linux-kernel - Re: brocken devfreq simple

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Date:   Mon, 29 Jun 2020 10:52:17 +0900
From:   Chanwoo Choi <cw00.choi@...sung.com>
To:     Sylwester Nawrocki <s.nawrocki@...sung.com>,
        Lukasz Luba <lukasz.luba@....com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Willy Wolff <willy.mh.wolff.ml@...il.com>
Cc:     MyungJoo Ham <myungjoo.ham@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Kukjin Kim <kgene@...nel.org>, linux-pm@...r.kernel.org,
        "linux-samsung-soc@...r.kernel.org" 
        <linux-samsung-soc@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: brocken devfreq simple_ondemand for Odroid XU3/4?

Hi Sylwester,

On 6/25/20 12:11 AM, 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 

It is good way to resolve the overflow issue.

> that the interconnect bus clock control seems to work much better.
>
-- 
Best Regards,
Chanwoo Choi
Samsung Electronics