Messages in this thread | | | Date | Fri, 10 Apr 2009 11:19:31 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 09/30] x86_64: ifdef out struct thread_struct::ip |
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* 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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