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

From: Mark Rutland <[email-masked]>
Date: Tue, 14 Jun 2022 17:53:25 +0100

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On Tue, Jun 14, 2022 at 06:42:14PM +0200, Peter Zijlstra wrote:
> 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) <[email-masked]>
> > > ---
> > >  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.

Ah, duh. Given that:

Acked-by: Mark Rutland <[email-masked]>

Mark.