[PATCH 1/2] cpuidle: Add new macro to enter a retention idle state

[Sudeep Holla]

On 07/11/17 17:35, Prashanth Prakash wrote:
> If a CPU is entering a low power idle state where it doesn't lose any
> context, then there is no need to call cpu_pm_enter()/cpu_pm_exit().
> Add a new macro(CPU_PM_CPU_IDLE_ENTER_RETENTION) to be used by cpuidle
> drivers when they are entering retention state. By not calling
> cpu_pm_enter and cpu_pm_exit we reduce the latency involved in
> entering and exiting the retention idle states.
> 
> On ARM64 based Qualcomm Server Platform we measured below overhead for
> for calling cpu_pm_enter and cpu_pm_exit for retention states.
> 
> workload: stress --hdd #CPUs --hdd-bytes 32M  -t 30
>         Average overhead of cpu_pm_enter - 1.2us
>         Average overhead of cpu_pm_exit  - 3.1us
> 
> Signed-off-by: Prashanth Prakash <pprakash at codeaurora.org>
> ---
>  include/linux/cpuidle.h | 38 +++++++++++++++++++++++---------------
>  1 file changed, 23 insertions(+), 15 deletions(-)
> 
> diff --git a/include/linux/cpuidle.h b/include/linux/cpuidle.h
> index 8f7788d..54cbd9d 100644
> --- a/include/linux/cpuidle.h
> +++ b/include/linux/cpuidle.h
> @@ -258,21 +258,29 @@ static inline int cpuidle_register_governor(struct cpuidle_governor *gov)
>  #endif
>  
>  #define CPU_PM_CPU_IDLE_ENTER(low_level_idle_enter, idx)	\
> -({								\
> -	int __ret;						\
> -								\
> -	if (!idx) {						\
> -		cpu_do_idle();					\
> -		return idx;					\
> -	}							\
> -								\
> -	__ret = cpu_pm_enter();					\
> -	if (!__ret) {						\
> -		__ret = low_level_idle_enter(idx);		\
> -		cpu_pm_exit();					\
> -	}							\
> -								\
> -	__ret ? -1 : idx;					\
> +	__CPU_PM_CPU_IDLE_ENTER(low_level_idle_enter, idx, 0)
> +
> +#define CPU_PM_CPU_IDLE_ENTER_RETENTION(low_level_idle_enter, idx)	\
> +	__CPU_PM_CPU_IDLE_ENTER(low_level_idle_enter, idx, 1)
> +
> +#define __CPU_PM_CPU_IDLE_ENTER(low_level_idle_enter, idx, is_retention) \
> +({									\
> +	int __ret = 0;							\
> +									\
> +	if (!idx) {							\
> +		cpu_do_idle();						\
> +		return idx;						\
> +	}								\
> +									\
> +	if (!is_retention)						\
> +		__ret =  cpu_pm_enter();				\

I am fine with this change as initial step. But I am wondering if we
will have a retention state which loses partial state ?

The specification has flags to specify that difference but will we see
that in reality is a different question. If we see such hardware, then
we may need to revert this and handle in the callbacks as we can't skip
cpu_pm notifier callbacks all together right ?

-- 
-- 
Regards,
Sudeep