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

Paul E. McKenney
2020-02-13T13:51:40Z

On Thu, Feb 13, 2020 at 09:27:16AM +0100, Peter Zijlstra wrote:
> 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?

The reason for the irq argument is to avoid invoking
rcu_prepare_for_idle() and rcu_dynticks_task_enter() from NMI context
from rcu_nmi_exit_common(). Similarly, we need to avoid invoking
rcu_dynticks_task_exit() and rcu_cleanup_after_idle() from NMI context
from rcu_nmi_enter_common().

It might well be that I could make these functions be NMI-safe, but
rcu_prepare_for_idle() in particular would be a bit ugly at best.
So, before looking into that, I have a question. Given these proposed
changes, will rcu_nmi_exit_common() and rcu_nmi_enter_common() be able
to just use in_nmi()?

Thanx, Paul