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

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Date:   Fri, 31 Jan 2020 09:42:24 +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

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

I'm developing the RFC patch and then I'll send it as soon as possible.

> 
> Regards,
> Lukasz
> 
> On 1/27/20 3:17 PM, lukasz.luba@....com wrote:
>> From: Lukasz Luba <lukasz.luba@....com>
>>
>> Hi all,
>>
>> This patch is a continuation of my previous work for fixing DEVFREQ monitoring
>> subsystem [1]. The issue is around DEFERRABLE_WORK, which uses TIMER_DEFERRABLE
>> under the hood which will work normally when the system is busy, but will not
>> cause a CPU to come out of idle and serve the DEVFREQ monitoring requests.
>>
>> This is especially important in the SMP systems with many CPUs, when the load
>> balance tries to keep some CPUs idle. The next service request could not be
>> triggered when the CPU went idle in the meantime.
>>
>> The DELAYED_WORK is going to be triggered even on an idle CPU. This will allow
>> to call the DEVFREQ monitoring in reliable intervals. Some of the drivers might
>> use internal counters to monitor their load, when the DEVFREQ work is not
>> triggered in a predictable way, these counters might overflow leaving the
>> device in undefined state.
>>
>> To observe the difference, the trace output might be used, i.e.
>>
>> echo 1 > /sys/kernel/debug/tracing/events/devfreq/enable
>> #your test starts here, i.e. 'sleep 5' or 'dd ' or 'gfxbench'
>> echo 0 > /sys/kernel/debug/tracing/events/devfreq/enable
>> cat /sys/kernel/debug/tracing/trace
>>
>> When there are some registered devfreq drivers, you should see the traces
>> 'devfreq_moniotor' triggered in reliable intervals.
>>
>> The patch set is based on Chanwoo's devfreq repository and branch
>> 'devfreq-next' [2].
>>
>> Regards,
>> Lukasz Luba
>>
>> [1] https://protect2.fireeye.com/url?k=d26154c0-8fb20fd4-d260df8f-0cc47a31ce4e-ba68a61e16ee1965&u=https://lkml.org/lkml/2019/2/12/1179
>> [2] https://git.kernel.org/pub/scm/linux/kernel/git/chanwoo/linux.git/log/?h=devfreq-next
>>
>>
>> Lukasz Luba (1):
>>    drivers: devfreq: add DELAYED_WORK to monitoring subsystem
>>
>>   drivers/devfreq/Kconfig   | 19 +++++++++++++++++++
>>   drivers/devfreq/devfreq.c |  6 +++++-
>>   2 files changed, 24 insertions(+), 1 deletion(-)
>>
> 
> 


-- 
Best Regards,
Chanwoo Choi
Samsung Electronics