linux-kernel - Re: [PATCH 15/36] cpuidle,cpu_pm: Remove RCU fiddling from cpu_pm

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Message-ID: <YqiznJL7qB9uSQ9c@FVFF77S0Q05N>
Date:   Tue, 14 Jun 2022 17:13:16 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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Subject: Re: [PATCH 15/36] cpuidle,cpu_pm: Remove RCU fiddling from
 cpu_pm_{enter,exit}()

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@...radead.org>
> ---
>  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);
>  }
> 
>