linux-kernel - Re: [patch 1/3]make readmostly section correctly align

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Message-Id: <20101201182854.dd70bdf2.akpm@linux-foundation.org>
Date:	Wed, 1 Dec 2010 18:28:54 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	"hpa@...or.com" <hpa@...or.com>, sam@...nborg.org,
	eric.dumazet@...il.com
Subject: Re: [patch 1/3]make readmostly section correctly align

On Thu, 02 Dec 2010 10:02:21 +0800 Shaohua Li <shaohua.li@...el.com> wrote:

> readmostly section should end at cache line aligned address, otherwise the last
> several data might share cachline with other data and make the readmostly data
> still have cache bounce.
> For example, in ia64, secpath_cachep is the last readmostly data, and it shares
> cacheline with init_uts_ns.
> a000000100e80480 d secpath_cachep
> a000000100e80488 D init_uts_ns
> 
> Signed-off-by: Shaohua Li <shaohua.li@...el.com>
> 
> ---
>  include/asm-generic/vmlinux.lds.h |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> Index: linux/include/asm-generic/vmlinux.lds.h
> ===================================================================
> --- linux.orig/include/asm-generic/vmlinux.lds.h	2010-12-01 16:49:48.000000000 +0800
> +++ linux/include/asm-generic/vmlinux.lds.h	2010-12-02 09:22:32.000000000 +0800
> @@ -192,7 +192,8 @@
>  
>  #define READ_MOSTLY_DATA(align)						\
>  	. = ALIGN(align);						\
> -	*(.data..read_mostly)
> +	*(.data..read_mostly)						\
> +	. = ALIGN(align);
>  
>  #define CACHELINE_ALIGNED_DATA(align)					\
>  	. = ALIGN(align);						\
> 

Surely the sane way to do this is to ensure that each section *starts*
at an at-least-cacheline aligned address and then not worry about how
the section ends.  So shouldn't we be fixing DATA_DATA?

With your approach, .data may end up sharing a cacheline with
some other section in some undesirable manner on a different arch.

"fixing" DATA_DATA would involve page-aligning it, which sucks a bit. 
Things would be better if include/asm-generic/vmlinux.lds.h had access
to a globally-agreed cacheline-size, as it does the page size.

otoh, .data.read_mostly is "special", in that it wants the tail end of
its last cacheline not to share with any other section.  Most other
sections aren't like that.

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