static keys: Introduce 'struct static_key', static_key_true()/false() and static_key_slow_[inc|dec]()

So here's a boot tested patch on top of Jason's series that does
all the cleanups I talked about and turns jump labels into a
more intuitive to use facility. It should also address the
various misconceptions and confusions that surround jump labels.

Typical usage scenarios:

        #include <linux/static_key.h>

        struct static_key key = STATIC_KEY_INIT_TRUE;

        if (static_key_false(&key))
                do unlikely code
        else
                do likely code

Or:

        if (static_key_true(&key))
                do likely code
        else
                do unlikely code

The static key is modified via:

        static_key_slow_inc(&key);
        ...
        static_key_slow_dec(&key);

The 'slow' prefix makes it abundantly clear that this is an
expensive operation.

I've updated all in-kernel code to use this everywhere. Note
that I (intentionally) have not pushed through the rename
blindly through to the lowest levels: the actual jump-label
patching arch facility should be named like that, so we want to
decouple jump labels from the static-key facility a bit.

On non-jump-label enabled architectures static keys default to
likely()/unlikely() branches.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Jason Baron <jbaron@redhat.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: a.p.zijlstra@chello.nl
Cc: mathieu.desnoyers@efficios.com
Cc: davem@davemloft.net
Cc: ddaney.cavm@gmail.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20120222085809.GA26397@elte.hu
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 20 insertions, 9 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