linux-kernel - Re: [RFC PATCH] clocksource: exynos_mct: use container_of() instead of this_cpu

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Date:	Thu, 10 Sep 2015 13:21:23 +0900
From:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
To:	Alexey Klimov <alexey.klimov@...aro.org>,
	daniel.lezcano@...aro.org, linux-samsung-soc@...r.kernel.org
Cc:	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	yury.norov@...il.com, klimov.linux@...il.com, kgene@...nel.org
Subject: Re: [RFC PATCH] clocksource: exynos_mct: use container_of() instead of
 this_cpu_ptr()

On 04.09.2015 08:49, Alexey Klimov wrote:
> Since evt structure is embedded in per-CPU mevt structure it's
> definitely faster to use container_of() to get access to mevt
> if we have evt (for example as incoming function argument) instead
> of more expensive approach with this_cpu_ptr(&percpu_mct_tick).
> this_cpu_ptr() on per-CPU mevt structure leads to access to cp15
> to get cpu id and arithmetic operations.
> Container_of() is cheaper since it's just one asm instruction.
> This should work if used evt pointer is correct and owned by
> local mevt structure.
> 
> For example, before this patch set_state_shutdown() looks like:
> 
>  4a4:	e92d4010 	push	{r4, lr}
>  4a8:	e3004000 	movw	r4, #0
>  4ac:	ebfffffe 	bl	0 <debug_smp_processor_id>
>  4b0:	e3003000 	movw	r3, #0
>  4b4:	e3404000 	movt	r4, #0
>  4b8:	e3403000 	movt	r3, #0
>  4bc:	e7933100 	ldr	r3, [r3, r0, lsl #2]
>  4c0:	e0844003 	add	r4, r4, r3
>  4c4:	e59400c0 	ldr	r0, [r4, #192]	; 0xc0
>  4c8:	ebffffd4 	bl	420 <exynos4_mct_tick_stop.isra.1>
>  4cc:	e3a00000 	mov	r0, #0
>  4d0:	e8bd8010 	pop	{r4, pc}
> 
> With this patch:
> 
>  4a4:	e92d4010 	push	{r4, lr}
>  4a8:	e59000c0 	ldr	r0, [r0, #192]	; 0xc0
>  4ac:	ebffffdb 	bl	420 <exynos4_mct_tick_stop.isra.1>
>  4b0:	e3a00000 	mov	r0, #0
>  4b4:	e8bd8010 	pop	{r4, pc}
> 
> Also, for me size of exynos_mct.o decreased from 84588 bytes
> to 83956.
> 
> Signed-off-by: Alexey Klimov <alexey.klimov@...aro.org>
> ---
>  drivers/clocksource/exynos_mct.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)

Looks good and sensible. Why you called this RFC? You are not sure if
this is correct?

One minor nit-pick below, but I am fine without it anyway:

Reviewed-by: Krzysztof Kozlowski <k.kozlowski@...sung.com>


> 
> diff --git a/drivers/clocksource/exynos_mct.c b/drivers/clocksource/exynos_mct.c
> index 029f96a..ff44082 100644
> --- a/drivers/clocksource/exynos_mct.c
> +++ b/drivers/clocksource/exynos_mct.c
> @@ -382,24 +382,28 @@ static void exynos4_mct_tick_start(unsigned long cycles,
>  static int exynos4_tick_set_next_event(unsigned long cycles,
>  				       struct clock_event_device *evt)
>  {
> -	struct mct_clock_event_device *mevt = this_cpu_ptr(&percpu_mct_tick);
> +	struct mct_clock_event_device *mevt;
>  
> +	mevt = container_of(evt, struct mct_clock_event_device, evt);
>  	exynos4_mct_tick_start(cycles, mevt);
> -

Actually I would prefer leaving the empty line here and add such in
function below. For me the code is more readable with
ending return separated by one line.

Best regards,
Krzysztof

>  	return 0;
>  }
>  
>  static int set_state_shutdown(struct clock_event_device *evt)
>  {
> -	exynos4_mct_tick_stop(this_cpu_ptr(&percpu_mct_tick));
> +	struct mct_clock_event_device *mevt;
> +
> +	mevt = container_of(evt, struct mct_clock_event_device, evt);
> +	exynos4_mct_tick_stop(mevt);
>  	return 0;
>  }
>  
>  static int set_state_periodic(struct clock_event_device *evt)
>  {
> -	struct mct_clock_event_device *mevt = this_cpu_ptr(&percpu_mct_tick);
> +	struct mct_clock_event_device *mevt;
>  	unsigned long cycles_per_jiffy;
>  
> +	mevt = container_of(evt, struct mct_clock_event_device, evt);
>  	cycles_per_jiffy = (((unsigned long long)NSEC_PER_SEC / HZ * evt->mult)
>  			    >> evt->shift);
>  	exynos4_mct_tick_stop(mevt);
> 

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