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

**From: **Michael Turquette

**Date: ** Thu Mar 10 2016 - 18:21:08 EST

Quoting Rafael J. Wysocki (2016-03-09 15:41:34)

>* On Wed, Mar 9, 2016 at 11:15 AM, Juri Lelli <juri.lelli@xxxxxxx> wrote:*

>* > Hi,*

>* >*

>* > sorry if I didn't reply yet. Trying to cope with jetlag and*

>* > talks/meetings these days :-). Let me see if I'm getting what you are*

>* > discussing, though.*

>* >*

>* > On 08/03/16 21:05, Rafael J. Wysocki wrote:*

>* >> 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:*

>* >*

>* > [...]*

>* >*

>* >> 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.*

>* >>*

>* >*

>* > Expanding terms again, your original formula (without the 1.1 factor of*

>* > the last version) was:*

>* >*

>* > next_freq = util / max_cap * max_freq*

>* >*

>* > and this doesn't work when we have freq invariance since util won't go*

>* > over curr_cap.*

>* *

>* Can you please remind me what curr_cap is?*

>* *

>* > What you propose above is to add another factor, so that we have:*

>* >*

>* > next_freq = util / max_cap * max_freq / curr_freq * max_freq*

>* >*

>* > which should give us the opportunity to reach max_freq also with freq*

>* > invariance.*

>* >*

>* > This should actually be the same of doing:*

>* >*

>* > next_freq = util / max_cap * max_cap / curr_cap * max_freq*

>* >*

>* > We are basically scaling how much the cpu is busy at curr_cap back to*

>* > the 0..1024 scale. And we use this to select next_freq. Also, we can*

>* > simplify this to:*

>* >*

>* > next_freq = util / curr_cap * max_freq*

>* >*

>* > and we save some ops.*

>* >*

>* > However, if that is correct, I think we might have a problem, as we are*

>* > skewing OPP selection towards higher frequencies. Let's suppose we have*

>* > a platform with 3 OPPs:*

>* >*

>* > freq cap*

>* > 1200 1024*

>* > 900 768*

>* > 600 512*

>* >*

>* > As soon a task reaches an utilization of 257 we will be selecting the*

>* > second OPP as*

>* >*

>* > next_freq = 257 / 512 * 1200 ~ 602*

>* >*

>* > While the cpu is only 50% busy in this case. And we will go at max OPP*

>* > when reaching ~492 (~64% of 768).*

>* >*

>* > That said, I guess this might work as a first solution, but we will*

>* > probably need something better in the future. I understand Rafael's*

>* > concerns regardin margins, but it seems to me that some kind of*

>* > additional parameter will be probably needed anyway to fix this.*

>* > Just to say again how we handle this in schedfreq, with a -20% margin*

>* > applied to the lowest OPP we will get to the next one when utilization*

>* > reaches ~410 (80% busy at curr OPP), and so on for the subsequent ones,*

>* > which is less aggressive and might be better IMHO.*

>* *

>* Well, Peter says that my idea is incorrect, so I'll go for*

>* *

>* next_freq = C * current_freq * util_raw / max*

>* *

>* where C > 1 (and likely C < 1.5) instead.*

>* *

>* That means C has to be determined somehow or guessed. The 80% tipping*

>* point condition seems reasonable to me, though, which leads to C =*

>* 1.25.*

Right, that is the same value used in the schedfreq series:

+/*

+ * Capacity margin added to CFS and RT capacity requests to provide

+ * some head room if task utilization further increases.

+ */

+unsigned int capacity_margin = 1280;

Regards,

Mike