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SubjectRe: [RFC] Stupid tracepoint ideas

* Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:

> * 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. :-)

There are some kernel developers who are also GCC developers - but i
have to say the choice for a good developer is rather obvious: in
the Linux kernel project the maximum latency until an obviously good
patch hits upstream is around 3 months. In the GCC space the
_minimum_ latency until an obviously good feature hits the compiler
tends to be more like 2-3 years in the typical case.

I think the solution is obvious: the kernel needs its own compiler.

Ingo


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