Re: [PATCH v2 0/2] drivers: devfreq: fix and optimize workqueue mechanism

[Lukasz Luba]

Hi Matthias,

On 2/12/19 8:32 PM, Matthias Kaehlcke wrote:
> Hi,
> 
> On Tue, Feb 12, 2019 at 02:46:24PM +0900, Chanwoo Choi wrote:
>> Hi Lukasz,
>>
>> On 19. 2. 12. ìì 12:30, Lukasz Luba wrote:
>>> This patch set changes workqueue related features in devfreq framework.
>>> First patch switches to delayed work instead of deferred.
>>> The second switches to regular system work and deletes custom 'devfreq'.
>>>
>>> Using deferred work in this context might harm the system performance.
>>> When the CPU enters idle, deferred work is not fired. The devfreq device's
>>> utilization does not have to be connected with a particular CPU.
>>> The drivers for GPUs, Network on Chip, cache L3 rely on devfreq governor.
>>> They all are missing opportunity to check the HW state and react when
>>> the deferred work is not fired.
>>> A corner test case, when Dynamic Memory Controller is utilized by CPUs running
>>> on full speed, might show x5 worse performance if the crucial CPU is in idle.
>>
>> The devfreq framework keeps the balancing between performance
>> and power-consumption. It is wrong to focus on only either
>> performance or power.
>>
>> This cover-letter focus on the only performance without any power-consumption
>> disadvantages. It is easy to raise the performance with short sampling rate
>> with polling modes. To get the performance, it is good as short as possible
>> of period.
>>
>> Sometimes, when cpu is idle, the device might require the busy state.
>> It is very difficult to catch the always right timing between them.
>>
>> Also, this patch cannot prevent the unneeded wakeup from idle state.
>> Apparently, it only focuses on performance without considering
>> the power-consumption disadvantage. In the embedded device,
>> the power-consumption is very important point. We can not ignore
>> the side effect.
>>
>> Always, I hope to improve the devfreq framwork more that older.
>> But, frankly, it is difficult to agree because it only consider
>> the performance without considering the side-effect.
>>
>> The power management framework always have to consider
>> the power-consumption issue. This point is always true.
> 
> I missed the impact of forcing a CPU out of an idle state and/or not
> allowing it to enter a more power efficient state. I agree that this
> should be avoided.
It would be good to have some real world scenarios for comparison:
w/ and w/o this change, i.e. it is 5% or 50% more power used.
I have patches that tries to mitigate wake-ups when there is small
utilization. Let's make it tunable and involve driver developers.
They will decide how much impact on the system power usage they
introduce.
> 
> I wonder if using a power-efficient workqueue could help here:
> 
>    Instead of running work on the local CPU, the workqueue core asks the
>    scheduler to provide the target CPU for the work queued on unbound
>    workqueues (which includes those marked as power-efficient). So they
>    will not get pinned on a single CPU as can happen with regular
>    workqueues.
> 
>    https://lwn.net/Articles/731052/
> 
> 
> Since this series also changes from a custom to system workqueue it
> seems worth to mention that there are power-efficient system workqueues:
> 
> system_power_efficient_wq
> system_freezable_power_efficient_wq
> 
> 
> In case a power-efficient workqueue is suitable in principle there
> would still be a problem though: the feature is currently disabled by
> default, hence devfreq couldn't really rely on it. It is enabled in
> the arm64 defconfig though, so at least devices on this architecture
> would benefit from it. Also power-efficient workqueues might be
> enabled by default in the future as the scheduler becomes more energy
> aware.
Regarding this CPU idle cost worries.
IIRC the new energy model does not even consider idle costs of the CPU.
It would be good to know the measurements, i.e. worst case scenario:
waking up 1 (of 4 or 8) CPU from idle 30 times per second for let's
say 100 us. It is 3 ms / 1000 ms * running energy cost i.e. 250mW.
Thus, 0.75mW.
In my opinion it is not a big cost. In most cases the system is still
doing some other work. It is worth to mention here that on mobiles
when the power button is hit the full suspend is called which freezes
all tasks, devices and power consumption is ~15mW. Thus, the system
suspend is out of scope here.

As I replayed to Chanwoon for the same email: in my opinion current
devfreq is broken.
It was probably developed in times where there was 1 CPU (maybe 2)
and idle state of CPU would be a good hint to not to check devfreq
devices.

Regards,
Lukasz
> 
> Cheers
> 
> Matthias
> 
>