Re: [PATCH 15/36] cpuidle,cpu_pm: Remove RCU fiddling from cpu_pm_{enter,exit}()

[Mark Rutland <mark.rutland@xxxxxxx>]

On Wed, Jun 08, 2022 at 04:27:38PM +0200, Peter Zijlstra wrote:
> All callers should still have RCU enabled.

IIUC with that true we should be able to drop the RCU_NONIDLE() from
drivers/perf/arm_pmu.c, as we only needed that for an invocation via a pm
notifier.

I should be able to give that a spin on some hardware.

> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  kernel/cpu_pm.c |    9 ---------
>  1 file changed, 9 deletions(-)
> 
> --- a/kernel/cpu_pm.c
> +++ b/kernel/cpu_pm.c
> @@ -30,16 +30,9 @@ static int cpu_pm_notify(enum cpu_pm_eve
>  {
>       int ret;
>  
> -     /*
> -      * This introduces a RCU read critical section, which could be
> -      * disfunctional in cpu idle. Copy RCU_NONIDLE code to let RCU know
> -      * this.
> -      */
> -     rcu_irq_enter_irqson();
>       rcu_read_lock();
>       ret = raw_notifier_call_chain(&cpu_pm_notifier.chain, event, NULL);
>       rcu_read_unlock();
> -     rcu_irq_exit_irqson();

To make this easier to debug, is it worth adding an assertion that RCU is
watching here? e.g.

        RCU_LOCKDEP_WARN(!rcu_is_watching(),
                         "cpu_pm_notify() used illegally from EQS");

>  
>       return notifier_to_errno(ret);
>  }
> @@ -49,11 +42,9 @@ static int cpu_pm_notify_robust(enum cpu
>       unsigned long flags;
>       int ret;
>  
> -     rcu_irq_enter_irqson();
>       raw_spin_lock_irqsave(&cpu_pm_notifier.lock, flags);
>       ret = raw_notifier_call_chain_robust(&cpu_pm_notifier.chain, event_up, 
> event_down, NULL);
>       raw_spin_unlock_irqrestore(&cpu_pm_notifier.lock, flags);
> -     rcu_irq_exit_irqson();


... and likewise here?

Thanks,
Mark.

>  
>       return notifier_to_errno(ret);
>  }
> 
>