linux-kernel - Re: [RFC] Stupid tracepoint ideas

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Message-ID: <alpine.DEB.2.00.0904201702430.10097@gandalf.stny.rr.com>
Date:	Mon, 20 Apr 2009 17:06:15 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	LKML <linux-kernel@...r.kernel.org>, 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: Re: [RFC] Stupid tracepoint ideas


On Mon, 20 Apr 2009, Mathieu Desnoyers wrote:

> * Steven Rostedt (rostedt@...dmis.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");				\
> > +		}							\
> 
> You are right, I thought of this.
> 
> gcc forbids jumping outside of inline assembly statements. Optimisations
> done by gcc are not aware of this sort of execution flow modification,
> and gcc has every rights to interleave unrelated code between the two
> inline assembly statements.

Yeah, I was afraid of that :-/

Would be nice to apply sections to code:

	__attribute__((section ".unlikely")) {
		/* code for .unlikely section */
	}

And have gcc do the jmps to and from the section.

This should not be too hard to implement.

> 
> And is it me or this sounds like an infinite loop ?
> 
> 42:
> ....
> jmp 42b
> 

Nope:

	jmp 43f
	42:
	.section ...
	43:
	jmp 42b
	.previous

is the same as:

	jmp 43f
	42:
	[...]


in the other section:

	43:
	jmp 42b

same as a return.

-- Steve

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