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SubjectRe: [RFC] Stupid tracepoint ideas

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.

>
> 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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