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SubjectRe: [PATCH 09/30] x86_64: ifdef out struct thread_struct::ip

* Matt Helsley <matthltc@us.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@gmail.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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