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Subject: Re: brocken devfreq simple_ondemand for Odroid XU3/4?
To:     Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Lukasz Luba <lukasz.luba@arm.com>
Cc:     Kamil Konieczny <k.konieczny@samsung.com>,
        Sylwester Nawrocki <s.nawrocki@samsung.com>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Willy Wolff <willy.mh.wolff.ml@gmail.com>,
        MyungJoo Ham <myungjoo.ham@samsung.com>,
        Kyungmin Park <kyungmin.park@samsung.com>,
        Kukjin Kim <kgene@kernel.org>, linux-pm@vger.kernel.org,
        "linux-samsung-soc@vger.kernel.org" 
        <linux-samsung-soc@vger.kernel.org>,
        linux-arm-kernel@lists.infradead.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
From:   Chanwoo Choi <cw00.choi@samsung.com>
Organization: Samsung Electronics
Message-ID: <691bc55c-5b04-b519-4575-6dce5ea9914c@samsung.com>
Date:   Mon, 29 Jun 2020 10:43:39 +0900
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Hi,

Sorry for late reply because of my perfornal issue. I count not check the email.

On 6/26/20 8:22 PM, Bartlomiej Zolnierkiewicz wrote:
> 
> On 6/25/20 2:12 PM, Kamil Konieczny wrote:
>> On 25.06.2020 14:02, Lukasz Luba wrote:
>>>
>>>
>>> On 6/25/20 12:30 PM, Kamil Konieczny wrote:
>>>> Hi Lukasz,
>>>>
>>>> On 25.06.2020 12:02, Lukasz Luba wrote:
>>>>> Hi Sylwester,
>>>>>
>>>>> On 6/24/20 4:11 PM, Sylwester Nawrocki wrote:
>>>>>> Hi All,
>>>>>>
>>>>>> On 24.06.2020 12:32, Lukasz Luba wrote:
>>>>>>> I had issues with devfreq governor which wasn't called by devfreq
>>>>>>> workqueue. The old DELAYED vs DEFERRED work discussions and my patches
>>>>>>> for it [1]. If the CPU which scheduled the next work went idle, the
>>>>>>> devfreq workqueue will not be kicked and devfreq governor won't check
>>>>>>> DMC status and will not decide to decrease the frequency based on low
>>>>>>> busy_time.
>>>>>>> The same applies for going up with the frequency. They both are
>>>>>>> done by the governor but the workqueue must be scheduled periodically.
>>>>>>
>>>>>> As I have been working on resolving the video mixer IOMMU fault issue
>>>>>> described here: https://patchwork.kernel.org/patch/10861757
>>>>>> I did some investigation of the devfreq operation, mostly on Odroid U3.
>>>>>>
>>>>>> My conclusions are similar to what Lukasz says above. I would like to add
>>>>>> that broken scheduling of the performance counters read and the devfreq
>>>>>> updates seems to have one more serious implication. In each call, which
>>>>>> normally should happen periodically with fixed interval we stop the counters,
>>>>>> read counter values and start the counters again. But if period between
>>>>>> calls becomes long enough to let any of the counters overflow, we will
>>>>>> get wrong performance measurement results. My observations are that
>>>>>> the workqueue job can be suspended for several seconds and conditions for
>>>>>> the counter overflow occur sooner or later, depending among others
>>>>>> on the CPUs load.
>>>>>> Wrong bus load measurement can lead to setting too low interconnect bus
>>>>>> clock frequency and then bad things happen in peripheral devices.
>>>>>>
>>>>>> I agree the workqueue issue needs to be fixed. I have some WIP code to use
>>>>>> the performance counters overflow interrupts instead of SW polling and with
>>>>>> that the interconnect bus clock control seems to work much better.
>>>>>>
>>>>>
>>>>> Thank you for sharing your use case and investigation results. I think
>>>>> we are reaching a decent number of developers to maybe address this
>>>>> issue: 'workqueue issue needs to be fixed'.
>>>>> I have been facing this devfreq workqueue issue ~5 times in different
>>>>> platforms.
>>>>>
>>>>> Regarding the 'performance counters overflow interrupts' there is one
>>>>> thing worth to keep in mind: variable utilization and frequency.
>>>>> For example, in order to make a conclusion in algorithm deciding that
>>>>> the device should increase or decrease the frequency, we fix the period
>>>>> of observation, i.e. to 500ms. That can cause the long delay if the
>>>>> utilization of the device suddenly drops. For example we set an
>>>>> overflow threshold to value i.e. 1000 and we know that at 1000MHz
>>>>> and full utilization (100%) the counter will reach that threshold
>>>>> after 500ms (which we want, because we don't want too many interrupts
>>>>> per sec). What if suddenly utilization drops to 2% (i.e. from 5GB/s
>>>>> to 250MB/s (what if it drops to 25MB/s?!)), the counter will reach the
>>>>> threshold after 50*500ms = 25s. It is impossible just for the counters
>>>>> to predict next utilization and adjust the threshold. [...]
>>>>
>>>> irq triggers for underflow and overflow, so driver can adjust freq
>>>>
>>>
>>> Probably possible on some platforms, depends on how many PMU registers
>>> are available, what information can be can assign to them and type of
>>> interrupt. A lot of hassle and still - platform and device specific.
>>> Also, drivers should not adjust the freq, governors (different types
>>> of them with different settings that they can handle) should do it.
>>>
>>> What the framework can do is to take this responsibility and provide
>>> generic way to monitor the devices (or stop if they are suspended).
>>> That should work nicely with the governors, which try to predict the
>>> next best frequency. From my experience the more fluctuating intervals
>>> the governors are called, the more odd decisions they make.
>>> That's why I think having a predictable interval i.e. 100ms is something
>>> desirable. Tuning the governors is easier in this case, statistics
>>> are easier to trace and interpret, solution is not to platform specific,
>>> etc.
>>>
>>> Kamil do you have plans to refresh and push your next version of the
>>> workqueue solution?
>>
>> I do not, as Bartek takes over my work,
>> +CC Bartek
> 
> Hi Lukasz,
> 
> As you remember in January Chanwoo has proposed another idea (to allow
> selecting workqueue type by devfreq device driver):
> 
> "I'm developing the RFC patch and then I'll send it as soon as possible."
> (https://lore.kernel.org/linux-pm/6107fa2b-81ad-060d-89a2-d8941ac4d17e@samsung.com/)
> 
> "After posting my suggestion, we can discuss it"
> (https://lore.kernel.org/linux-pm/f5c5cd64-b72c-2802-f6ea-ab3d28483260@samsung.com/)
> 
> so we have been waiting on the patch to be posted..

Sorry for this. I'll send it within few days.


> 
> Similarly we have been waiting on (any) feedback for exynos-bus/nocp
> fixes for Exynos5422 support (which have been posted by Kamil also in
> January):
> 
> https://lore.kernel.org/linux-pm/8f82d8d5-927b-afb4-272f-45c16b5a23b9@samsung.com/
> 
> Considering the above and how hard it has been to push the changes
> through review/merge process last year we are near giving up when it
> comes to upstream devfreq contributions. Sylwester is still working on
> exynos-bus & interconnect integration (continuation of Artur Swigon's
> work from last year) & related issues (IRQ support for PPMU)  but
> I'm seriously considering putting it all on-hold..

The Sylwester's patches (originally Artus Swigon's path) were reviewed
and I agreed this approach about devfreq/interconnect. It needs
the review from interconnect maintainer.

> 
> Best regards,
> --
> Bartlomiej Zolnierkiewicz
> Samsung R&D Institute Poland
> Samsung Electronics
> 
> 


-- 
Best Regards,
Chanwoo Choi
Samsung Electronics