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Message-ID: <20241015112224.KdvzKo80@linutronix.de>
Date: Tue, 15 Oct 2024 13:22:24 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
Ankur Arora <ankur.a.arora@...cle.com>,
linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...nel.org,
juri.lelli@...hat.com, vincent.guittot@...aro.org,
dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
mgorman@...e.de, vschneid@...hat.com, frederic@...nel.org,
efault@....de
Subject: Re: [PATCH 2/7] rcu: limit PREEMPT_RCU configurations
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
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