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

[Ankur Arora]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:

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

Isn't the behaviour identical to how it behaves when you choose
PREEMPT_NONE/VOLUNTARY and enable/disable PREEMPT_DYNAMIC?

This option is just choosing the RCU model suited for the kinds of
preemption supported.

--
ankur