Messages in this thread | | | Date | Mon, 20 Apr 2009 17:06:15 -0400 (EDT) | From | Steven Rostedt <> | Subject | Re: [RFC] Stupid tracepoint ideas |
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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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