linux-kernel - Re: [TEGRA194_CPUFREQ Patch 2/3] cpufreq: Add Tegra194 cpufreq driver

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Message-ID: <64b609f1-efb1-425f-a91a-27a492bd3ec4@nvidia.com>
Date:   Mon, 13 Apr 2020 17:50:00 +0530
From:   Sumit Gupta <sumitg@...dia.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
CC:     <rjw@...ysocki.net>, <catalin.marinas@....com>, <will@...nel.org>,
        <thierry.reding@...il.com>, <jonathanh@...dia.com>,
        <talho@...dia.com>, <linux-pm@...r.kernel.org>,
        <linux-tegra@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <bbasu@...dia.com>,
        <mperttunen@...dia.com>
Subject: Re: [TEGRA194_CPUFREQ Patch 2/3] cpufreq: Add Tegra194 cpufreq driver



On 13/04/20 11:51 AM, Viresh Kumar wrote:
> External email: Use caution opening links or attachments
> 
> 
> On 09-04-20, 16:51, Sumit Gupta wrote:
>> We are using "read_counters_work" as local variable. So every invocation the
>> function will have its own copy of counters for corresponding cpu. That's
>> why are doing INIT_WORK_ONSTACK here.
> 
> Why? To support parallel calls to reading the freq ?
> 
Yes.

>>>>>>>>>> +     queue_work_on(cpu, read_counters_wq, &read_counters_work.work);
>>>>>>>>>> +     flush_work(&read_counters_work.work);
>>>>>>>>>
>>>>>>>>> Why can't this be done in current context ?
>>>>>>>>>
>>>>>>>> We used work queue instead of smp_call_function_single() to have long delay.
>>>>>>>
>>>>>>> Please explain completely, you have raised more questions than you
>>>>>>> answered :)
>>>>>>>
>>>>>>> Why do you want to have long delays ?
>>>>>>>
>>>>>> Long delay value is used to have the observation window long enough for
>>>>>> correctly reconstructing the CPU frequency considering noise.
>>>>>> In next patch version, changed delay value to 500us which in our tests is
>>>>>> considered reliable.
>>>>>
>>>>> I understand that you need to put a udelay() while reading the freq from
>>>>> hardware, that is fine, but why do you need a workqueue for that? Why can't you
>>>>> just read the values directly from the same context ?
>>>>>
>>>> The register to read frequency is per core and not accessible to other
>>>> cores. So, we have to execute the function remotely as the target core to
>>>> read frequency might be different from current.
>>>> The functions for that are smp_call_function_single or queue_work_on.
>>>> We used queue_work_on() to avoid long delay inside ipi interrupt context
>>>> with interrupts disabled.
>>>
>>> Okay, I understand this now, finally :)
>>>
>>> But if the interrupts are disabled during some call, won't workqueues face the
>>> same problem ?
>>>
>> Yes, we are trying to minimize the case.
> 
> But how do you know workqueues will perform better than
> smp_call_function_single() ? Just asking for clarity on this as normally
> everyone tries to do smp_call_function_single().
> 
This was done considering long delay value as explained previously.
Do you think that smp_call_function_single() would be better than work 
queue here?

> --
> viresh
>