linux-kernel - [PATCH] Cache miss eliminating in WARN_ON

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Date:	Wed, 4 Oct 2006 17:19:42 +0400
From:	"Ananiev, Leonid I" <leonid.i.ananiev@...el.com>
To:	"Andrew Morton" <akpm@...l.org>, <andrew.j.wade@...il.com>
Cc:	<tim.c.chen@...ux.intel.com>, <herbert@...dor.apana.org.au>,
	<linux-kernel@...r.kernel.org>
Subject:  [PATCH] Cache miss eliminating in WARN_ON_ONCE

The static variable __warn_once was "never" read (until there is no bug)
before patch "Let WARN_ON/WARN_ON_ONCE return the condition"
http://kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commi
t;h=684f978347deb42d180373ac4c427f82ef963171
 in WARN_ON_ONCE's line 
- if (unlikely((condition) && __warn_once)) { \
because 'condition' is false. There was no cache miss as a result.

Cache miss for __warn_once is happened in new lines
+ if (likely(__warn_once)) \
+ if (WARN_ON(__ret_warn_once)) \

Proposed patch is tested by tbench.

>From Leonid Ananiev

Cache miss eliminating in WARN_ON_ONCE  by testing expression value
before static variable value.

Signed-off-by: Leonid Ananiev <leonid.i.ananiev@...el.com>
--- linux-2.6.18-git14/include/asm-generic/bug.h
+++ linux-2.6.18-git14b/include/asm-generic/bug.h
@@ -45,9 +45,12 @@
        static int __warn_once = 1;                     \
        typeof(condition) __ret_warn_once = (condition);\
                                                        \
-       if (likely(__warn_once))                        \
-               if (WARN_ON(__ret_warn_once))           \
+       /* check 'condition' first to avoid cache miss with __warn_once
*/\
+       if (unlikely(__ret_warn_once))                  \
+               if (__warn_once) {                      \
                        __warn_once = 0;                \
+                       WARN_ON(1);                     \
+               }                                       \
        unlikely(__ret_warn_once);                      \
 })
-
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