linux-kernel - Re: [PATCH 0/1] drivers: devfreq: use DELAYED

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Message-ID: <931bb376-0045-9929-6e67-15d34ebf3fbe@samsung.com>
Date:   Mon, 3 Feb 2020 10:10:00 +0900
From:   Chanwoo Choi <cw00.choi@...sung.com>
To:     Lukasz Luba <lukasz.luba@....com>, myungjoo.ham@...sung.com,
        kyungmin.park@...sung.com, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     b.zolnierkie@...sung.com, Kamil Konieczny <k.konieczny@...sung.com>
Subject: Re: [PATCH 0/1] drivers: devfreq: use DELAYED_WORK in DEVFREQ
 monitoring subsystem

On 1/31/20 6:38 PM, Lukasz Luba wrote:
> Hi Chanwoo,
> 
> On 1/31/20 12:47 AM, Chanwoo Choi wrote:
>> On 1/31/20 9:42 AM, Chanwoo Choi wrote:
>>> Hi Lukasz,
>>>
>>> On 1/30/20 8:47 PM, Lukasz Luba wrote:
>>>> Hi Chanwoo, MyungJoo,
>>>>
>>>> Gentle ping. The issue is not only in the devfreq itself,
>>>> but also it affects thermal. The devfreq cooling rely on
>>>> busy_time and total_time updated by the devfreq monitoring
>>>> (in simple_ondemand).
>>>> Thermal uses DELAYED_WORK and is more reliable, but uses stale
>>>> data from devfreq_dev_stats. It is especially visible when
>>>> you have cgroup spanning one cluster. Android uses cgroups
>>>> heavily. You can make easily this setup using 'taskset',
>>>> run some benchmarks and observe 'devfreq_monitor' traces and
>>>> timestamps, i.e. for your exynos-bus.
>>>>
>>>> The patch is really non-invasive and simple. It can be a good starting
>>>> point for testing and proposing other solutions.
>>>
>>> Sorry for late reply. I'm preparing the RFC patch about my approach
>>> to support this requirement as following:
>>>
>>> As you knew, DEFERRABLE_WORK with CONFIG_NO_HZ focuses on removing
>>> the redundant of power-consumption by preventing the unneeded wakeup
>>> from idle state if there are no any interrupts and runnable threads.
>>>
>>> Finally, I agree the requirement of delaywd_work for devfreq subsystem.
>>> But, I would like to support both deferrable_work and delayed_work
>>> on devfreq subsystem. It is better to select either deferrable_work
>>> or delayed_work by user like Kamil's suggestion[1].
>>> [1] https://lore.kernel.org/patchwork/patch/1164317/
>>> - [2/4] PM / devfreq: add possibility for delayed work
>>>
>>> But, I want to change the timer type for devfreq device
>>> using simple_ondemand governor via sysfs as following:
>>>
>>> Example:
>>>
>>> 1.
>>> enum work_timer_type {
>>>     DEVFREQ_WORK_TIMER_DEFERRABLE = 0,
>>>     DEVFREQ_WORK_TIMER_DELAYED = 0,
>>> };
>>>
>>> struct devfreq_simple_ondemand_data {
>>>     unsigned int upthreshold;
>>>     unsigned int downdifferential;
>>>     enum work_timer_type timer_type;
>>> };
>>>
>>> The developer of devfreq device driver can choose
>>> the default work time type by initializing the 'timer_type of
>>> struct devfreq_simple_ondemand_data'.
>>>
>>> 2. Change the work timer type at the runtime
>>> - Change the work timer type from 'deferrable' to 'delayed'
>>> $ echo delayed > /sys/class/devfreq/devfreq0/work_timer_type
>>> $ cat /sys/class/devfreq/devfreq0/work_timer_type
>>> delayed
>>>
>>> - Change the work timer type from 'delayed' to 'deferrable'
>>> $ echo deferrable > /sys/class/devfreq/devfreq0/work_timer_type
>>> $ cat /sys/class/devfreq/devfreq0/work_timer_type
>>> deferrable
>>>
>>
>> And
>> Only show '/sys/class/devfreq/devfreq0/work_timer_type' sysfs attribute,
>> if devfreq device uses the simple_ondemand. Because this 'work_timer_type'
>> sysfs attribute only depends on simple_ondemand governor and are useful.
>>
>> So, 'work_timer_type' sysfs attribute will be handled
>> at drivers/devfreq/governor_simpleondemand.c.
>>
>> After posting my suggestion, we can discuss it.
>>
>>
>>> I'm developing the RFC patch and then I'll send it as soon as possible.
> 
> Good, thank you for the explanation. For the first glance the design
> looks OK, we can discuss it a bit more in you RFC series.
> I would recommend to not make it conditional on simple_ondemand governor
> just add a comment that for i.e. performance or passive governors it has
> less sense to use this setting. There might be some other governors
> loaded as modules, which could benefit from it, or in Android e.g.
> https://android.googlesource.com/kernel/msm/+/refs/heads/android-msm-coral-4.14-android10/drivers/devfreq/governor_msm_adreno_tz.c

OK. Instead, I'll add the flag for governors. The governor flag
indicates each sysfs entries like polling_interval, work_timer_type.

If each governor want to use the specific sysfs attributes,
just set the flag when governor is defined.

Thanks.

> 
> It would be good if it can land in mainline before v5.8-v5.9.
> 
> Regards,
> Lukasz
> 
> 
> 
> 


-- 
Best Regards,
Chanwoo Choi
Samsung Electronics