Re: [PATCH] cpufreq: schedutil: add up/down frequency transition rate limits

[Juri Lelli <Juri.Lelli@arm.com>]

On 21/11/16 13:26, Peter Zijlstra wrote:
> On Mon, Nov 21, 2016 at 12:14:32PM +0000, Juri Lelli wrote:
> > On 21/11/16 11:19, Peter Zijlstra wrote:
> 
> > > So no tunables and rate limits here at all please.
> > > 
> > > During LPC we discussed the rampup and decay issues and decided that we
> > > should very much first address them by playing with the PELT stuff.
> > > Morton was going to play with capping the decay on the util signal. This
> > > should greatly improve the ramp-up scenario and cure some other wobbles.
> > > 
> > > The decay can be set by changing the over-all pelt decay, if so desired.
> > > 
> > 
> > Do you mean we might want to change the decay (make it different from
> > ramp-up) once for all, or maybe we make it tunable so that we can
> > address different power/perf requirements?
> 
> So the limited decay would be the dominant factor in ramp-up time,
> leaving the regular PELT period the dominant factor for ramp-down.
> 

Hmmm, AFAIU the limited decay will help not forgetting completely the
contribution of tasks that sleep for a long time, but it won't modify
the actual ramp-up of the signal. So, for new tasks we will need to play
with a sensible initial value (trading off perf and power as usual).

> (Note that the decay limit would only be applied on the per-task signal,
> not the accumulated signal.)
> 

Right, and since schedutil consumes the latter, we could still suffer
from too frequent frequency switch events I guess (this is where the
down threshold thing came as a quick and dirty fix). Maybe we can think
of some smoothing applied to the accumulated signal, or make it decay
slower (don't really know what this means in practice, though :) ?

> It could be an option, for some, to build the kernel with a PELT window
> of 16ms or so (half its current size), this of course means regenerating
> all the constants etc.. And this very much is a compile time thing.
> 

Right. I seem to remember that helped a bit for mobile type of
workloads. But never did a thorough evaluation.

> We could fairly easy; if this is so desired; make the PELT window size a
> CONFIG option (hidden by default).
> 
> But like everything; patches should come with numbers justifying them
> etc..
> 

Sure. :)

> > > Also, there was the idea of; once the above ideas have all been
> > > explored; tying the freq ram rate to the power curve.
> > > 
> > 
> > Yep. That's an interesting one to look at, but it might require some
> > time.
> 
> Sure, just saying that we should resist knobs until all other avenues
> have been explored. Never start with a knob.