@ -69,7 +71,7 @@ static struct cpuidle_driver arm_idle_driver = {
>  	 * handler for idle state index 0.
>  	 */
>  	.states[0] = {
> -		.enter                  = arm_enter_idle_state,
> +		.enter                  = arm_enter_wfi_state,
>  		.exit_latency           = 1,
>  		.target_residency       = 1,
>  		.power_usage		= UINT_MAX,
> -- 
> 2.8.0.rc3
>