Re: [RFC patch 3/5] ftrace trace event add missing semicolumn

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Frederic Weisbecker (fweisbec@gmail.com) wrote:
> On Tue, Jan 04, 2011 at 07:18:37PM -0500, Mathieu Desnoyers wrote:
> > * Frederic Weisbecker (fweisbec@gmail.com) wrote:
> > > On Tue, Jan 04, 2011 at 06:16:32PM -0500, Mathieu Desnoyers wrote:
> > > > Add a missing semicolumn at the end of a ftrace definition.
> > > > 
> > > > We currently are not seeing any impact of this missing semicolumn because extra
> > > > semicolumns appear all over the place in the code generated from TRACE_EVENT
> > > > within ftrace stages.
> > > > 
> > > > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > > > CC: Steven Rostedt <rostedt@goodmis.org>
> > > > CC: Frederic Weisbecker <fweisbec@gmail.com>
> > > > CC: Ingo Molnar <mingo@elte.hu>
> > > > CC: Thomas Gleixner <tglx@linutronix.de>
> > > > ---
> > > >  include/trace/ftrace.h |    2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > Index: linux-2.6-lttng/include/trace/ftrace.h
> > > > ===================================================================
> > > > --- linux-2.6-lttng.orig/include/trace/ftrace.h
> > > > +++ linux-2.6-lttng/include/trace/ftrace.h
> > > > @@ -69,7 +69,7 @@
> > > >  #undef DEFINE_EVENT
> > > >  #define DEFINE_EVENT(template, name, proto, args)	\
> > > >  	static struct ftrace_event_call	__used		\
> > > > -	__attribute__((__aligned__(4))) event_##name
> > > > +	__attribute__((__aligned__(4))) event_##name;
> > > 
> > > But DEFINE_EVENT() calls are supposed to be ";" terminated, no?
> > 
> > Currently yes, but if you look at the preprocessor output currently generated by
> > the current TRACE_EVENT()/DEFINE_EVENT() scheme, there are useless ";" added all
> > over the place. I have a patch later in my queue that proposes removal of these
> > extra ";" as a cleanup of the TRACE_EVENT() semantic, but I'm keeping it for
> > later because it removes the extra ";" at the end of each TRACE_EVENT()
> > instance (and thus is more intrusive code-wise).
> > 
> > Adding this semicolumn here ensures that all Ftrace macros are consistent wrt
> > semicolumns. We can get away without consistency currently exactly because the
> > current scheme adds many useless semicolumns between each TRACE_EVENT().
> 
> Are you sure you want to put so much time on this?

We are about to spend more time arguing about it that the time it takes cleaning
it up. But here we go.

> 
> This will require a massive change for the sole win of removing double ";"
> in generated code. This won't optimize much the build, and it will make the things
> not so much more readable for very rare people who dare to have interest into the
> TRACE_EVENT generated code. That notwithstanding the obfuscation of that generated
> code resides more in the lack of indentation and newlines than in double
> semicolons that we barely notice.

Building:

make kernel/sched.o V=1

taking the gcc invokation, changing it to:

gcc -Wp,-MD,kernel/.sched.o.d  -nostdinc -isystem /usr/local/lib/gcc/x86_64-unknown-linux-gnu/4.5.1/include -I/home/compudj/git/linux-tip/arch/x86/include -Iinclude  -include include/generated/autoconf.h -D__KERNEL__ -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -Wno-format-security -fno-delete-null-pointer-checks -O2 -m64 -march=core2 -mno-red-zone -mcmodel=kernel -funit-at-a-time -maccumulate-outgoing-args -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 -DCONFIG_AS_CFI_SECTIONS=1 -pipe -Wno-sign-compare -fno-asynchronous-unwind-tables -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -Wframe-larger-than=1024 -fno-stack-protector -fno-omit-frame-pointer -fno-optimize-sibling-calls -Wdeclaration-after-statement -Wno-pointer-sign -fno-strict-overflow -fconserve-stack    -D"KBUILD_STR(s)=#s" -D"KBUILD_BASENAME=KBUILD_STR(sched)"  -D"KBUILD_MODNAME=KBUILD_STR(sched)"  -E -o kernel/sched.pp kernel/sched.c

An exerpt of the output, fed through "indent" for readability:

static inline __attribute__ ((always_inline))
     void
# 275 "include/trace/events/sched.h"
       check_trace_callback_type_sched_process_fork
# 252 "include/trace/events/sched.h"
      
       (void (*cb)
        (void *__data, struct task_struct * parent,
         struct task_struct * child))
{
}
# 275 "include/trace/events/sched.h"
 ;






# 305 "include/trace/events/sched.h"
 ;


As we can notice, a few extra ";" are added between each "entity" created by the
ftrace trace event phase. This works only as long as we declare
semicolumn-separated C structure elements, functions, and statements, because
the compiler just skips the extra semicolumns, but forbids creation of arrays of
events, which need to be comma-separated.

These extra semicolumns we see here are simply polluting the compiler input, and
I don't see any reason why we should leave them there.

Thanks,

Mathieu

> 
> Hm?

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com