linux-kernel - Re: [RFC 02/15] x86_64: Fold pda into per cpu area

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Date:	Sun, 13 Jul 2008 19:54:09 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Mike Travis <travis@....com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC 02/15] x86_64: Fold pda into per cpu area


* Eric W. Biederman <ebiederm@...ssion.com> wrote:

> Mike Travis <travis@....com> writes:
> 
> > WARNING: there is still a FIXME in this patch (see arch/x86/kernel/acpi/sleep.c)
> >
> >   * Declare the pda as a per cpu variable.
> >
> >   * Make the x86_64 per cpu area start at zero.
> >
> >   * Relocate the initial pda and per_cpu(gdt_page) in head_64.S for the
> >     boot cpu (0).  For secondary cpus, do_boot_cpu() sets up the correct
> >     initial pda and gdt_page pointer.
> >
> >   * Initialize per_cpu_offset to point to static pda in the per_cpu area
> >     (@ __per_cpu_load).
> >
> >   * After allocation of the per cpu area for the boot cpu (0), reload the
> >     gdt page pointer.
> >
> > Based on linux-2.6.tip/master
> 
> Given that we have not yet understood the weird failure case.  This patch needs
> to be split in two.  
> - make the current per cpu variable section zero based.
> - Move the pda into the per cpu variable section.
> 
> There are too many variables at present the reported failure cases to
> guess what is really going on.
> 
> We can not optimize the per cpu variable accesses until the pda moves
> but we can easily test for linker and tool chain bugs with zero
> based pda segment itself.

agreed, a patch of this gravity and with a diffstat:

 12 files changed, 112 insertions(+), 142 deletions(-)

is indeed too large. Test failures that get bisected to this patch will 
still cause people to guess about which aspect of the large patch caused 
the problem.

	Ingo
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