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

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Date:	Fri, 10 Apr 2009 11:20:00 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jaswinder Singh Rajput <jaswinder@...nel.org>
Cc:	Matt Helsley <matthltc@...ibm.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	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,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH 09/30] x86_64: ifdef out struct thread_struct::ip


* Jaswinder Singh Rajput <jaswinder@...nel.org> wrote:

> On Thu, 2009-04-09 at 20:53 -0700, Matt Helsley wrote:
> > 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 */
> > 	};
> > 
> 
> Can we do it like this:
>   	unsigned long		ip_fs; /* ip: i386, fs: x86_64 */
> 
> I am using same variable for both cases, or we can use some better 
> name than ip_fs. I am assuming either it is i386 or x86_64 machine 
> ;-)

This is the least clean variant amongst all the suggestions.

	Ingo
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