Re: RFC: call_rcu_outstanding (was Re: WARNING in __mmdrop)

From: Paul E. McKenney
Date: Sun Jul 21 2019 - 19:33:51 EST

On Sun, Jul 21, 2019 at 02:08:37PM -0700, Matthew Wilcox wrote:
> On Sun, Jul 21, 2019 at 06:17:25AM -0700, Paul E. McKenney wrote:
> > Also, the overhead is important. For example, as far as I know,
> > current RCU gracefully handles close(open(...)) in a tight userspace
> > loop. But there might be trouble due to tight userspace loops around
> > lighter-weight operations.
> I thought you believed that RCU was antifragile, in that it would scale
> better as it was used more heavily?

You are referring to this?

If so, the last few paragraphs might be worth re-reading. ;-)

And in this case, the heuristics RCU uses to decide when to schedule
invocation of the callbacks needs some help. One component of that help
is a time-based limit to the number of consecutive callback invocations
(see my crude prototype and Eric Dumazet's more polished patch). Another
component is an overload warning.

Why would an overload warning be needed if RCU's callback-invocation
scheduling heurisitics were upgraded? Because someone could boot a
100-CPU system with the rcu_nocbs=0-99, bind all of the resulting
rcuo kthreads to (say) CPU 0, and then run a callback-heavy workload
on all of the CPUs. Given the constraints, CPU 0 cannot keep up.

So warnings are required as well.

> Would it make sense to have call_rcu() check to see if there are many
> outstanding requests on this CPU and if so process them before returning?
> That would ensure that frequent callers usually ended up doing their
> own processing.

Unfortunately, no. Here is a code fragment illustrating why:

void my_cb(struct rcu_head *rhp)
unsigned long flags;

spin_lock_irqsave(&my_lock, flags);
spin_unlock_irqrestore(&my_lock, flags);

. . .

spin_lock_irqsave(&my_lock, flags);
p = look_something_up();
call_rcu(p, my_cb);
spin_unlock_irqrestore(&my_lock, flags);

Invoking the extra callbacks directly from call_rcu() would thus result
in self-deadlock. Documentation/RCU/UP.txt contains a few more examples
along these lines.