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

[Juri Lelli]

On 03/03/16 17:37, Peter Zijlstra wrote:
> On Thu, Mar 03, 2016 at 05:24:32PM +0100, Rafael J. Wysocki wrote:
> > On Thu, Mar 3, 2016 at 1:20 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > On Wed, Mar 02, 2016 at 11:49:48PM +0100, Rafael J. Wysocki wrote:
> > >> >>> +       min_f = sg_policy->policy->cpuinfo.min_freq;
> > >> >>> +       max_f = sg_policy->policy->cpuinfo.max_freq;
> > >> >>> +       next_f = util > max ? max_f : min_f + util * (max_f - min_f) / max;
> > >
> > >> In case a more formal derivation of this formula is needed, it is
> > >> based on the following 3 assumptions:
> > >>
> > >> (1) Performance is a linear function of frequency.
> > >> (2) Required performance is a linear function of the utilization ratio
> > >> x = util/max as provided by the scheduler (0 <= x <= 1).
> > >
> > >> (3) The minimum possible frequency (min_freq) corresponds to x = 0 and
> > >> the maximum possible frequency (max_freq) corresponds to x = 1.
> > >>
> > >> (1) and (2) combined imply that
> > >>
> > >> f = a * x + b
> > >>
> > >> (f - frequency, a, b - constants to be determined) and then (3) quite
> > >> trivially leads to b = min_freq and a = max_freq - min_freq.
> > >
> > > 3 is the problem, that just doesn't make sense and is probably the
> > > reason why you see very little selection of the min freq.
> > 
> > It is about mapping the entire [0,1] interval to the available frequency range.
> 
> Yeah, but I don't see why that makes sense..
> 
> > I till overprovision things (the smaller x the more), but then it may
> > help the race-to-idle a bit in theory.
> 
> So, since we also have the cpuidle information, could we not make a
> better guess at race-to-idle?
> 
> > > Suppose a machine with the following frequencies:
> > >
> > >         500, 750, 1000
> > >
> > > And a utilization of 0.4, how does asking for 500 + 0.4 * (1000-500) =
> > > 700 make any sense? Per your point 1, it should should be asking for
> > > 0.4 * 1000 = 400.
> > >
> > > Because, per 1, at 500 it runs exactly half as fast as at 1000, and we
> > > only need 0.4 times as much. Therefore 500 is more than sufficient.
> > 
> > OK, but then I don't see why this reasoning only applies to the lower
> > bound of the frequency range.  Is there any reason why x = 1 should be
> > the only point mapping to max_freq?
> 
> Well, everything that goes over the second to last freq would end up at
> the last (max) freq.
> 
> Take again the 500,750,1000 example, everything that's >750 would end up
> at 1000 (for relation_l, >875 for _c).
> 
> But given the platform's cpuidle information, maybe coupled with an avg
> idle est, we can compute the benefit of race-to-idle and over provision
> based on that, right?
> 

Shouldn't this kind of considerations be a scheduler thing? I'm not
really getting why we want to put more "intelligence" in a new governor.
Also, if I understand Ingo's point correctly, I think we want to make
this kind of policy decisions inside the scheduler.