Re: [PATCH][2.6] Completely out of line spinlocks / x86_64

[Andi Kleen <ak@suse.de>]

On Sun, 8 Aug 2004 02:08:30 -0400 (EDT)
Zwane Mwaikambo <zwane@linuxpower.ca> wrote:

>  arch/x86_64/Kconfig           |   10 ++++++++++
>  arch/x86_64/lib/Makefile      |    1 +
>  arch/x86_64/lib/spinlock.c    |   38 ++++++++++++++++++++++++++++++++++++++
>  include/asm-x86_64/spinlock.h |   22 ++++++++++++++++++++--
>  4 files changed, 69 insertions(+), 2 deletions(-)
> 
> Index: linux-2.6.8-rc3-mm1-amd64/arch/x86_64/Kconfig
> ===================================================================
> RCS file: /home/cvsroot/linux-2.6.8-rc3-mm1/arch/x86_64/Kconfig,v
> retrieving revision 1.1.1.1
> diff -u -p -B -r1.1.1.1 Kconfig
> --- linux-2.6.8-rc3-mm1-amd64/arch/x86_64/Kconfig	5 Aug 2004 16:37:48 -0000	1.1.1.1
> +++ linux-2.6.8-rc3-mm1-amd64/arch/x86_64/Kconfig	7 Aug 2004 22:47:30 -0000
> @@ -438,6 +438,16 @@ config DEBUG_SPINLOCK
>  	  best used in conjunction with the NMI watchdog so that spinlock
>  	  deadlocks are also debuggable.
> 
> +config COOL_SPINLOCK
> +	bool "Completely out of line spinlocks"
> +	depends on SMP
> +	default y
> +	help
> +	  Say Y here to build spinlocks which have common text for contended
> +	  and uncontended paths. This reduces kernel text size by at least
> +	  50k on most configurations, plus there is the additional benefit
> +	  of better cache utilisation.

I think the 50k number is wrong. I took a look at it and the big 
difference is only seen when you enable interrupts during spinning, which
we didn't do before.  If you compare it to the old implementation the
difference is much less.

I don't really like the config option. Either it's a good idea
then it should be done by default without option or it should not be done at all.

Did you do any lock intensive benchmarks that could show a slowdown?

> Index: linux-2.6.8-rc3-mm1-amd64/arch/x86_64/lib/spinlock.c
> ===================================================================
> RCS file: linux-2.6.8-rc3-mm1-amd64/arch/x86_64/lib/spinlock.c
> diff -N linux-2.6.8-rc3-mm1-amd64/arch/x86_64/lib/spinlock.c
> --- /dev/null	1 Jan 1970 00:00:00 -0000
> +++ linux-2.6.8-rc3-mm1-amd64/arch/x86_64/lib/spinlock.c	8 Aug 2004 05:39:04 -0000
> @@ -0,0 +1,38 @@
> +#include <linux/module.h>

You should make this file assembly only. 


-Andi