linux-kernel - [tip:tracing/printk] tracing: optimize trace

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Date:	Mon, 9 Mar 2009 09:12:34 GMT
From:	Ingo Molnar <mingo@...e.hu>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...hat.com,
	peterz@...radead.org, fweisbec@...il.com, rostedt@...dmis.org,
	tglx@...utronix.de, mingo@...e.hu
Subject: [tip:tracing/printk] tracing: optimize trace_printk()

Commit-ID:  7bffc23e56e92c14b787bf4d95249a32085bfed5
Gitweb:     http://git.kernel.org/tip/7bffc23e56e92c14b787bf4d95249a32085bfed5
Author:     "Ingo Molnar" <mingo@...e.hu>
AuthorDate: Mon, 9 Mar 2009 10:11:36 +0100
Commit:     Ingo Molnar <mingo@...e.hu>
CommitDate: Mon, 9 Mar 2009 10:11:36 +0100

tracing: optimize trace_printk()

Impact: micro-optimization

trace_printk() does this unconditionally:

	trace_printk_fmt = fmt;

Where trace_printk_fmt is an entry into a global array. This is
very SMP-unfriendly.

So only write it once per bootup.

Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Peter Zijlstra <peterz@...radead.org>
LKML-Reference: <1236356510-8381-5-git-send-email-fweisbec@...il.com>
Signed-off-by: Ingo Molnar <mingo@...e.hu>


---
 include/linux/kernel.h |   10 ++++++++--
 1 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 4e726b9..7742798 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -454,7 +454,10 @@ do {									\
 do {									\
 	static const char *trace_printk_fmt				\
 	__attribute__((section("__trace_printk_fmt")));			\
-	trace_printk_fmt = fmt;					\
+									\
+	if (!trace_printk_fmt)						\
+		trace_printk_fmt = fmt;					\
+									\
 	__trace_printk_check_format(fmt, ##args);			\
 	__trace_printk(_THIS_IP_, trace_printk_fmt, ##args);		\
 } while (0)
@@ -467,7 +470,10 @@ __trace_printk(unsigned long ip, const char *fmt, ...)
 do {									\
 	static const char *trace_printk_fmt				\
 	__attribute__((section("__trace_printk_fmt")));			\
-	trace_printk_fmt = fmt;					\
+									\
+	if (!trace_printk_fmt)						\
+		trace_printk_fmt = fmt;					\
+									\
 	__ftrace_vprintk(_THIS_IP_, trace_printk_fmt, vargs);		\
 } while (0)
 
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