Re: [PATCH 7/5] tracing: Make sure rcu_irq_enter() can work for trace_*_rcuidle() trace events

From: Steven Rostedt
Date: Fri Apr 07 2017 - 13:55:30 EST


On Fri, 7 Apr 2017 17:49:17 +0000 (UTC)
Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:

> > Welcome to MACRO MAGIC!

Somebody is not wizardly happy.

> >
> >>
> >> as one argument to the __DO_TRACE() macro. To me it's a bit unexpected
> >> coding-style wise. Am I the only one not comfortable with the proposed
> >> syntax ?
> >
> > The entire TRACE_EVENT()/__DO_TRACE() is special.
> >
> > I thought about add yet another parameter, but as it doesn't change
> > much, I figured this was good enough. We could beak it up if you like:
> >
> > #define RCU_IRQ_ENTER_CHECK \
> > if (WARN_ON_ONCE(rcu_irq_enter_disabled()) \
> > return; \
> > rcu_irq_enter_irqson();
> >
> > [..]
> > __DO_TRACE(&__tracepoint_##name, \
> > TP_PROTO(data_proto), \
> > TP_ARGS(data_args), \
> > TP_CONDITION(cond), \
> > PARAMS(RCU_IRQ_ENTER_CHECK), \
> > rcu_irq_exit_irqson()); \
> >
> >
> > Would that make you feel more comfortable?
>
> No, it's almost worse and adds still adds a return that apply within __DO_TRACE(),
> but which is passed as an argument (code as macro argument), which I find really
> unsettling.

/me finds it strangely enjoyable to make Mathieu unsettled.

>
> I would prefer to add a new argument to __DO_TRACE, which we can call
> "checkrcu", e.g.:
>
> #define __DO_TRACE(tp, proto, args, cond, checkrcu, prercu, postrcu) \

Grumble. I was trying to avoid making the patch more intrusive. But I
do understand your concern.

> do { \
> struct tracepoint_func *it_func_ptr; \
> void *it_func; \
> void *__data; \
> \
> if (!((cond) && (checkrcu))) \
> return; \
> prercu; \
> rcu_read_lock_sched_notrace(); \
> it_func_ptr = rcu_dereference_sched((tp)->funcs); \
> if (it_func_ptr) { \
> do { \
> it_func = (it_func_ptr)->func; \
> __data = (it_func_ptr)->data; \
> ((void(*)(proto))(it_func))(args); \
> } while ((++it_func_ptr)->func); \
> } \
> rcu_read_unlock_sched_notrace(); \
> postrcu; \
> } while (0)
>
> And use it like this:
>
> #define __DECLARE_TRACE_RCU(name, proto, args, cond, data_proto, data_args) \
> static inline void trace_##name##_rcuidle(proto) \
> { \
> if (static_key_false(&__tracepoint_##name.key)) \
> __DO_TRACE(&__tracepoint_##name, \
> TP_PROTO(data_proto), \
> TP_ARGS(data_args), \
> TP_CONDITION(cond), \
> !WARN_ON_ONCE(rcu_irq_enter_disabled()),\
> rcu_irq_enter_irqson(), \
> rcu_irq_exit_irqson()); \
> }
>
> This way we only pass evaluated expression (not code with "return" that
> changes the flow) as arguments to __DO_TRACE, which makes it behave more
> like a "sub-function", which is what we usually expect.

I understand what you are getting at, and I will concede your point.
OK, I'll do it your way, but I still think you take all the fun out of
it. ;-)

-- Steve