Messages in this thread | | | Date | Tue, 21 Apr 2009 09:21:47 +0200 | From | Ingo Molnar <> | Subject | Re: [RFC] Stupid tracepoint ideas |
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* 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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