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

From: Stephen Rothwell <sfr@canb.auug.org.au>
To: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: akpm@linux-foundation.org, gregkh@linuxfoundation.org,
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Subject: Re: [PATCH RFC 01/10] rcu: Make CONFIG_SRCU unconditionally enabled
Date: Wed, 8 Aug 2018 21:04:50 +1000	[thread overview]
Message-ID: <20180808210450.02108372@canb.auug.org.au> (raw)
In-Reply-To: <9ac119a7-4142-3a5a-6c4b-6f35ad026cb0@virtuozzo.com>

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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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