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

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Message-ID: <55F7DB35.9070803@linaro.org>
Date:	Tue, 15 Sep 2015 10:47:49 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Alexey Klimov <alexey.klimov@...aro.org>, k.kozlowski@...sung.com,
	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 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@...aro.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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