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

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Date:   Tue, 14 Jun 2022 18:42:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mark Rutland <mark.rutland@....com>
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Subject: Re: [PATCH 15/36] cpuidle,cpu_pm: Remove RCU fiddling from
 cpu_pm_{enter,exit}()

On Tue, Jun 14, 2022 at 05:13:16PM +0100, Mark Rutland wrote:
> 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");
> 

My understanding is that rcu_read_lock() implies something along those
lines when PROVE_RCU.