Re: [PATCH v2 3/3] cpufreq: schedutil: map raw required frequency to driver frequency

[Rafael J. Wysocki]

On Thu, May 26, 2016 at 9:16 AM, Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
> On 25-05-16, 19:53, Steve Muckle wrote:
>> The slow-path frequency transition path is relatively expensive as it
>> requires waking up a thread to do work. Should support be added for
>> remote CPU cpufreq updates that is also expensive since it requires an
>> IPI. These activities should be avoided if they are not necessary.
>>
>> To that end, calculate the actual driver-supported frequency required by
>> the new utilization value in schedutil by using the recently added
>> cpufreq_driver_resolve_freq callback. If it is the same as the
>> previously requested driver frequency then there is no need to continue
>> with the update assuming the cpu frequency limits have not changed. This
>> will have additional benefits should the semantics of the rate limit be
>> changed to apply solely to frequency transitions rather than to
>> frequency calculations in schedutil.

[cut]

> I also have a doubt (I am quite sure Rafael will have a reason for
> that, which I am failing to understand now), on why we are doing
> next_freq == UINT_MAX in sugov_should_update_freq().
>
> I understand that because the limits might have changed,
> need_freq_update would have been set to true. We should evaluate
> next-freq again without worrying about the load or the time since last
> evaluation.

This is in response to the "limits" event (or to the ->limits call
after my recent patches).  That event basically means "something has
changed, so if you have cached anything, invalidate it" to the
governor.  Accordingly, it invalidates next_freq, because that's a
cached value.

> But what will happen by forcefully calling the cpufreq routines to
> change the frequency, if next_freq hasn't changed even after limits
> updates?

I can't really parse the above question, so I'm not going to try to
answer it. :-)

> Wouldn't that call always return early because the new freq
> and the current freq are going to be same ?
>
> @Rafael: Sorry for asking this so late :(

It is not too late.  If there's a problem somewhere, it needs to be
fixed, but at this point I have no idea what you are asking about.