Re: [PATCH v4 16/27] tracing: Remove regular RCU context for _rcuidle tracepoints (again)

Joel Fernandes
2020-03-06T19:14:10Z

On Fri, Mar 06, 2020 at 01:59:25PM -0500, Steven Rostedt wrote:
[snip]
> > > - rcu_irq_enter_irqson(); \
> > > - } \
> > > \
> > > it_func_ptr = rcu_dereference_raw((tp)->funcs); \
> > > \
> > > if (it_func_ptr) { \
> > > do { \
> > > + int rcu_flags; \
> > > it_func = (it_func_ptr)->func; \
> > > + if (rcuidle && \
> > > + (it_func_ptr)->requires_rcu) \
> > > + rcu_flags = trace_rcu_enter(); \
> > > __data = (it_func_ptr)->data; \
> > > ((void(*)(proto))(it_func))(args); \
> > > + if (rcuidle && \
> > > + (it_func_ptr)->requires_rcu) \
> > > + trace_rcu_exit(rcu_flags); \
> >
> > Nit: If we have incurred the cost of trace_rcu_enter() once, we can call
> > it only once and then call trace_rcu_exit() after the do-while loop. That way
> > we pay the price only once.
> >
>
> I thought about that, but the common case is only one callback attached at
> a time. To make the code complex for the non common case seemed too much
> of an overkill. If we find that it does help, it's best to do that as a
> separate patch because then if something goes wrong we know where it
> happened.
>
> Currently, this provides the same overhead as if each callback did it
> themselves like we were proposing (but without the added need to do it for
> all instances of the callback).

That's ok, it could be a separate patch.

thanks,

- Joel