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

[Paul E. McKenney]

On Fri, Oct 11, 2024 at 04:43:41PM +0200, Sebastian Andrzej Siewior wrote:
> On 2024-10-11 06:59:44 [-0700], Paul E. McKenney wrote:
> > > > If you have PREEMPT_DYNAMIC, presumably you would like to boot with
> > > > preemption enabled, and would like it to act as if you had built the
> > > > kernel to be unconditionally preemptible, so again you need preemptible
> > > > RCU, and so the defaults should supply it.
> > > > 
> > > > If you started off building a non-preemptible kernel, then you are not
> > > > using one of the major distros (last I checked).  There is a good chance
> > > > that you have a large number of systems, and are thus deeply interested
> > > > in minimizing memory cost.  In which case, you need non-preemptible
> > > > RCU in the new-age lazy-preemptible kernel.
> > > > 
> > > > Hence the choice of non-preemptible RCU as the default in a kernel that,
> > > > without lazy preemption, would use non-preemptible RCU.
> > > 
> > > I *think* I slowly begin to understand. So we have LAZY and DYNAMIC
> > > enabled then and PREEMPT_RCU depends on the default choice which is
> > > either FULL (yes, please) or NONE/VOLUNTARY (no, thank you). But then if
> > > you change the model at runtime (or boottime) from (default) NONE to
> > > FULL you don't have preemptible RCU anymore.
> > 
> > Almost.  If you are able to change the model at boot time or at run time,
> > then you *always* have preemptible RCU.
> 
> 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?

> > > 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.

> > 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?

							Thanx, Paul