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Date: Mon, 20 Apr 2009 21:17:55 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>, mingo@goodmis.org,
       Frederic Weisbecker <fweisbec@gmail.com>,
       "H. Peter Anvin" <hpa@zytor.com>, Jeremy Fitzhardinge <jeremy@goop.org>,
       Masami Hiramatsu <mhiramat@redhat.com>
Subject: Re: [RFC] Stupid tracepoint ideas
Message-ID: <20090420191755.GA15571@elte.hu>
References: <alpine.DEB.2.00.0904201456470.26147@gandalf.stny.rr.com>
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* Steven Rostedt <rostedt@goodmis.org> wrote:

> 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))	\

does this boot fine? If yes then it would be nice to have a 
measurement.

	Ingo