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

Mark Rutland
2022-06-14T16:15:37Z

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@infradead.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);
> }
>
>