linux-kernel - Re: [PATCH 09/30] x86_64: ifdef out struct thread

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Message-ID: <20090410035328.GB29496@us.ibm.com>
Date:	Thu, 9 Apr 2009 20:53:28 -0700
From:	Matt Helsley <matthltc@...ibm.com>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	akpm@...ux-foundation.org, containers@...ts.linux-foundation.org,
	xemul@...allels.com, linux-kernel@...r.kernel.org,
	dave@...ux.vnet.ibm.com, hch@...radead.org, mingo@...e.hu,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH 09/30] x86_64: ifdef out struct thread_struct::ip

On Fri, Apr 10, 2009 at 06:35:22AM +0400, Alexey Dobriyan wrote:
> struct thread_struct::ip isn't used on x86_64, struct pt_regs::ip is used
> instead. 
> 
> kgdb should be reading 0, but I can't check it.
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
> ---
> 
>  arch/x86/include/asm/processor.h |    2 ++
>  arch/x86/kernel/kgdb.c           |    2 +-
>  2 files changed, 3 insertions(+), 1 deletion(-)
> 
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -421,7 +421,9 @@ struct thread_struct {
>  	unsigned short		fsindex;
>  	unsigned short		gsindex;
>  #endif
> +#ifdef CONFIG_X86_32
>  	unsigned long		ip;
> +#endif
>  #ifdef CONFIG_X86_64
>  	unsigned long		fs;
>  #endif

Do these make struct thread_struct behave better in cachelines (smaller,
less aliasing)? Can we really fit more in the slab du jour?

Otherwise it seems like we're littering these structs with #ifdefs
and not really saving anything. If these #ifdefs don't save any space why not 
just put in a comment:

>  	unsigned long		ip; /* Used only on i386 */

Or maybe even:

	union {
	  	unsigned long		ip; /* Used only on i386 */
	  	unsigned long		fs; /* Used only on x86_64 */
	};

Cheers,
	-Matt Helsley
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