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

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2024-10-11 08:59:14 [-0700], Paul E. McKenney wrote:
> On Fri, Oct 11, 2024 at 04:43:41PM +0200, Sebastian Andrzej Siewior wrote:
>
> > Okay, this eliminates PREEMPT_DYNAMIC then.
> > With PeterZ current series, PREEMPT_LAZY (without everything else
> > enabled) behaves as PREEMPT without the "forced" wake up for the fair
> > class. It is preemptible after all, with preempt_disable() actually
> > doing something. This might speak for preemptible RCU.
> > And assuming in this condition you that "low memory overhead RCU" which
> > is not PREEMPT_RCU. This might require a config option.
> 
> The PREEMPT_DYNAMIC case seems to work well as-is for the intended users,
> so I don't see a need to change it.  In particular, we already learned
> that we need to set PREEMPT_DYNAMIC=n.  Yes, had I caught this in time, I
> would have argued against changing the default, but this was successfully
> slid past me.
> 
> As for PREEMPT_LAZY, you seem to be suggesting a more intrusive change
> than just keeping non-preemptible RCU when the Kconfig options are
> consistent with this being expected.  If this is the case, what are the
> benefits of this more-intrusive change?

As far as I understand you are only concerned about PREEMPT_LAZY and
everything else (PREEMPT_LAZY + PREEMPT_DYNAMIC or PREEMPT_DYNAMIC
without PREEMPT_LAZY) is fine. 
In the PREEMPT_LAZY + !PREEMPT_DYNAMIC the suggested change

| config PREEMPT_RCU
| 	bool
| 	default y if (PREEMPT || PREEMPT_RT || PREEMPT_DYNAMIC)
| 	select TREE_RCU
| 	help

would disable PREEMPT_RCU while the default model is PREEMPT. You argue
that only people on small embedded would do such a thing and they would
like to safe additional memory. 
I don't think this is always the case because the "preemptible" users
would also get this and this is an unexpected change for them.

> > > > If you would like to add some relief to memory constrained systems,
> > > > wouldn't BASE_SMALL be something you could hook to? With EXPERT_RCU to
> > > > allow to override it?
> > > 
> > > Does BASE_SMALL affect anything but log buffer sizes?  Either way, we
> > > would still need to avoid the larger memory footprint of preemptible
> > > RCU that shows up due to RCU readers being preempted.
> > 
> > It only reduces data structures where possible. So lower performance is
> > probably due to things like futex hashmap (and others) are smaller.
> 
> Which is still counterproductive for use cases other than small deep
> embedded systems.

Okay, so that option is gone.

> > > Besides, we are not looking to give up performance vs BASE_SMALL's
> > > "may reduce performance" help text.
> > > 
> > > Yes, yes, it would simplify things to just get rid of non-preemptible RCU,
> > > but that is simply not in the cards at the moment.
> > 
> > Not sure what the time frame is here. If we go for LAZY and remove NONE
> > and VOLUNTARY then making PREEMPT_RCU would make sense to lower the
> > memory footprint (and not attaching to BASE_SMALL).
> > 
> > Is this what you intend or did misunderstand something here?
> 
> My requirement is that LAZY not remove/disable/whatever non-preemptible
> RCU.  Those currently using non-preemptible RCU should continue to be able
> to be able to use it, with or without LAZY.  So why is this requirement
> a problem for you?  Or am I missing your point?

Those who were using non-preemptible RCU, whish to use LAZY_PREEPMT +
!PREEMPT_DYNAMIC should be able to disable PREEMPT_RCU only in this case. 
Would the following work?

diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index 8cf8a9a4d868c..2183c775e7808 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -121,6 +121,7 @@ config PREEMPT_COUNT
 config PREEMPTION
        bool
        select PREEMPT_COUNT
+       select PREEMPT_RCU if PREEMPT_DYNAMIC
 
 config PREEMPT_DYNAMIC
 	bool "Preemption behaviour defined on boot"
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index 3e079de0f5b43..9e4bdbbca4ff9 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -17,7 +17,7 @@ config TREE_RCU
 	  smaller systems.
 
 config PREEMPT_RCU
-	bool
+	bool "Preemptible RCU"
 	default y if PREEMPTION
 	select TREE_RCU
 	help
@@ -91,7 +91,7 @@ config NEED_TASKS_RCU
 
 config TASKS_RCU
 	bool
-	default NEED_TASKS_RCU && (PREEMPTION || PREEMPT_AUTO)
+	default NEED_TASKS_RCU && PREEMPTION
 	select IRQ_WORK
 
 config FORCE_TASKS_RUDE_RCU

I added TASKS_RCU to the hunk since I am not sure if you wish to follow
PREEMPTION (which is set by LAZY) or PREEMPT_RCU.

> 							Thanx, Paul

Sebastian