Re: [PATCH 1/3] x86-64: Convert the PDA to percpu.

[Brian Gerst]

On Sat, Dec 27, 2008 at 5:41 AM, Ingo Molnar <mingo@xxxxxxx> wrote:
>
> (Cc:-ed a few more people who might be interested in this)
>
> * Brian Gerst <brgerst@xxxxxxxxx> 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@xxxxxxxxx>
>> ---
>>  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.

--
Brian Gerst
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