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

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Message-ID: <20241017070710.U9bTJFMS@linutronix.de>
Date: Thu, 17 Oct 2024 09:07:10 +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 2024-10-15 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.

> > 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.

> >  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.

> 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