Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler utilization data

[Rafael J. Wysocki]

On Tue, Mar 8, 2016 at 8:26 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Tue, Mar 08, 2016 at 07:00:57PM +0100, Rafael J. Wysocki wrote:
>> On Tue, Mar 8, 2016 at 12:27 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
>> > Seeing how frequency invariance is an arch feature, and cpufreq drivers
>> > are also typically arch specific, do we really need a flag at this
>> > level?
>>
>> The next frequency is selected by the governor and that's why.  The
>> driver gets a frequency to set only.
>>
>> Now, the governor needs to work with different platforms, so it needs
>> to know how to deal with the given one.
>
> Ah, indeed. In any case, the availability of arch_sched_scale_freq() is
> a compile time thingy, so we can, at compile time, know what to use.
>
>> > In any case, I think the only difference between the two formula should
>> > be the addition of (1) for the platforms that do not already implement
>> > frequency invariance.
>>
>> OK
>>
>> So I'm reading this as a statement that linear is a better
>> approximation for frequency invariant utilization.
>
> Well, (1) is what the scheduler does with frequency invariance, except
> that allows a more flexible definition of 'current frequency' by asking
> for it every time we update the util stats.
>
> But if a platform doesn't need this, ie. it has a fixed frequency, or
> simply doesn't provide anything like this, assuming we run at the
> frequency we asked for is a reasonable assumption no?
>
>> This means that on platforms where the utilization is frequency
>> invariant we should use
>>
>>   next_freq = a * x
>>
>> (where x is given by (2) above) and for platforms where the
>> utilization is not frequency invariant
>>
>>   next_freq = a * x * current_freq / max_freq
>>
>> and all boils down to finding a.
>
> Right.

However, that doesn't seem to be in agreement with the Steve's results
posted earlier in this thread.

Also theoretically, with frequency invariant, the only way you can get
to 100% utilization is by running at the max frequency, so the closer
to 100% you get, the faster you need to run to get any further.  That
indicates nonlinear to me.

>> Now, it seems reasonable for a to be something like (1 + 1/n) *
>> max_freq, so for non-frequency invariant we get
>>
>>   nex_freq = (1 + 1/n) * current_freq * x
>
> This seems like a big leap; where does:
>
>   (1 + 1/n) * max_freq
>
> come from? And what is 'n'?

a = max_freq gives next_freq = max_freq for x = 1, but with that
choice of a you may never get to x = 1 with frequency invariant
because of the feedback effect mentioned above, so the 1/n produces
the extra boost needed for that (n is a positive integer).

Quite frankly, to me it looks like linear really is a better
approximation for "raw" utilization.  That is, for frequency invariant
x we should take:

  next_freq = a * x * max_freq / current_freq

(and if x is not frequency invariant, the right-hand side becomes a *
x).  Then, the extra boost needed to get to x = 1 for frequency
invariant is produced by the (max_freq / current_freq) factor that is
greater than 1 as long as we are not running at max_freq and a can be
chosen as max_freq.