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

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

On 2024-10-15 15:13:46 [-0700], Ankur Arora wrote:
> Sebastian Andrzej Siewior <bigeasy@linutronix.de> writes:
> 
> >>
> >> 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
> 
> With PREEMPT_LAZY=y, PREEMPT_DYNAMIC=n, isn't the default model
> PREEMPT_LAZY, which has PREEMPTION=y, but PREEMPT=n?

Correct.

> > 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.
> 
> Can you clarify this? The intent with lazy is to be preemptible but
> preempt less often. In that it is meant to be quite different from
> CONFIG_PREEMPT.

A wake up with PREEMPT may not always lead to a preemption but will lead
to preemption once the time slice is up. With LAZY this changes to the
point that a preemption point will be delayed to the sched tick. Tasks
which are not based on the fail class (SCHED_DL, FIFO, …) will receive a
wake up right away.

If in the long term NONE and VOL goes away you could argue that everyone
using LAZY + !DYNAMIC is one of those. If additionally PREEMPT goes away
then you can not. Therefore I would prefer to have the RCU model to be
selectable rather than forced. While !PREEMPT_RCU may save memory, it
also disable preemption for the read section. 

> Ankur

Sebastian