Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934107AbZDJLRp (ORCPT ); Fri, 10 Apr 2009 07:17:45 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759871AbZDJLRe (ORCPT ); Fri, 10 Apr 2009 07:17:34 -0400 Received: from mail-fx0-f158.google.com ([209.85.220.158]:42552 "EHLO mail-fx0-f158.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757909AbZDJLRb (ORCPT ); Fri, 10 Apr 2009 07:17:31 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=date:from:to:cc:subject:message-id:references:mime-version :content-type:content-disposition:in-reply-to:user-agent; b=Ry/yTsl7yBRS0eCSz0pn6ZyTOGXllTxOnPu7XhrEiBp/nQtB5ldCd7QXrRa1FOIxAB bxP8NOTFDRor4A5v5ZWhKQuCVzjnnuZ3xbPBERllfbM/RlveeDln3xivb43MKjGam0v5 JyX9wA4zPpai96aHe6/N6DSuXjw3OgjJjGc2c= Date: Fri, 10 Apr 2009 15:17:33 +0400 From: Alexey Dobriyan To: Ingo Molnar Cc: Matt Helsley , Tejun Heo , Jeremy Fitzhardinge , akpm@linux-foundation.org, containers@lists.linux-foundation.org, xemul@parallels.com, linux-kernel@vger.kernel.org, dave@linux.vnet.ibm.com, hch@infradead.org, torvalds@linux-foundation.org Subject: Re: [PATCH 09/30] x86_64: ifdef out struct thread_struct::ip Message-ID: <20090410111733.GA3311@x200.localdomain> References: <20090410023522.GJ27788@x200.localdomain> <20090410035328.GB29496@us.ibm.com> <20090410091931.GF17962@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090410091931.GF17962@elte.hu> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2229 Lines: 69 On Fri, Apr 10, 2009 at 11:19:31AM +0200, Ingo Molnar wrote: > > * 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 > > > --- > > > > > > 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. You should have thought about it when i386/x86_64 unification was introduced. pagefault code was full of ifdefs (it's less now), and this is trivial ifdef in a header. > > [...] 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/