Messages in this thread | | | Date | Sat, 27 Dec 2008 16:53:39 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 1/3] x86-64: Convert the PDA to percpu. |
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* Brian Gerst <brgerst@gmail.com> wrote:
> On Sat, Dec 27, 2008 at 5:41 AM, Ingo Molnar <mingo@elte.hu> wrote: > > > > (Cc:-ed a few more people who might be interested in this) > > > > * Brian Gerst <brgerst@gmail.com> wrote: > > > >> This patch makes the PDA a normal per-cpu variable, allowing the > >> removal of the special allocator code. %gs still points to the > >> base of the PDA. > >> > >> Tested on a dual-core AMD64 system. > >> > >> Signed-off-by: Brian Gerst <brgerst@gmail.com> > >> --- > >> arch/x86/include/asm/pda.h | 3 -- > >> arch/x86/include/asm/percpu.h | 3 -- > >> arch/x86/include/asm/setup.h | 1 - > >> arch/x86/kernel/cpu/common.c | 6 ++-- > >> arch/x86/kernel/dumpstack_64.c | 8 ++-- > >> arch/x86/kernel/head64.c | 23 +------------ > >> arch/x86/kernel/irq.c | 2 +- > >> arch/x86/kernel/nmi.c | 2 +- > >> arch/x86/kernel/setup_percpu.c | 70 ++++++++-------------------------------- > >> arch/x86/kernel/smpboot.c | 58 +-------------------------------- > >> arch/x86/xen/enlighten.c | 2 +- > >> arch/x86/xen/smp.c | 12 +------ > >> 12 files changed, 27 insertions(+), 163 deletions(-) > > > > the simplification factor is significant. I'm wondering, have you measured > > the code size impact of this on say the defconfig x86 kernel? That will > > generally tell us how much worse optimizations the compiler does under > > this scheme. > > > > Ingo > > > > Patch #1 by itself doesn't change how the PDA is accessed, only how it > is allocated. The text size goes down significantly with patch #1, > but data goes up. Changing the PDA to cacheline-aligned (1a) brings > it back in line. > > text data bss dec hex filename > 7033648 1754476 758508 9546632 91ab88 vmlinux.0 (vanilla 2.6.28) > 7029563 1758428 758508 9546499 91ab03 vmlinux.1 (with patch #1) > 7029563 1754460 758508 9542531 919b83 vmlinux.1a (with patch #1 cache align) > 7036694 1758428 758508 9553630 91c6de vmlinux.3 (with all three patches) > > I think the first patch (with the alignment fix) is a clear win. As for > the other patches, they add about 8 bytes per use of a PDA variable. > cpu_number is used 903 times in this compile, so this is likely the most > extreme example. I have an idea to optimize this particular case > further that I'd like to look at which would lessen the impact.
curious, what idea is that?
Ingo
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