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

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Date:   Tue, 12 Feb 2019 14:46:24 +0900
From:   Chanwoo Choi <cw00.choi@...sung.com>
To:     Lukasz Luba <l.luba@...tner.samsung.com>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Cc:     b.zolnierkie@...sung.com, myungjoo.ham@...sung.com,
        kyungmin.park@...sung.com, m.szyprowski@...sung.com,
        s.nawrocki@...sung.com, joel@...lfernandes.org,
        chris.diamand@....com, mka@...omium.org
Subject: Re: [PATCH v2 0/2] drivers: devfreq: fix and optimize workqueue
 mechanism

Hi Lukasz,

On 19. 2. 12. 오전 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.

> 
> Changes:
> v2:
> - single patch split into two
> - added cover letter
> 
> link for the previous version and discussion:
> https://marc.info/?l=linux-pm&m=154904631226997&w=2
> 
> Regards,
> Lukasz Luba
> 
> Lukasz Luba (2):
>   drivers: devfreq: change devfreq workqueue mechanism
>   drivers: devfreq: change deferred work into delayed
> 
>  drivers/devfreq/devfreq.c | 27 +++++++--------------------
>  1 file changed, 7 insertions(+), 20 deletions(-)
> 

-- 
Best Regards,
Chanwoo Choi
Samsung Electronics