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

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Date:   Mon, 13 Apr 2020 11:51:41 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Sumit Gupta <sumitg@...dia.com>
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 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 ?

> > > > > > > > > +     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().

-- 
viresh