Re: [patch 2/7] Immediate Values - Kconfig menu in EMBEDDED

Randy Dunlap
2007-09-18T22:07:28Z

On Tue, 18 Sep 2007 17:07:49 -0400 Mathieu Desnoyers wrote:

> Immediate values provide a way to use dynamic code patching to update variables
> sitting within the instruction stream. It saves caches lines normally used by
> static read mostly variables. Enable it by default, but let users disable it
> through the EMBEDDED menu with the "Disable immediate values" submenu entry.
>
> Note: Since I think that I really should let embedded systems developers using
> RO memory the option to disable the immediate values, I choose to leave this
> menu option there, in the EMBEDDED menu. Also, the "CONFIG_IMMEDIATE" makes
> sense because we want to compile out all the immediate code when we decide not
> to use optimized immediate values at all (it removes otherwise unused code).
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> CC: Adrian Bunk <bunk@stusta.de>
> CC: Andi Kleen <andi@firstfloor.org>
> CC: Alexey Dobriyan <adobriyan@gmail.com>
> CC: Christoph Hellwig <hch@infradead.org>
> ---
> init/Kconfig | 21 +++++++++++++++++++++
> 1 file changed, 21 insertions(+)
>
> Index: linux-2.6-lttng/init/Kconfig
> ===================================================================
> --- linux-2.6-lttng.orig/init/Kconfig 2007-09-18 10:09:40.000000000 -0400
> +++ linux-2.6-lttng/init/Kconfig 2007-09-18 13:14:58.000000000 -0400
> @@ -407,6 +407,17 @@ config CC_OPTIMIZE_FOR_SIZE
> config SYSCTL
> bool
>
> +config IMMEDIATE
> + default y if !DISABLE_IMMEDIATE

So DISABLE_IMMEDIATE is just an intermediate symbol to make this option
be displayed in the EMBEDDED menu instead of in the General Setup menu ?

> + depends on X86_32 || PPC || PPC64
> + bool
> + help
> + Immediate values are used as read-mostly variables that are rarely
> + updated. They use code patching to modify the values inscribed in the
> + instruction stream. It provides a way to save precious cache lines
> + that would otherwise have to be used by these variables. They can be
> + disabled through the EMBEDDED menu.
> +
> menuconfig EMBEDDED
> bool "Configure standard kernel features (for small systems)"
> help
> @@ -646,6 +657,16 @@ config PROC_KPAGEMAP
> information on page-level memory usage. Disabling this interface
> will reduce the size of the kernel by around 600 bytes.
>
> +config DISABLE_IMMEDIATE
> + default y if EMBEDDED
> + bool "Disable immediate values" if EMBEDDED
> + depends on X86_32 || PPC || PPC64
> + help
> + Disable code patching based immediate values for embedded systems. It
> + consumes slightly more memory and requires to modify the instruction
> + stream each time a variable is updated. Should really be disabled for
> + embedded systems with read-only text.
> +
> endmenu # General setup
>
> config RT_MUTEXES

---
~Randy
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