Re: [RFC] Stupid tracepoint ideas

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Steven Rostedt (rostedt@goodmis.org) wrote:
> 
> On Mon, 20 Apr 2009, Mathieu Desnoyers wrote:
> 
> > * 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");				\
> > > +		}							\
> > 
> > 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.
> 

Yes, but for some reason no kernel developer I know seems to be very
keen of digging into gcc's internals. :-)

> > 
> > 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.
> 

Ah ! I knew I had to be missing something :)

Mathieu


> -- Steve
> 

-- 
Mathieu Desnoyers
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