linux-kernel - Re: [RFC PATCH] Tracepoint: introduce tracepoint() API

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Date:	Thu, 17 Nov 2011 16:58:18 -0500
From:	Jason Baron <jbaron@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [RFC PATCH] Tracepoint: introduce tracepoint() API

On Thu, Nov 17, 2011 at 04:06:54PM -0500, Steven Rostedt wrote:
> On Thu, 2011-11-17 at 15:50 -0500, Mathieu Desnoyers wrote:
> > Introduce:
> > 
> >   tracepoint(event_name, arg1, arg2, ...)
> > 
> > while keeping the old tracepoint API in place, e.g.:
> > 
> >   trace_event_name(arg1, arg2, ...)
> > 
> > This allows skipping parameter side-effects (pointer dereference,
> > function calls, ...) when the tracepoint is not dynamically activated.
> > 
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > ---
> > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> > index d530a44..c9c73f7 100644
> > --- a/include/linux/tracepoint.h
> > +++ b/include/linux/tracepoint.h
> > @@ -107,6 +107,12 @@ void tracepoint_update_probe_range(struct tracepoint * const *begin,
> >  
> >  #ifdef CONFIG_TRACEPOINTS
> >  
> > +#define tracepoint(name, args...)					\
> > +	do {								\
> > +		if (static_branch(&__tracepoint_##name.key))		\
> > +			__trace_##name(args);				\
> > +	} while (0)
> > +
> >  /*
> >   * it_func[0] is never NULL because there is at least one element in the array
> >   * when the array itself is non NULL.
> > @@ -144,13 +150,17 @@ void tracepoint_update_probe_range(struct tracepoint * const *begin,
> >   */
> >  #define __DECLARE_TRACE(name, proto, args, cond, data_proto, data_args)	\
> >  	extern struct tracepoint __tracepoint_##name;			\
> > +	static inline void __trace_##name(proto)			\
> > +	{								\
> > +		__DO_TRACE(&__tracepoint_##name,			\
> > +			TP_PROTO(data_proto),				\
> > +			TP_ARGS(data_args),				\
> > +			TP_CONDITION(cond));				\
> > +	}								\
> 
> I wrote a patch earlier today that does almost the exact same thing, but
> I had more in macro part, which I would have cleaned up after the RFC. I
> didn't add another static inline, but I think this approach is a little
> cleaner (with the second static inline).
> 
> I didn't post mine because I was still analyzing the assembly to make
> sure it did what I expected. But I got side tracked on other things (RT
> related) and didn't quite finish the analysis.
> 
> Did you do a compare of kmem_cache_alloc() to see if this fixes the
> reported problem?
> 
> -- Steve
> 
> >  	static inline void trace_##name(proto)				\
> >  	{								\
> >  		if (static_branch(&__tracepoint_##name.key))		\
> > -			__DO_TRACE(&__tracepoint_##name,		\
> > -				TP_PROTO(data_proto),			\
> > -				TP_ARGS(data_args),			\
> > -				TP_CONDITION(cond));			\
> > +			__trace_##name(args);				\
> >  	}								\
> >  	static inline int						\
> >  	register_trace_##name(void (*probe)(data_proto), void *data)	\
> > @@ -193,7 +203,12 @@ void tracepoint_update_probe_range(struct tracepoint * const *begin,
> >  	EXPORT_SYMBOL(__tracepoint_##name)
> >  
> >  #else /* !CONFIG_TRACEPOINTS */
> > +
> > +#define tracepoint(name, args...)	__trace_##name(args)
> > +
> >  #define __DECLARE_TRACE(name, proto, args, cond, data_proto, data_args)	\
> > +	static inline void __trace_##name(proto)			\
> > +	{ }								\
> >  	static inline void trace_##name(proto)				\
> >  	{ }								\
> >  	static inline int						\
> > 
> 

I brought this issue up a while back, with a very similar patch to what
Mathieu wrote. Please see:

http://thread.gmane.org/gmane.linux.kernel/838911

One conclusion from that thread was that it would be nice if gcc could
optimize the load so that it only happens in the unlikely path. I filed
a gcc bug for that: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40207,
but it doesn't seem to be implemented yet...

Thanks,

-Jason
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