Re: [PATCH v4] cpuidle: psci: Do not suspend topology CPUs on PREEMPT_RT

[Daniel Lezcano <daniel.lezcano@linaro.org>]

Hi Krzysztof,


On 25/01/2023 12:34, 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 idle callbacks in cpuidle-psci and mark the domain as
> always on.  This is a trade-off between making PREEMPT_RT working and
> still having a proper power domain hierarchy in the system.

Wouldn't make sense to rely on the latency constraint framework ?


> Cc: Adrien Thierry <athierry@redhat.com>
> Cc: Brian Masney <bmasney@redhat.com>
> Cc: linux-rt-users@vger.kernel.org
> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
> 
> ---
> 
> Changes since v3:
> 1. Rework - disable idle states, mark as always on (Ulf).
> 2. Extend Kconfig warning (Ulf).
> 
> 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           | 8 ++++++++
>   drivers/cpuidle/cpuidle-psci-domain.c | 7 +++++--
>   drivers/cpuidle/cpuidle-psci.c        | 3 +++
>   3 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/cpuidle/Kconfig.arm b/drivers/cpuidle/Kconfig.arm
> index 747aa537389b..8deaa2e05206 100644
> --- a/drivers/cpuidle/Kconfig.arm
> +++ b/drivers/cpuidle/Kconfig.arm
> @@ -24,6 +24,14 @@ 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 has limitations when used with 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).
> +
>   config ARM_PSCI_CPUIDLE_DOMAIN
>   	bool "PSCI CPU idle Domain"
>   	depends on ARM_PSCI_CPUIDLE
> diff --git a/drivers/cpuidle/cpuidle-psci-domain.c b/drivers/cpuidle/cpuidle-psci-domain.c
> index c80cf9ddabd8..6ad2954948a5 100644
> --- a/drivers/cpuidle/cpuidle-psci-domain.c
> +++ b/drivers/cpuidle/cpuidle-psci-domain.c
> @@ -64,8 +64,11 @@ static int psci_pd_init(struct device_node *np, bool use_osi)
>   
>   	pd->flags |= GENPD_FLAG_IRQ_SAFE | GENPD_FLAG_CPU_DOMAIN;
>   
> -	/* Allow power off when OSI has been successfully enabled. */
> -	if (use_osi)
> +	/*
> +	 * Allow power off when OSI has been successfully enabled.
> +	 * PREEMPT_RT is not yet ready to enter domain idle states.
> +	 */
> +	if (use_osi && !IS_ENABLED(CONFIG_PREEMPT_RT))
>   		pd->power_off = psci_pd_power_off;
>   	else
>   		pd->flags |= GENPD_FLAG_ALWAYS_ON;
> diff --git a/drivers/cpuidle/cpuidle-psci.c b/drivers/cpuidle/cpuidle-psci.c
> index 312a34ef28dc..6de027f9f6f5 100644
> --- a/drivers/cpuidle/cpuidle-psci.c
> +++ b/drivers/cpuidle/cpuidle-psci.c
> @@ -222,6 +222,9 @@ static int psci_dt_cpu_init_topology(struct cpuidle_driver *drv,
>   	if (!psci_has_osi_support())
>   		return 0;
>   
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> +		return 0;
> +
>   	data->dev = psci_dt_attach_cpu(cpu);
>   	if (IS_ERR_OR_NULL(data->dev))
>   		return PTR_ERR_OR_ZERO(data->dev);

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