Re: [PATCH v3] cpuidle: psci: Do not suspend topology CPUs on PREEMPT_RT

[Krzysztof Kozlowski]

On 24/01/2023 11:33, Ulf Hansson wrote:
> On Thu, 19 Jan 2023 at 19:42, Krzysztof Kozlowski
> <krzysztof.kozlowski@xxxxxxxxxx> wrote:
>>
>> The runtime Power Management of CPU topology is not compatible with
>> PREEMPT_RT:
>> 1. Core cpuidle path disables IRQs.
>> 2. Core cpuidle calls cpuidle-psci.
>> 3. cpuidle-psci in __psci_enter_domain_idle_state() calls
>>    pm_runtime_put_sync_suspend() and pm_runtime_get_sync() which use
>>    spinlocks (which are sleeping on PREEMPT_RT).
>>
>> Deep sleep modes are not a priority of Realtime kernels because the
>> latencies might become unpredictable.  On the other hand the PSCI CPU
>> idle power domain is a parent of other devices and power domain
>> controllers, thus it cannot be simply skipped (e.g. on Qualcomm SM8250).
>>
>> Disable the runtime PM calls from cpuidle-psci, which effectively stops
>> suspending the cpuidle PSCI domain.  This is a trade-off between making
>> PREEMPT_RT working and still having a proper power domain hierarchy in
>> the system.
> 
> I think this sounds like a reasonable compromise, at least at this point.
> 
>>
>> Cc: Adrien Thierry <athierry@xxxxxxxxxx>
>> Cc: Brian Masney <bmasney@xxxxxxxxxx>
>> Cc: linux-rt-users@xxxxxxxxxxxxxxx
>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxx>
>>
>> ---
>>
>> Changes since v1:
>> 1. Re-work commit msg.
>> 2. Add note to Kconfig.
>>
>> Several other patches were dropped, as this is the only one actually
>> needed.  It effectively stops PSCI cpuidle power domains from suspending
>> thus solving all other issues I experienced.
> 
> I like this approach better, thanks!
> 
>> ---
>>  drivers/cpuidle/Kconfig.arm    | 3 +++
>>  drivers/cpuidle/cpuidle-psci.c | 4 ++--
>>  2 files changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/cpuidle/Kconfig.arm b/drivers/cpuidle/Kconfig.arm
>> index 747aa537389b..24429b5bfd1c 100644
>> --- a/drivers/cpuidle/Kconfig.arm
>> +++ b/drivers/cpuidle/Kconfig.arm
>> @@ -24,6 +24,9 @@ config ARM_PSCI_CPUIDLE
>>           It provides an idle driver that is capable of detecting and
>>           managing idle states through the PSCI firmware interface.
>>
>> +         The driver is not yet compatible with PREEMPT_RT: no idle states will
>> +         be entered by CPUs on such kernel.
> 
> This isn't entirely correct. In principle your suggested change ends
> up providing the below updated behaviour for PREEMPT_RT.
> 
> *) If the idle states are described with the non-hierarchical layout,
> all idle states are still available.
> **) If the idle states are described with the hierarchical layout,
> only the idle states defined per CPU are available, but not the ones
> being shared among a group of CPUs (aka cluster idle states).
> 
> Perhaps there is an easier way to summarize what I stated above?

Yes, I'll correct the message.

> 
>> +
>>  config ARM_PSCI_CPUIDLE_DOMAIN
>>         bool "PSCI CPU idle Domain"
>>         depends on ARM_PSCI_CPUIDLE
>> diff --git a/drivers/cpuidle/cpuidle-psci.c b/drivers/cpuidle/cpuidle-psci.c
>> index 312a34ef28dc..c25592718984 100644
>> --- a/drivers/cpuidle/cpuidle-psci.c
>> +++ b/drivers/cpuidle/cpuidle-psci.c
>> @@ -66,7 +66,7 @@ static __cpuidle int __psci_enter_domain_idle_state(struct cpuidle_device *dev,
>>         /* Do runtime PM to manage a hierarchical CPU toplogy. */
>>         if (s2idle)
>>                 dev_pm_genpd_suspend(pd_dev);
>> -       else
>> +       else if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> 
> Rather than doing this (and the below) in
> __psci_enter_domain_idle_state(), I suggest replacing this with a
> bailout point in psci_dt_cpu_init_topology(). That would prevent the
> __psci_enter_domain_idle_state() from being called altogether, which
> is really what we need.

Ack

> 
> Moreover, I think it would make sense to set the GENPD_FLAG_ALWAYS_ON
> for the corresponding genpd, when CONFIG_PREEMPT_RT is set. See
> psci_pd_init().

Makes sense.


Best regards,
Krzysztof