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

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?