Re: [PATCH 2/7] rcu: limit PREEMPT_RCU configurations

[Paul E. McKenney]

On Thu, Oct 10, 2024 at 09:58:50AM +0200, Peter Zijlstra wrote:
> On Wed, Oct 09, 2024 at 02:16:43PM -0700, Paul E. McKenney wrote:
> 
> > Because we don't need a bunch of people surprised by this change in
> > behavior.
> 
> I'm not sure what change in behaviour; Lazy is brand spanking new, it
> will have new behaviour not in line with previous modes. Just put in the
> help text that if you're looking for a reality closer to the old
> Voluntary to use PREEMPT_RCU=n.
> 
> In reality very few people will care, because distros will be running
> PREEMPT_DYNAMIC and the 3 people doing their own thing ought to be savvy
> enough to deal with things, no?

No.

Sure, a number of them have proven themselves capable of debugging back
from an otherwise inexplicable series of OOMs not obviously caused by
RCU Kconfig settings, but why put them through that?  And especially,
why put others through such a "learning experience"?

Why not instead tag the Kconfig help text noting that lazy preemption
does not automatically apply to RCU read-side critical sections, and
that if you want it to, there is a Kconfig option that is there for you?

							Thanx, Paul