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

[Sumit Gupta]

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