Re: [PATCH v2 3/5] cpuidle: psci: Do not suspend topology CPUs on PREEMPT_RT

[Krzysztof Kozlowski]

On 12/01/2023 12:09, Sebastian Andrzej Siewior wrote:
> On 2022-12-19 16:15:01 [+0100], Krzysztof Kozlowski wrote:
>> diff --git a/drivers/cpuidle/cpuidle-psci.c b/drivers/cpuidle/cpuidle-psci.c
>> index 57bc3e3ae391..9d971cc4b12b 100644
>> --- a/drivers/cpuidle/cpuidle-psci.c
>> +++ b/drivers/cpuidle/cpuidle-psci.c
>> @@ -72,7 +72,7 @@ static int __psci_enter_domain_idle_state(struct cpuidle_device *dev,
>>  	ct_irq_enter_irqson();
>>  	if (s2idle)
>>  		dev_pm_genpd_suspend(pd_dev);
>> -	else
>> +	else if (!IS_ENABLED(CONFIG_PREEMPT_RT))
>>  		pm_runtime_put_sync_suspend(pd_dev);
> 
> So based on the commit description you run into a sleeping lock in
> pm_runtime_put_sync_suspend() while the CPU is in an IRQ-off region.
> Why is it okay to skip it on PREEMPT_RT?

It is okay to skip it everywhere, you just don't get a suspended CPU.
Why PREEMPT_RT is different here - having suspended CPU is a great way
to have longer or even unpredictable (as it goes to firmware which is
out of control of Linux) latencies.

Best regards,
Krzysztof