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

Mathieu Desnoyers
2007-09-19T11:13:29Z

* Randy Dunlap (randy.dunlap@oracle.com) wrote:
> 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 ?
>

I simply prefer to use a positive logic (CONFIG_IMMEDIATE) rather than
the opposite. It makes makefiles and ifdefs much cleaner. Actually, the
IMMEDIATE config option is not shown in any menu at all; it is totally
controlled by DISABLE_IMMEDIATE. And yes, DISABLE_IMMEDIATE appears in
the embedded 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
> *** Remember to use Documentation/SubmitChecklist when testing your code ***