linux-kernel - Re: [PATCH 2/7] rcu: limit PREEMPT

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Message-ID: <0313c8c5-a6a0-4d09-9f85-ac5afa379041@paulmck-laptop>
Date: Fri, 18 Oct 2024 10:38:15 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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 Thu, Oct 17, 2024 at 09:07:10AM +0200, Sebastian Andrzej Siewior wrote:
> On 2024-10-15 16:11:55 [-0700], Paul E. McKenney wrote:
> > > | 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 am more worried about large datacenter deployments than small embedded
> > systems.  Larger systems, but various considerations often limit the
> > amount of memory on a given system.
> 
> okay.
> 
> > > 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.
> > 
> > Is this series now removing PREEMPT_NONE and PREEMPT_VOLUNTARY?
> no, not yet. It is only adding PREEMPT_LAZY as new model, next to
> PREEMPT_NONE and PREEMPT_VOLUNTARY. But is is likely to be on schedule.
> 
> > As conceived last time around, the change would affect only kernels
> > built with one of the other of those two Kconfig options, which will
> > not be users expecting preemption.
> 
> If you continue to use PREEMPT_NONE/ PREEMPT_VOLUNTARY nothing changes
> right now.

Good, thank you!

Presumably PREEMPT_NONE=y && PREEMPT_LAZY=y enables lazy preemption,
but retains non-preemptible RCU.

> > > 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
> > 
> > If PREEMPT_NONE and PREEMPT_VOLUNTARY are still around, it would be
> > far better to make PREEMPT_RCU depend on neither of those being set.
> > That would leave the RCU Kconfig settings fully automatic, and this
> > automation is not to be abandoned lightly.
> 
> Yes, that was my intention - only to make is selectable with
> LAZY-preemption enabled but without dynamic.
> So you are not complete against it.

Help me out here.  In what situation is it beneficial to make PREEMPT_RCU
visible, given that PREEMPT_NONE, PREEMPT_VOLUNTARY, PREEMPT, and
PREEMPT_FULL already take care of this automatically?

> > >  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.
> > 
> > TASKS_RCU needs to be selected when there is preemption of any kind,
> > lazy or otherwise, regardless of the settign of PREEMPT_RCU.
> 
> Okay. In that case PREEMPT_AUTO can be removed.

Makes sense to me!

							Thanx, Paul

> > The current substition of vanilla RCU for Tasks RCU works only in
> > kernels that are guaranteed non-preemptible, which does not include
> > kernels built with lazy preemption.
> > 
> > 							Thanx, Paul
> 
> Sebastian