Re: [patch 01/15] Kernel Tracepoints

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

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

Hrm, you are right, the implementation I just proposed is bogus. (but so
was yours) ;)

func is an iterator on the funcs array. My typing of func is thus wrong,
it should be void **. Otherwise I'm just incrementing the function
address which is plain wrong.

The read barrier is included in rcu_dereference() now. But given that we
have to take a pointer to the array as an iterator, we would have to
rcu_dereference() our iterator multiple times and then have many read
barrier depends, which we don't need. This is why I would go back to a
smp_read_barrier_depends().

Also, I use a NULL entry at the end of the funcs array as an end of
array identifier. However, I cannot use this in the for loop both as a
check for NULL array and check for NULL array element. This is why a if
() test is needed in addition to the for loop test. (this is actually
what is wrong in the implementation you proposed : you treat func both
as a pointer to the function pointer array and as a function pointer)

Something like this seems better :

#define __DO_TRACE(tp, proto, args)                                     \
        do {                                                            \
                void **it_func;                                         \
                                                                        \
                preempt_disable();                                      \
                it_func = (tp)->funcs;                                  \
                if (it_func) {                                          \
                        smp_read_barrier_depends();                     \
                        for (; *it_func; it_func++)                     \
                                ((void(*)(proto))(*it_func))(args);     \
                }                                                       \
                preempt_enable();                                       \
        } while (0)

What do you think ?

Mathieu

> 
> 

-- 
Mathieu Desnoyers
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