Re: [PATCH 01/10] rcu: Directly lock rdp->nocb_lock on nocb code entrypoints

Joel Fernandes
2020-05-26T21:28:00Z

On Tue, May 26, 2020 at 02:09:47PM -0700, Paul E. McKenney wrote:
[...]
> > > > BTW, I'm really itching to give it a try to make the scheduler more deadlock
> > > > resilient (that is, if the scheduler wake up path detects a deadlock, then it
> > > > defers the wake up using timers, or irq_work on its own instead of passing
> > > > the burden of doing so to the callers). Thoughts?
> > >
> > > I have used similar approaches within RCU, but on the other hand the
> > > scheduler often has tighter latency constraints than RCU does. So I
> > > think that is a better question for the scheduler maintainers than it
> > > is for me. ;-)
> >
> > Ok, it definitely keeps coming up in my radar first with the
> > rcu_read_unlock_special() stuff, and now the nocb ;-). Perhaps it could also
> > be good for a conference discussion!
>
> Again, please understand that RCU has way looser latency constraints
> than the scheduler does. Adding half a jiffy to wakeup latency might
> not go over well, especially in the real-time application area.

Yeah, agreed that the "deadlock detection" code should be pretty light weight
if/when it is written.

> But what did the scheduler maintainers say about this idea?

Last I remember when it came up during the rcu_read_unlock_special() deadlock
discussions, there's no way to know for infra like RCU to know that it was
invoked from the scheduler.

The idea I am bringing up now (about the scheduler itself detecting a
recursion) was never brought up (not yet) with the sched maintainers (at
least not by me).

thanks,

- Joel