linux-kernel - [RFC] Stupid tracepoint ideas

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Date:	Mon, 20 Apr 2009 15:04:22 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	LKML <linux-kernel@...r.kernel.org>
cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	mingo@...dmis.org, Frederic Weisbecker <fweisbec@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Masami Hiramatsu <mhiramat@...hat.com>
Subject: [RFC] Stupid tracepoint ideas


Mathieu,

You may have tried this in your creation of tracepoints, but I figured I 
would ask before wasting too much time on it.

I'm looking at ways to make tracepoints even lighter weight when disabled. 
And I thought of doing section code. I'm playing with the following idea 
(see below patch) but I'm afraid gcc is allowed to think that the code it 
produces will not move to different sections.

Any thoughts on how we could do something similar to this.

Note, this patch is purely proof-of-concept. I'm fully aware that it is an 
x86 solution only.

-- Steve

[ no Signed-off-by: because this patch is crap ]

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 4353f3f..6953f78 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -65,9 +65,18 @@ struct tracepoint {
 	extern struct tracepoint __tracepoint_##name;			\
 	static inline void trace_##name(proto)				\
 	{								\
-		if (unlikely(__tracepoint_##name.state))		\
+		if (unlikely(__tracepoint_##name.state)) {		\
+			asm volatile ("jmp 43f\n"			\
+				      "42:\n"				\
+				      ".section .unlikely,\"ax\"\n"	\
+				      "43:\n"				\
+				      ::: "memory");			\
 			__DO_TRACE(&__tracepoint_##name,		\
-				TP_PROTO(proto), TP_ARGS(args));	\
+				   TP_PROTO(proto), TP_ARGS(args));	\
+			asm volatile ("jmp 42b\n"			\
+			     ".previous\n"				\
+			     ::: "memory");				\
+		}							\
 	}								\
 	static inline int register_trace_##name(void (*probe)(proto))	\
 	{								\
--
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