Re: [RFC PATCH v2 3/8] cpuidle: add a new predict interface

Li, Aubrey
2017-10-16T08:04:19Z

On 2017/10/14 8:45, Rafael J. Wysocki wrote:
> On Saturday, September 30, 2017 9:20:29 AM CEST Aubrey Li wrote:
>> For the governor has predict functionality, add a new predict
>> interface in cpuidle framework to call and use it.
>
> Care to describe how it is intended to work?
>
> Also this patch uses data structures introduced in the previous one
> (and the previous one didn't use them), so maybe merge the two?

okay, will refine in the next version.

>
>> ---
>> drivers/cpuidle/cpuidle.c | 34 ++++++++++++++++++++++++++++++++++
>> drivers/cpuidle/governors/menu.c | 7 +++++++
>> include/linux/cpuidle.h | 3 +++
>> kernel/sched/idle.c | 1 +
>> 4 files changed, 45 insertions(+)
>>
>> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
>> index 4066308..ef6f7dd 100644
>> --- a/drivers/cpuidle/cpuidle.c
>> +++ b/drivers/cpuidle/cpuidle.c
>> @@ -336,6 +336,40 @@ void cpuidle_entry_end(void)
>> }
>>
>> /**
>> + * cpuidle_predict - predict whether the coming idle is a fast idle or not
>> + */
>> +void cpuidle_predict(void)
>> +{
>> + struct cpuidle_device *dev = cpuidle_get_device();
>> + unsigned int overhead_threshold;
>> +
>> + if (!dev)
>> + return;
>> +
>> + overhead_threshold = dev->idle_stat.overhead;
>> +
>> + if (cpuidle_curr_governor->predict) {
>> + dev->idle_stat.predicted_us = cpuidle_curr_governor->predict();
>> + /*
>> + * notify idle governor to avoid reduplicative
>> + * prediction computation
>> + */
>
> This comment is hard to understand.
>
> What does it mean, really?
>

predict() does a lot of computation. If it's called in cpuidle_predict(),
we don't want it to be called in menu governor again. So here I use a flag to
tell menu governor if predict computation is already done for the coming idle
this time.

>> + dev->idle_stat.predicted = true;
>> + if (dev->idle_stat.predicted_us < overhead_threshold) {
>> + /*
>> + * notify tick subsystem to keep ticking
>> + * for the coming idle
>> + */
>> + dev->idle_stat.fast_idle = true;
>> + } else
>> + dev->idle_stat.fast_idle = false;
>
> What about
>
> dev->idle_stat.fast_idle = dev->idle_stat.predicted_us < overhead_threshold;

Ack

>
> Also why don't you use dev->idle_stat.overhead directly here?

Yeah, I should merge a few patches, because dev->idle_stat.overhead is referenced many times,
for irq timings, for scheduler idle statistics and for idle governor.

>
> And what is the basic idea here? Why do we compare the predicted
> idle duration with the entry/exit overhead from the previous cycle
> (if I understood this correctly, that is)?

entry/exit overhead is an average here, and the variance should not be big.

>
>> + } else {
>> + dev->idle_stat.predicted = false;
>> + dev->idle_stat.fast_idle = false;
>> + }
>> +}
>> +
>> +/**
>> * cpuidle_install_idle_handler - installs the cpuidle idle loop handler
>> */
>> void cpuidle_install_idle_handler(void)
>> diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
>> index 6bed197..90b2a10 100644
>> --- a/drivers/cpuidle/governors/menu.c
>> +++ b/drivers/cpuidle/governors/menu.c
>> @@ -344,6 +344,12 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev)
>> if (unlikely(latency_req == 0))
>> return 0;
>>
>> + /*don't predict again if idle framework already did it */
>> + if (!dev->idle_stat.predicted)
>> + menu_predict();
>> + else
>> + dev->idle_stat.predicted = false;
>
> We provide the callback which is going to be used by the core if present,
> so why would the core not use it after all?

There is a case that in the loop

while (!need_resched()) {
cpuidle_idle_call()
}

The CPU receives a timer interrupt and wake up and find nothing to be scheduled
and back to call menu then mwait to sleep again.

Under this case, cpuidle_predict() is not reached but the idle data for the
prediction needs to be updated.

Thanks,
-Aubrey