Messages in this thread Patch in this message | | | Date | Thu, 16 Apr 2009 23:05:30 -0400 | From | Mathieu Desnoyers <> | Subject | [PATCH] tracepoints : let subsystem nop-out the tracepoints at build time |
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* Jeremy Fitzhardinge (jeremy@goop.org) wrote: > Mathieu Desnoyers wrote: >> "all this code" is actually : >> >> rcu_read_lock_sched_notrace(); \ >> it_func = rcu_dereference((tp)->funcs); \ >> if (it_func) { \ >> do { \ >> ((void(*)(proto))(*it_func))(args); \ >> } while (*(++it_func)); \ >> } \ >> rcu_read_unlock_sched_notrace(); \ >> >> Which does nothing more than disabling preemption and a for loop to >> call all the tracepoint handlers. I don't see the big win in laying out >> the stack to call this code out-of-line; we would just remove the >> preempt disable and the loop, which are minimal compared to most >> call stacks. >> > > Well, look at it from my perspective: Ingo has been repeatedly beating > me up for the overhead pvops adds to a native kernel, where it really is > just a (direct) function call. I want to instrument each pvop site with > a tracepoint so I can actually work out which calls are being called how > frequently to look for new optimisation opportunities. > > I would guess the tracepoint code sequence is going to increase the > impact of each pvop call site by a fair bit, and that's not counting the > effects the extra register pressure will have. That's a pile of code to > add. > > And frankly, that's fine by me, because I would expect this degree of > introspection to have some performance hit. But it does make the need > for per-subsystem tracing Kconfig entries fairly important, because I > don't think this would be acceptable to ship in a non-debug-everything > kernel build, even though other tracepoints might be. >
Agreed. Tracepoints might change the code surrounding the pvops in a similar fashion as the pvops themselves would change the code. Therefore, it makes sense to have a Kconfig option to enable the pvops tracepoints.
In terms of tracepoints (with the DECLARE_TRACE/DEFINE_TRACE semantic), we could have something like :
in include/trace/pvops.h :
#include <linux/tracepoint.h>
#ifdef CONFIG_PVOPS_TRACEPOINTS
#define DECLARE_PVOPS_TRACE DECLARE_TRACE #define DEFINE_PVOPS_TRACE DEFINE_TRACE #define EXPORT_PVOPS_TRACEPOINT_SYMBOL_GPL EXPORT_TRACEPOINT_SYMBOL_GPL #define EXPORT_PVOPS_TRACEPOINT_SYMBOL EXPORT_TRACEPOINT_SYMBOL
#else /* !CONFIG_PVOPS_TRACEPOINTS */
#define DECLARE_PVOPS_TRACE DECLARE_TRACE_NOP #define DEFINE_PVOPS_TRACE DEFINE_TRACE_NOP #define EXPORT_PVOPS_TRACEPOINT_SYMBOL_GPL EXPORT_TRACEPOINT_SYMBOL_GPL_NOP #define EXPORT_PVOPS_TRACEPOINT_SYMBOL EXPORT_TRACEPOINT_SYMBOL_NOP
#endif /* CONFIG_PVOPS_TRACEPOINTS */
And then do the declarations/definitions using the new
DECLARE_PVOPS_TRACE / DEFINE_PVOPS_TRACE.
For that you'll need the patch I am attaching below. I'll let Steven figure out how to tweak TRACE_EVENT() to support this new tracepoint feature.
>> So basically, tracepoints are already just doing a function call, with a >> few more bytes for preempt disable and multiple handler support. >> >> About the compiler deciding to put the unlikely branch out-of-line, I've >> never seen any function calls generated just for the sake of saving >> those few bytes, that would be crazy of the part of the compiler. >> However, it can (and should) freely put the stack setup in the coldest >> cache-lines possible, which are reachable by a near jump. >> > > No, it wouldn't generate a call. But if its going to put the code out > of line into cold cache-lines, then it may as well generate a call. >
Jumping out-of-line was somewhat faster than calling a function if I recall well my performance tests. But that's all been done long ago.
And note that whenever the tracer becomes active, the out-of-line code of busy tracepoints becomes cache-hot, which means that there is no more cache line fetch to perform, which leaves the stack setup and other overhead of function call/return vs 2*jump very measurable.
> Anyway, the important point from my perspective is that tracepoint.h > have no #include dependencies beyond linux/types.h (compiler.h, etc). >
Is preempt.h a problem ?
Here is the patch.
Mathieu
tracepoints : let subsystem nop-out the tracepoints at build time
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> CC: Jeremy Fitzhardinge <jeremy@goop.org> CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> CC: Steven Rostedt <rostedt@goodmis.org> CC: Ingo Molnar <mingo@elte.hu> CC: Andrew Morton <akpm@linux-foundation.org> CC: Christoph Hellwig <hch@lst.de> --- include/linux/tracepoint.h | 38 ++++++++++++++++++++++---------------- 1 file changed, 22 insertions(+), 16 deletions(-)
Index: linux.trees.git/include/linux/tracepoint.h =================================================================== --- linux.trees.git.orig/include/linux/tracepoint.h 2009-04-16 22:40:26.000000000 -0400 +++ linux.trees.git/include/linux/tracepoint.h 2009-04-16 22:40:33.000000000 -0400 @@ -37,6 +37,24 @@ struct tracepoint { #define TP_PROTO(args...) args #define TP_ARGS(args...) args +#define DECLARE_TRACE_NOP(name, proto, args) \ + static inline void _do_trace_##name(struct tracepoint *tp, proto) \ + { } \ + static inline void trace_##name(proto) \ + { } \ + static inline int register_trace_##name(void (*probe)(proto)) \ + { \ + return -ENOSYS; \ + } \ + static inline int unregister_trace_##name(void (*probe)(proto)) \ + { \ + return -ENOSYS; \ + } + +#define DEFINE_TRACE_NOP(name) +#define EXPORT_TRACEPOINT_SYMBOL_GPL_NOP(name) +#define EXPORT_TRACEPOINT_SYMBOL_NOP(name) + #ifdef CONFIG_TRACEPOINTS /* @@ -95,23 +113,11 @@ extern void tracepoint_update_probe_rang struct tracepoint *end); #else /* !CONFIG_TRACEPOINTS */ -#define DECLARE_TRACE(name, proto, args) \ - static inline void _do_trace_##name(struct tracepoint *tp, proto) \ - { } \ - static inline void trace_##name(proto) \ - { } \ - static inline int register_trace_##name(void (*probe)(proto)) \ - { \ - return -ENOSYS; \ - } \ - static inline int unregister_trace_##name(void (*probe)(proto)) \ - { \ - return -ENOSYS; \ - } -#define DEFINE_TRACE(name) -#define EXPORT_TRACEPOINT_SYMBOL_GPL(name) -#define EXPORT_TRACEPOINT_SYMBOL(name) +#define DECLARE_TRACE DECLARE_TRACE_NOP +#define DEFINE_TRACE DEFINE_TRACE_NOP +#define EXPORT_TRACEPOINT_SYMBOL_GPL EXPORT_TRACEPOINT_SYMBOL_GPL_NOP +#define EXPORT_TRACEPOINT_SYMBOL EXPORT_TRACEPOINT_SYMBOL_NOP static inline void tracepoint_update_probe_range(struct tracepoint *begin, struct tracepoint *end)
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