linux-kernel - Re: [RFC PATCH] Kernel Tracepoints

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Date:	Fri, 27 Jun 2008 09:30:10 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Takashi Nishiie <t-nishiie@...css.fujitsu.com>,
	'Alexey Dobriyan' <adobriyan@...il.com>,
	'Peter Zijlstra' <peterz@...radead.org>,
	'Steven Rostedt' <rostedt@...dmis.org>,
	"'Frank Ch. Eigler'" <fche@...hat.com>,
	'Ingo Molnar' <mingo@...e.hu>,
	'LKML' <linux-kernel@...r.kernel.org>,
	'systemtap-ml' <systemtap@...rces.redhat.com>,
	'Hideo AOKI' <haoki@...hat.com>
Subject: Re: [RFC PATCH] Kernel Tracepoints

* Masami Hiramatsu (mhiramat@...hat.com) wrote:
 >
> > Implementation of kernel tracepoints. Inspired from the Linux Kernel Markers.
> 
> What would you think redesigning markers on tracepoints? because most of the
> logic (scaning sections, multiple probe and activation) seems very similar
> to markers.
> 

We could, although markers, because they use var args, allow to put the
iteration on the multi probe array out-of-line. Tracepoints cannot
afford this and the iteration must be done at the initial call-site.

>From what I see in your proposal, it's mostly to extract the if() call()
code from the inner __trace_mark() macro and to put it in a separate
macro, am I correct ? This would make the macro more readable.

> For example, (not complete, I just thought :-))
> 
>  struct tracepoint {
>  	const char *name;		/* Tracepoint name */
>  	DEFINE_IMV(char, state);	/* Immediate value state. */
>  	struct tracepoint_probe_closure *multi;	/* Closures */
> 	void * callsite_data;		/* private date from call site */
>  } __attribute__((aligned(8)));
> 
>  #define __tracepoint_block(generic, name, data, func, args)
>  	static const char __tpstrtab_##name[]			\
>  	__attribute__((section("__tracepoints_strings")))	\
>  	= #name;						\
>  	static struct tracepoint __tracepoint_##name		\
>  	__attribute__((section("__tracepoints"), aligned(8))) =	\
>  	{ __tpstrtab_##name, 0, NULL, data};			\
>  	if (!generic) {						\
>  		if (unlikely(imv_cond(__tracepoint_##name.state))) { \
>  			imv_cond_end();				\
>  			func(&__tracepoint_##name, args); \
>  		} else						\
>  			imv_cond_end();				\
>  	} else {						\
>  		if (unlikely(_imv_read(__tracepoint_##name.state))) \
>  			func(&__tracepoint_##name, args); \
>  	}
> 
>  struct marker {
>         const char *name;       /* Marker name */
>         const char *format;     /* Marker format string, describing the
>                                  * variable argument list.
>                                  */
>  } __attribute__((aligned(8)));
> 
>  #define trace_mark(name, fmt, args...)			\
>  do {							\
>  	static const char __mstrtab_##name[]		\
>  	__attribute__((section("__markers_strings")))	\
>  	= #name "\0" fmt;				\
>  	static struct marker __marker_##name		\
>  	__attribute__((section("__markers"), aligned(8))) =	\
>  	{ __mstrtab_##name, &__mstrtab_##name[sizeof(#name)]};	\ 	
>  	__tracepoint_block(1, name, __marker_##name, marker_probe_cb, args)	\
>  } while (0)
> 
> >
[...]
> > +	static inline int register_trace_##name(			\
> > +		void (*probe)(void *private_data, proto),		\
> > +		void *private_data)					\
> > +	{								\
> > +		return tracepoint_probe_register(#name, (void *)probe,	\
> > +			private_data);					\
> > +	}								\
> > +	static inline void unregister_trace_##name(			\
> > +		void (*probe)(void *private_data, proto),		\
> > +		void *private_data)					\
> > +	{								\
> > +		tracepoint_probe_unregister(#name, (void *)probe,	\
> > +			private_data);					\
> > +	}
> 
> Out of curiousity, what the private_data is for?
> 

When a probe is registered, it gives more flexibility to be able to pass
a pointer to private data associated with that probe. For instance, if a
tracer needs to register the same probe to many different tracepoints,
but having a different context associated with each, it will pass the
same function pointer with different private_data to the registration
function.

> > +
> > +extern void tracepoint_update_probe_range(struct tracepoint *begin,
> > +	struct tracepoint *end);
> > +
> > +#else /* !CONFIG_TRACEPOINTS */
> > +#define DEFINE_TRACE(name, proto, args)			\
> > +	static inline void _do_trace_##name(struct tracepoint *tp, proto) \
> > +	{ }								\
> > +	static inline void __trace_##name(int generic, proto)		\
> > +	{ }								\
> > +	static inline void trace_##name(proto)				\
> > +	{ }								\
> > +	static inline void _trace_##name(proto)				\
> > +	{ }
> 
> By the way, I think you'd better add below two inlines.
> 
> 	static inline int register_trace_##name(			\
> 		void (*probe)(void *private_data, proto),		\
> 		void *private_data)					\
> 	{ return -ENOSYS; }
> 	static inline void unregister_trace_##name(			\
> 		void (*probe)(void *private_data, proto),		\
> 		void *private_data)					\
> 	{ }
> 

My original thought was that if tracepoints are disabled, the probe
objects should not even be built. But I can foresee that they will not
always be in separate objects, so it makes sense. Will add.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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