linux-kernel - [PATCH] jump labels: Explain the .config option better

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Date:	Wed, 22 Feb 2012 10:03:18 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"H. Peter Anvin" <hpa@...or.com>, Jason Baron <jbaron@...hat.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Jason Baron <jbaron@...hat.com>, a.p.zijlstra@...llo.nl,
	mathieu.desnoyers@...icios.com, davem@...emloft.net,
	ddaney.cavm@...il.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
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@...e.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
--
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