Re: [PATCH v4 1/1] powerpc: enable dynamic preemption

[Shrikanth Hegde]

Hi Paul, Christophe,

On 7/27/26 11:59 PM, Christophe Leroy (CS GROUP) wrote:
> Hi Shrikanth,

> That's right with CONFIG_PREEMPT_DYNAMIC=y
>
> With CONFIG_PREEMT_DYNAMIC=n:
> - CONFIG_PREEMPT (full) implies CONFIG_PREEMPT_RCU
> - CONFIG_PREEMPT_LAZY (lazy) doesn't imply CONFIG_PREEMPT_RCU
>
> because of kernel/rcu/Kconfig,
>
> config PREEMPT_RCU
>      bool
>      default y if (PREEMPT || PREEMPT_RT || PREEMPT_DYNAMIC)
>      select TREE_RCU
>
> Christophe

Right.

Looks like __rcu_read_unlock can set need_resched bit if it find irq were disabled
and rcu_read_unlock_special.s is set. I don't have much clue about it.
So, after seeing __rcu_read_unlock for PREEMPT_RCU I have these concerns now.

In current upstream, lazy is a preemption mode with preempt_enable being a preemptible point,

1. If PREEMPT_LAZY=y and PREEMPT_DYNAMIC=n, then PREEMPT_RCU=n. That seems wrong. It is supposed to
   do the preemption checks, since preempt_count is actual count with lazy preemption.

2. PREEMPT_LAZY=y and PREEMPT_DYNAMIC=n and PREEMPT_LAZY=y and PREEMPT_DYNAMIC=y are supposed
   to be similar w.r.t to performance. Promise of PREEMPT_DYNAMIC was the ability to
   switch at runtime based on the user workload and his/her preference.
   But with  PREEMPT_LAZY=y and PREEMPT_DYNAMIC=y, there is additional cost due to PREEMPT_RCU.
   lazy being the middle ground w.r.t for decent performance, either PREEMPT_RCU should either kick in
   for both cases or bail out for both.

Before 7.0
3. the same concerns would be true to PREEMPT_VOLUNTARY and PREEMPT_NONE with PREEMPT_DYNAMIC=y.
   Those model never wanted a preemption, but PREEMPT_RCU could have forced a need_resched to be
   set. Which is also likely wrong. No?