linux-kernel - Re: [patch 01/15] Kernel Tracepoints

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Date:	Tue, 15 Jul 2008 15:59:16 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	Masami Hiramatsu <mhiramat@...hat.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Hideo AOKI <haoki@...hat.com>,
	Takashi Nishiie <t-nishiie@...css.fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
Subject: Re: [patch 01/15] Kernel Tracepoints

On Tue, 2008-07-15 at 09:25 -0400, Mathieu Desnoyers wrote:
> * Peter Zijlstra (peterz@...radead.org) wrote:
> > On Wed, 2008-07-09 at 10:59 -0400, Mathieu Desnoyers wrote:

> > > +#define __DO_TRACE(tp, proto, args)					\
> > > +	do {								\
> > > +		int i;							\
> > > +		void **funcs;						\
> > > +		preempt_disable();					\
> > > +		funcs = (tp)->funcs;					\
> > > +		smp_read_barrier_depends();				\
> > > +		if (funcs) {						\
> > > +			for (i = 0; funcs[i]; i++) {			\
> > 
> > can't you get rid of 'i' and write:
> > 
> >   void **func;
> > 
> >   preempt_disable();
> >   func = (tp)->funcs;
> >   smp_read_barrier_depends();
> >   for (; func; func++)
> >     ((void (*)(proto))func)(args);
> >   preempt_enable();
> > 
> 
> Yes, I though there would be an optimization to do here, I'll use your
> proposal. This code snippet is especially important since it will
> generate instructions near every tracepoint side. Saving a few bytes
> becomes important.
> 
> Given that (tp)->funcs references an array of function pointers and that
> it can be NULL, the if (funcs) test must still be there and we must use
> 
> #define __DO_TRACE(tp, proto, args)					\
> 	do {								\
> 		void *func;						\
> 									\
> 		preempt_disable();					\
> 		if ((tp)->funcs) {					\
> 			func = rcu_dereference((tp)->funcs);		\
> 			for (; func; func++) {				\
> 				((void(*)(proto))(func))(args);		\
> 			}						\
> 		}							\
> 		preempt_enable();					\
> 	} while (0)
> 
> 
> The resulting assembly is a bit more dense than my previous
> implementation, which is good :

My version also has that if ((tp)->funcs), but its hidden in the 
for (; func; func++) loop. The only thing your version does is an extra
test of tp->funcs but without read depends barrier - not sure if that is
ok.



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