Re: [linus:master] [rcu/nocb] 7625926086: WARNING:at_kernel/rcu/tree_nocb.h:#rcu_nocb_try_bypass

[Frederic Weisbecker]

On Tue, Jul 11, 2023 at 09:41:58AM -0700, Paul E. McKenney wrote:
> On Tue, Jul 11, 2023 at 06:06:11PM +0200, Frederic Weisbecker wrote:
> Heh!
> 
> The purpose was to see if this lock was ever contended.  I guess we now
> have an answer, which is "Yes, but rarely."
> 
> It looks like the victim commit increased the size of the ->nocb_lock
> critical section, just enough to make this happen sometimes.
> 
> Removing the WARN_ON_ONCE() seems appropriate, especially given that
> this only happens when shrinking.

Ok, I'll check that.

> Should we add something that monitors that lock's contention?  It is
> often the case that lock contention rises over time as new features and
> optimizations are added.

I'm not sure. Should we keep the current ->nocb_lock_contended based engine
to report contention somehow somewhere? Also does it behave better than our
current spinlock slow path implementations?

Thanks.