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

[Sebastian Andrzej Siewior]

On 2024-10-10 10:10:32 [+0200], Peter Zijlstra wrote:
> On Thu, Oct 10, 2024 at 08:32:07AM +0200, Sebastian Andrzej Siewior wrote:
> > On 2024-10-09 11:24:09 [-0700], Paul E. McKenney wrote:
> > > In order to support systems that currently run CONFIG_PREEMPT=n that
> > …
> > > Or am I once again missing your point?
> > 
> > The change is:
> > | config PREEMPT_RCU
> > |        bool
> > |-       default y if PREEMPTION
> > |+       default y if (PREEMPT || PREEMPT_RT || PREEMPT_DYNAMIC)
> > 
> > Now:
> > - CONFIG_PREEMPT select PREEMPT_BUILD
> > - PREEMPT_RT select CONFIG_PREEMPTION
> > - PREEMPT_DYNAMIC selects PREEMPT_BUILD
> > 
> > and PREEMPT_BUILD select CONFIG_PREEMPTION
> > 
> > so in the end, this change is a nop, right?
> 
> PREEMPT_RT selects PREEMPTION *and* has one of PREEMPT / PREEMPT_LAZY /
> PREEMPT_DYNAMIC, all of which in turn select PREEMPT_BUILD, which
> selects PREEMPTION.
> 
> (arguably we can remove the select PREEMPTION from PREEMPT_RT)
> 
> The proposed change is not a nop because the config: PREEMPT_LAZY=y
> PREEMPT_DYNAMIC=n will result in false, while it will have PREEMPTION.

I have a config with PREEMPT_LAZY=y PREEMPT_DYNAMIC=n  and
CONFIG_PREEMPT_RCU=y.

I can't deselect CONFIG_PREEMPT_RCU=y. This is because LAZY selects
PREEMPT_BUILD and PREEMPT_RCU selects itself once PREEMPTION is on.

> That said, I really do not agree with the change, it makes the condition
> complicated for no reason.

Sebastian