[RFC PATCH] clocksource: exynos_mct: use container_of() instead of this_cpu_ptr()

[RFC PATCH] clocksource: exynos_mct: use container_of() instead of this_cpu_ptr()

[Alexey Klimov]

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@linaro.org>
---
 drivers/clocksource/exynos_mct.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

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);
-
 	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);
-- 
2.1.4

Re: [RFC PATCH] clocksource: exynos_mct: use container_of() instead of this_cpu_ptr()

[Krzysztof Kozlowski]

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@linaro.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@samsung.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);
> 

Re: [RFC PATCH] clocksource: exynos_mct: use container_of() instead of this_cpu_ptr()

[Daniel Lezcano]

On 09/04/2015 01:49 AM, 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@linaro.org>
> ---

Despite the RFC I applied the patch as it seems simple enough and it has 
been reviewed by Krzysztof.

   -- Daniel


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