lkml.org 
[lkml]   [2012]   [Feb]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[PATCH] jump labels: Explain the .config option better

- update the help text as it's not only about trace points anymore

- explain the functional effects of the option prominently,
treating the implementational details as a second level of
information

Additionally we might also consider renaming CONFIG_JUMP_LABEL
to:

CONFIG_OPTIMIZE_STATIC_BRANCHES=y

or so, to make it clearer what it's all really about and makes
it part of the family of similarly named optimizations - but I'm
not too concerned about that, as CONFIG_JUMP_LABEL does not
appear in actual static-key usage sites.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
arch/Kconfig | 27 +++++++++++++++++++--------
1 files changed, 19 insertions(+), 8 deletions(-)
diff --git a/arch/Kconfig b/arch/Kconfig
index 4f55c73..5b448a7 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -47,18 +47,29 @@ config KPROBES
If in doubt, say "N".

config JUMP_LABEL
- bool "Optimize trace point call sites"
+ bool "Optimize very unlikely/likely branches"
depends on HAVE_ARCH_JUMP_LABEL
help
+ This option enables a transparent branch optimization that
+ makes certain almost-always-true or almost-always-false branch
+ conditions even cheaper to execute within the kernel.
+
+ Certain performance-sensitive kernel code, such as trace points,
+ scheduler functionality, networking code and KVM have such
+ branches and include support for this optimization technique.
+
If it is detected that the compiler has support for "asm goto",
- the kernel will compile trace point locations with just a
- nop instruction. When trace points are enabled, the nop will
- be converted to a jump to the trace function. This technique
- lowers overhead and stress on the branch prediction of the
- processor.
-
- On i386, options added to the compiler flags may increase
- the size of the kernel slightly.
+ the kernel will compile such branches with just a nop
+ instruction. When the condition flag is toggled to true, the
+ nop will be converted to a jump instruction to execute the
+ conditional block of instructions.
+
+ This technique lowers overhead and stress on the branch prediction
+ of the processor and generally makes the kernel faster. The update
+ of the condition is slower, but those are always very rare.
+
+ ( On 32-bit x86, the necessary options added to the compiler
+ flags may increase the size of the kernel slightly. )

config OPTPROBES
def_bool y

\
 
 \ /
  Last update: 2012-02-22 10:05    [from the cache]
©2003-2011 Jasper Spaans