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

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Date:	Fri, 10 Apr 2009 11:19:31 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Matt Helsley <matthltc@...ibm.com>, Tejun Heo <tj@...nel.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	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


* Matt Helsley <matthltc@...ibm.com> 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. [...]

Removing fields always saves memory (even if it does not show up 
currently due to allocators cache-aligning sizes).

But the #ifdef ugliness is a real worry.

> [...] If these #ifdefs don't save any 
> space why not just put in a comment:
> 
> >  	unsigned long		ip; /* Used only on i386 */

Yes.

> Or maybe even:
> 
> 	union {
> 	  	unsigned long		ip; /* Used only on i386 */
> 	  	unsigned long		fs; /* Used only on x86_64 */
> 	};

Maybe. If this ever gets misunderstood somewhere in platform code we 
will get ugly failure modes and zero compiler help.

	Ingo
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