Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}()

Peter Zijlstra
2020-02-13T08:27:57Z

On Wed, Feb 12, 2020 at 06:20:05PM -0500, Joel Fernandes wrote:
> On Wed, Feb 12, 2020 at 10:01:42PM +0100, Peter Zijlstra wrote:

> > +#define trace_rcu_enter() \
> > +({ \
> > + unsigned long state = 0; \
> > + if (!rcu_is_watching()) { \
> > + if (in_nmi()) { \
> > + state = __TR_NMI; \
> > + rcu_nmi_enter(); \
> > + } else { \
> > + state = __TR_IRQ; \
> > + rcu_irq_enter_irqsave(); \
>
> I think this can be simplified. You don't need to rely on in_nmi() here. I
> believe for NMI's, you can just call rcu_irq_enter_irqsave() and that should
> be sufficient to get RCU watching. Paul can correct me if I'm wrong, but I am
> pretty sure that would work.
>
> In fact, I think a better naming for rcu_irq_enter_irqsave() pair could be
> (in the first patch):
>
> rcu_ensure_watching_begin();
> rcu_ensure_watching_end();

So I hadn't looked deeply into rcu_irq_enter(), it seems to call
rcu_nmi_enter_common(), but with @irq=true.

What exactly is the purpose of that @irq argument, and how much will it
hurt to lie there? Will it come apart if we have @irq != !in_nmi()
for example?

There is a comment in there that says ->dynticks_nmi_nesting ought to be
odd only if we're in NMI. The only place that seems to care is
rcu_nmi_exit_common(), and that does indeed do something different for
IRQs vs NMIs.

So I don't think we can blindly unify this. But perhaps Paul sees a way?