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

[Sudeep Holla]

On Thu, Jan 19, 2023 at 07:42:28PM +0100, Krzysztof Kozlowski 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.
> 
> 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.
> ---
>  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.
> +

Any particular reason for even compiling this file in or allowing the
ARM_PSCI_CPUIDLE when PREEMPT_RT=y ? If we can't enter idle states, we
can as well compile this file out ?

-- 
Regards,
Sudeep