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

[Prakash, Prashanth]

Hi Sudeep,

On 11/8/2017 7:38 AM, Sudeep Holla wrote:
>
> 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 ?
I agree. This is a initial and quick first step and will help only fully retention states.

If there are devices with partial retention states and necessary support in cpu
PM framework we can revert this and handle it in more generic way.

--
Thanks,
Prashanth