Re: [PATCH RFC 01/10] rcu: Make CONFIG_SRCU unconditionally enabled

Stephen Rothwell
2018-08-08T11:05:05Z

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Hi Kirill,

On Wed, 8 Aug 2018 12:59:40 +0300 Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>
> On 08.08.2018 04:08, Stephen Rothwell wrote:
> >
> > So what sort of overheads (in terms of code size and performance) are
> > we adding by having SRCU enabled where it used not to be?
>
> SRCU is unconditionally enabled for x86, so I had to use another arch (sparc64)
> to check the size difference. The config, I used to compile, is attached, SRCU
> was enabled via:
>
> diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
> index 2d58c26bff9a..6e9116e356d4 100644
> --- a/arch/sparc/Kconfig
> +++ b/arch/sparc/Kconfig
> @@ -15,6 +15,7 @@ config SPARC
> select ARCH_MIGHT_HAVE_PC_PARPORT if SPARC64 && PCI
> select ARCH_MIGHT_HAVE_PC_SERIO
> select OF
> + select SRCU
> select OF_PROMTREE
> select HAVE_IDE
> select HAVE_OPROFILE
>
> $ size image.srcu.disabled
> text data bss dec hex filename
> 5117546 8030506 1968104 15116156 e6a77c image.srcu.disabled
>
> $ size image.srcu.enabled
> text data bss dec hex filename
> 5126175 8064346 1968104 15158625 e74d61 image.srcu.enabled
>
> The difference is: 15158625-15116156 = 42469 ~41Kb

Thanks for that.

> I have not ideas about performance overhead measurements. If you have ideas,
> where they may occur, please say. At the first sight, there should not be
> a problem, since SRCU is enabled in x86 by default.

I have no idea, just asking questions that might be relevant for
platforms where SRCU is normally disabled.

--
Cheers,
Stephen Rothwell

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