linux-kernel - Re: [PATCH v2 3/5] cpuidle: psci: Do not suspend topology CPUs on PREEMPT

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Message-ID: <3d7c3a52-baa4-ce7b-e6d1-06a4d885d445@linaro.org>
Date:   Thu, 12 Jan 2023 12:34:35 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kevin Hilman <khilman@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Adrien Thierry <athierry@...hat.com>,
        Brian Masney <bmasney@...hat.com>,
        linux-rt-users@...r.kernel.org
Subject: Re: [PATCH v2 3/5] cpuidle: psci: Do not suspend topology CPUs on
 PREEMPT_RT

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